com.google.code.gson/gson/2.8.5 : com/google/gson/Gson.java

Gson
https://github.com/google/gson/gson: Gson JSON library
Apache 2.0
/*
 * Copyright (C) 2008 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.gson;

import java.io.EOFException;
import java.io.IOException;
import java.io.Reader;
import java.io.StringReader;
import java.io.StringWriter;
import java.io.Writer;
import java.lang.reflect.Type;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.text.DateFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicLongArray;

import com.google.gson.internal.ConstructorConstructor;
import com.google.gson.internal.Excluder;
import com.google.gson.internal.GsonBuildConfig;
import com.google.gson.internal.Primitives;
import com.google.gson.internal.Streams;
import com.google.gson.internal.bind.ArrayTypeAdapter;
import com.google.gson.internal.bind.CollectionTypeAdapterFactory;
import com.google.gson.internal.bind.DateTypeAdapter;
import com.google.gson.internal.bind.JsonAdapterAnnotationTypeAdapterFactory;
import com.google.gson.internal.bind.JsonTreeReader;
import com.google.gson.internal.bind.JsonTreeWriter;
import com.google.gson.internal.bind.MapTypeAdapterFactory;
import com.google.gson.internal.bind.ObjectTypeAdapter;
import com.google.gson.internal.bind.ReflectiveTypeAdapterFactory;
import com.google.gson.internal.bind.SqlDateTypeAdapter;
import com.google.gson.internal.bind.TimeTypeAdapter;
import com.google.gson.internal.bind.TypeAdapters;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;
import com.google.gson.stream.MalformedJsonException;

This is the main class for using Gson. Gson is typically used by first constructing a Gson instance and then invoking toJson(Object) or fromJson(String, Class<Object>) methods on it. Gson instances are Thread-safe so you can reuse them freely across multiple threads. You can create a Gson instance by invoking new Gson() if the default configuration is all you need. You can also use GsonBuilder to build a Gson instance with various configuration options such as versioning support, pretty printing, custom JsonSerializers, JsonDeserializers, and InstanceCreators.
Here is an example of how Gson is used for a simple Class:
Gson gson = new Gson(); // Or use new GsonBuilder().create();
MyType target = new MyType();
String json = gson.toJson(target); // serializes target to Json
MyType target2 = gson.fromJson(json, MyType.class); // deserializes json into target2

If the object that your are serializing/deserializing is a ParameterizedType (i.e. contains at least one type parameter and may be an array) then you must use the toJson(Object, Type) or fromJson(String, Type) method. Here is an example for serializing and deserializing a ParameterizedType: 
Type listType = new TypeToken<List<String>>() {}.getType();
List<String> target = new LinkedList<String>();
target.add("blah");
Gson gson = new Gson();
String json = gson.toJson(target, listType);
List<String> target2 = gson.fromJson(json, listType);

See the Gson User Guide
for a more complete set of examples.
Author: Inderjeet Singh, Joel Leitch, Jesse Wilson
See Also: TypeToken/**
 * This is the main class for using Gson. Gson is typically used by first constructing a
 * Gson instance and then invoking {@link #toJson(Object)} or {@link #fromJson(String, Class)}
 * methods on it. Gson instances are Thread-safe so you can reuse them freely across multiple
 * threads.
 *
 * <p>You can create a Gson instance by invoking {@code new Gson()} if the default configuration
 * is all you need. You can also use {@link GsonBuilder} to build a Gson instance with various
 * configuration options such as versioning support, pretty printing, custom
 * {@link JsonSerializer}s, {@link JsonDeserializer}s, and {@link InstanceCreator}s.</p>
 *
 * <p>Here is an example of how Gson is used for a simple Class:
 *
 * <pre>
 * Gson gson = new Gson(); // Or use new GsonBuilder().create();
 * MyType target = new MyType();
 * String json = gson.toJson(target); // serializes target to Json
 * MyType target2 = gson.fromJson(json, MyType.class); // deserializes json into target2
 * </pre></p>
 *
 * <p>If the object that your are serializing/deserializing is a {@code ParameterizedType}
 * (i.e. contains at least one type parameter and may be an array) then you must use the
 * {@link #toJson(Object, Type)} or {@link #fromJson(String, Type)} method.  Here is an
 * example for serializing and deserializing a {@code ParameterizedType}:
 *
 * <pre>
 * Type listType = new TypeToken&lt;List&lt;String&gt;&gt;() {}.getType();
 * List&lt;String&gt; target = new LinkedList&lt;String&gt;();
 * target.add("blah");
 *
 * Gson gson = new Gson();
 * String json = gson.toJson(target, listType);
 * List&lt;String&gt; target2 = gson.fromJson(json, listType);
 * </pre></p>
 *
 * <p>See the <a href="https://sites.google.com/site/gson/gson-user-guide">Gson User Guide</a>
 * for a more complete set of examples.</p>
 *
 * @see com.google.gson.reflect.TypeToken
 *
 * @author Inderjeet Singh
 * @author Joel Leitch
 * @author Jesse Wilson
 */
public final class Gson {
  static final boolean DEFAULT_JSON_NON_EXECUTABLE = false;
  static final boolean DEFAULT_LENIENT = false;
  static final boolean DEFAULT_PRETTY_PRINT = false;
  static final boolean DEFAULT_ESCAPE_HTML = true;
  static final boolean DEFAULT_SERIALIZE_NULLS = false;
  static final boolean DEFAULT_COMPLEX_MAP_KEYS = false;
  static final boolean DEFAULT_SPECIALIZE_FLOAT_VALUES = false;

  private static final TypeToken<?> NULL_KEY_SURROGATE = TypeToken.get(Object.class);
  private static final String JSON_NON_EXECUTABLE_PREFIX = ")]}'\n";

  This thread local guards against reentrant calls to getAdapter(). In
certain object graphs, creating an adapter for a type may recursively
require an adapter for the same type! Without intervention, the recursive
lookup would stack overflow. We cheat by returning a proxy type adapter.
The proxy is wired up once the initial adapter has been created.
/**
   * This thread local guards against reentrant calls to getAdapter(). In
   * certain object graphs, creating an adapter for a type may recursively
   * require an adapter for the same type! Without intervention, the recursive
   * lookup would stack overflow. We cheat by returning a proxy type adapter.
   * The proxy is wired up once the initial adapter has been created.
   */
  private final ThreadLocal<Map<TypeToken<?>, FutureTypeAdapter<?>>> calls
      = new ThreadLocal<Map<TypeToken<?>, FutureTypeAdapter<?>>>();

  private final Map<TypeToken<?>, TypeAdapter<?>> typeTokenCache = new ConcurrentHashMap<TypeToken<?>, TypeAdapter<?>>();

  private final ConstructorConstructor constructorConstructor;
  private final JsonAdapterAnnotationTypeAdapterFactory jsonAdapterFactory;

  final List<TypeAdapterFactory> factories;

  final Excluder excluder;
  final FieldNamingStrategy fieldNamingStrategy;
  final Map<Type, InstanceCreator<?>> instanceCreators;
  final boolean serializeNulls;
  final boolean complexMapKeySerialization;
  final boolean generateNonExecutableJson;
  final boolean htmlSafe;
  final boolean prettyPrinting;
  final boolean lenient;
  final boolean serializeSpecialFloatingPointValues;
  final String datePattern;
  final int dateStyle;
  final int timeStyle;
  final LongSerializationPolicy longSerializationPolicy;
  final List<TypeAdapterFactory> builderFactories;
  final List<TypeAdapterFactory> builderHierarchyFactories;

  Constructs a Gson object with default configuration. The default configuration has the
following settings:

  The JSON generated by toJson methods is in compact representation. This means that all the unneeded white-space is removed. You can change this behavior with GsonBuilder.setPrettyPrinting(). 
  The generated JSON omits all the fields that are null. Note that nulls in arrays are kept as is since an array is an ordered list. Moreover, if a field is not null, but its generated JSON is empty, the field is kept. You can configure Gson to serialize null values by setting GsonBuilder.serializeNulls().
  Gson provides default serialization and deserialization for Enums, Map, URL, URI, Locale, Date, BigDecimal, and BigInteger classes. If you would prefer to change the default representation, you can do so by registering a type adapter through GsonBuilder.registerTypeAdapter(Type, Object). 
  The default Date format is same as DateFormat.DEFAULT. This format ignores the millisecond portion of the date during serialization. You can change this by invoking GsonBuilder.setDateFormat(int) or GsonBuilder.setDateFormat(String). 
  By default, Gson ignores the Expose annotation. You can enable Gson to serialize/deserialize only those fields marked with this annotation through GsonBuilder.excludeFieldsWithoutExposeAnnotation(). 
  By default, Gson ignores the Since annotation. You can enable Gson to use this annotation through GsonBuilder.setVersion(double).
  The default field naming policy for the output Json is same as in Java. So, a Java class
  field versionNumber will be output as "versionNumber" in Json. The same rules are applied for mapping incoming Json to the Java classes. You can change this policy through GsonBuilder.setFieldNamingPolicy(FieldNamingPolicy).
  By default, Gson excludes transient or static fields from consideration for serialization and deserialization. You can change this behavior through GsonBuilder.excludeFieldsWithModifiers(int...).

/**
   * Constructs a Gson object with default configuration. The default configuration has the
   * following settings:
   * <ul>
   *   <li>The JSON generated by <code>toJson</code> methods is in compact representation. This
   *   means that all the unneeded white-space is removed. You can change this behavior with
   *   {@link GsonBuilder#setPrettyPrinting()}. </li>
   *   <li>The generated JSON omits all the fields that are null. Note that nulls in arrays are
   *   kept as is since an array is an ordered list. Moreover, if a field is not null, but its
   *   generated JSON is empty, the field is kept. You can configure Gson to serialize null values
   *   by setting {@link GsonBuilder#serializeNulls()}.</li>
   *   <li>Gson provides default serialization and deserialization for Enums, {@link Map},
   *   {@link java.net.URL}, {@link java.net.URI}, {@link java.util.Locale}, {@link java.util.Date},
   *   {@link java.math.BigDecimal}, and {@link java.math.BigInteger} classes. If you would prefer
   *   to change the default representation, you can do so by registering a type adapter through
   *   {@link GsonBuilder#registerTypeAdapter(Type, Object)}. </li>
   *   <li>The default Date format is same as {@link java.text.DateFormat#DEFAULT}. This format
   *   ignores the millisecond portion of the date during serialization. You can change
   *   this by invoking {@link GsonBuilder#setDateFormat(int)} or
   *   {@link GsonBuilder#setDateFormat(String)}. </li>
   *   <li>By default, Gson ignores the {@link com.google.gson.annotations.Expose} annotation.
   *   You can enable Gson to serialize/deserialize only those fields marked with this annotation
   *   through {@link GsonBuilder#excludeFieldsWithoutExposeAnnotation()}. </li>
   *   <li>By default, Gson ignores the {@link com.google.gson.annotations.Since} annotation. You
   *   can enable Gson to use this annotation through {@link GsonBuilder#setVersion(double)}.</li>
   *   <li>The default field naming policy for the output Json is same as in Java. So, a Java class
   *   field <code>versionNumber</code> will be output as <code>&quot;versionNumber&quot;</code> in
   *   Json. The same rules are applied for mapping incoming Json to the Java classes. You can
   *   change this policy through {@link GsonBuilder#setFieldNamingPolicy(FieldNamingPolicy)}.</li>
   *   <li>By default, Gson excludes <code>transient</code> or <code>static</code> fields from
   *   consideration for serialization and deserialization. You can change this behavior through
   *   {@link GsonBuilder#excludeFieldsWithModifiers(int...)}.</li>
   * </ul>
   */
  public Gson() {
    this(Excluder.DEFAULT, FieldNamingPolicy.IDENTITY,
        Collections.<Type, InstanceCreator<?>>emptyMap(), DEFAULT_SERIALIZE_NULLS,
        DEFAULT_COMPLEX_MAP_KEYS, DEFAULT_JSON_NON_EXECUTABLE, DEFAULT_ESCAPE_HTML,
        DEFAULT_PRETTY_PRINT, DEFAULT_LENIENT, DEFAULT_SPECIALIZE_FLOAT_VALUES,
        LongSerializationPolicy.DEFAULT, null, DateFormat.DEFAULT, DateFormat.DEFAULT,
        Collections.<TypeAdapterFactory>emptyList(), Collections.<TypeAdapterFactory>emptyList(),
        Collections.<TypeAdapterFactory>emptyList());
  }

  Gson(final Excluder excluder, final FieldNamingStrategy fieldNamingStrategy,
      final Map<Type, InstanceCreator<?>> instanceCreators, boolean serializeNulls,
      boolean complexMapKeySerialization, boolean generateNonExecutableGson, boolean htmlSafe,
      boolean prettyPrinting, boolean lenient, boolean serializeSpecialFloatingPointValues,
      LongSerializationPolicy longSerializationPolicy, String datePattern, int dateStyle,
      int timeStyle, List<TypeAdapterFactory> builderFactories,
      List<TypeAdapterFactory> builderHierarchyFactories,
      List<TypeAdapterFactory> factoriesToBeAdded) {
    this.excluder = excluder;
    this.fieldNamingStrategy = fieldNamingStrategy;
    this.instanceCreators = instanceCreators;
    this.constructorConstructor = new ConstructorConstructor(instanceCreators);
    this.serializeNulls = serializeNulls;
    this.complexMapKeySerialization = complexMapKeySerialization;
    this.generateNonExecutableJson = generateNonExecutableGson;
    this.htmlSafe = htmlSafe;
    this.prettyPrinting = prettyPrinting;
    this.lenient = lenient;
    this.serializeSpecialFloatingPointValues = serializeSpecialFloatingPointValues;
    this.longSerializationPolicy = longSerializationPolicy;
    this.datePattern = datePattern;
    this.dateStyle = dateStyle;
    this.timeStyle = timeStyle;
    this.builderFactories = builderFactories;
    this.builderHierarchyFactories = builderHierarchyFactories;

    List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();

    // built-in type adapters that cannot be overridden
    factories.add(TypeAdapters.JSON_ELEMENT_FACTORY);
    factories.add(ObjectTypeAdapter.FACTORY);

    // the excluder must precede all adapters that handle user-defined types
    factories.add(excluder);

    // users' type adapters
    factories.addAll(factoriesToBeAdded);

    // type adapters for basic platform types
    factories.add(TypeAdapters.STRING_FACTORY);
    factories.add(TypeAdapters.INTEGER_FACTORY);
    factories.add(TypeAdapters.BOOLEAN_FACTORY);
    factories.add(TypeAdapters.BYTE_FACTORY);
    factories.add(TypeAdapters.SHORT_FACTORY);
    TypeAdapter<Number> longAdapter = longAdapter(longSerializationPolicy);
    factories.add(TypeAdapters.newFactory(long.class, Long.class, longAdapter));
    factories.add(TypeAdapters.newFactory(double.class, Double.class,
            doubleAdapter(serializeSpecialFloatingPointValues)));
    factories.add(TypeAdapters.newFactory(float.class, Float.class,
            floatAdapter(serializeSpecialFloatingPointValues)));
    factories.add(TypeAdapters.NUMBER_FACTORY);
    factories.add(TypeAdapters.ATOMIC_INTEGER_FACTORY);
    factories.add(TypeAdapters.ATOMIC_BOOLEAN_FACTORY);
    factories.add(TypeAdapters.newFactory(AtomicLong.class, atomicLongAdapter(longAdapter)));
    factories.add(TypeAdapters.newFactory(AtomicLongArray.class, atomicLongArrayAdapter(longAdapter)));
    factories.add(TypeAdapters.ATOMIC_INTEGER_ARRAY_FACTORY);
    factories.add(TypeAdapters.CHARACTER_FACTORY);
    factories.add(TypeAdapters.STRING_BUILDER_FACTORY);
    factories.add(TypeAdapters.STRING_BUFFER_FACTORY);
    factories.add(TypeAdapters.newFactory(BigDecimal.class, TypeAdapters.BIG_DECIMAL));
    factories.add(TypeAdapters.newFactory(BigInteger.class, TypeAdapters.BIG_INTEGER));
    factories.add(TypeAdapters.URL_FACTORY);
    factories.add(TypeAdapters.URI_FACTORY);
    factories.add(TypeAdapters.UUID_FACTORY);
    factories.add(TypeAdapters.CURRENCY_FACTORY);
    factories.add(TypeAdapters.LOCALE_FACTORY);
    factories.add(TypeAdapters.INET_ADDRESS_FACTORY);
    factories.add(TypeAdapters.BIT_SET_FACTORY);
    factories.add(DateTypeAdapter.FACTORY);
    factories.add(TypeAdapters.CALENDAR_FACTORY);
    factories.add(TimeTypeAdapter.FACTORY);
    factories.add(SqlDateTypeAdapter.FACTORY);
    factories.add(TypeAdapters.TIMESTAMP_FACTORY);
    factories.add(ArrayTypeAdapter.FACTORY);
    factories.add(TypeAdapters.CLASS_FACTORY);

    // type adapters for composite and user-defined types
    factories.add(new CollectionTypeAdapterFactory(constructorConstructor));
    factories.add(new MapTypeAdapterFactory(constructorConstructor, complexMapKeySerialization));
    this.jsonAdapterFactory = new JsonAdapterAnnotationTypeAdapterFactory(constructorConstructor);
    factories.add(jsonAdapterFactory);
    factories.add(TypeAdapters.ENUM_FACTORY);
    factories.add(new ReflectiveTypeAdapterFactory(
        constructorConstructor, fieldNamingStrategy, excluder, jsonAdapterFactory));

    this.factories = Collections.unmodifiableList(factories);
  }

  Returns a new GsonBuilder containing all custom factories and configuration used by the current
instance.
Returns: a GsonBuilder instance./**
   * Returns a new GsonBuilder containing all custom factories and configuration used by the current
   * instance.
   *
   * @return a GsonBuilder instance.
   */
  public GsonBuilder newBuilder() {
    return new GsonBuilder(this);
  }

  public Excluder excluder() {
    return excluder;
  }

  public FieldNamingStrategy fieldNamingStrategy() {
    return fieldNamingStrategy;
  }

  public boolean serializeNulls() {
    return serializeNulls;
  }

  public boolean htmlSafe() {
    return htmlSafe;
  }

  private TypeAdapter<Number> doubleAdapter(boolean serializeSpecialFloatingPointValues) {
    if (serializeSpecialFloatingPointValues) {
      return TypeAdapters.DOUBLE;
    }
    return new TypeAdapter<Number>() {
      @Override public Double read(JsonReader in) throws IOException {
        if (in.peek() == JsonToken.NULL) {
          in.nextNull();
          return null;
        }
        return in.nextDouble();
      }
      @Override public void write(JsonWriter out, Number value) throws IOException {
        if (value == null) {
          out.nullValue();
          return;
        }
        double doubleValue = value.doubleValue();
        checkValidFloatingPoint(doubleValue);
        out.value(value);
      }
    };
  }

  private TypeAdapter<Number> floatAdapter(boolean serializeSpecialFloatingPointValues) {
    if (serializeSpecialFloatingPointValues) {
      return TypeAdapters.FLOAT;
    }
    return new TypeAdapter<Number>() {
      @Override public Float read(JsonReader in) throws IOException {
        if (in.peek() == JsonToken.NULL) {
          in.nextNull();
          return null;
        }
        return (float) in.nextDouble();
      }
      @Override public void write(JsonWriter out, Number value) throws IOException {
        if (value == null) {
          out.nullValue();
          return;
        }
        float floatValue = value.floatValue();
        checkValidFloatingPoint(floatValue);
        out.value(value);
      }
    };
  }

  static void checkValidFloatingPoint(double value) {
    if (Double.isNaN(value) || Double.isInfinite(value)) {
      throw new IllegalArgumentException(value
          + " is not a valid double value as per JSON specification. To override this"
          + " behavior, use GsonBuilder.serializeSpecialFloatingPointValues() method.");
    }
  }

  private static TypeAdapter<Number> longAdapter(LongSerializationPolicy longSerializationPolicy) {
    if (longSerializationPolicy == LongSerializationPolicy.DEFAULT) {
      return TypeAdapters.LONG;
    }
    return new TypeAdapter<Number>() {
      @Override public Number read(JsonReader in) throws IOException {
        if (in.peek() == JsonToken.NULL) {
          in.nextNull();
          return null;
        }
        return in.nextLong();
      }
      @Override public void write(JsonWriter out, Number value) throws IOException {
        if (value == null) {
          out.nullValue();
          return;
        }
        out.value(value.toString());
      }
    };
  }

  private static TypeAdapter<AtomicLong> atomicLongAdapter(final TypeAdapter<Number> longAdapter) {
    return new TypeAdapter<AtomicLong>() {
      @Override public void write(JsonWriter out, AtomicLong value) throws IOException {
        longAdapter.write(out, value.get());
      }
      @Override public AtomicLong read(JsonReader in) throws IOException {
        Number value = longAdapter.read(in);
        return new AtomicLong(value.longValue());
      }
    }.nullSafe();
  }

  private static TypeAdapter<AtomicLongArray> atomicLongArrayAdapter(final TypeAdapter<Number> longAdapter) {
    return new TypeAdapter<AtomicLongArray>() {
      @Override public void write(JsonWriter out, AtomicLongArray value) throws IOException {
        out.beginArray();
        for (int i = 0, length = value.length(); i < length; i++) {
          longAdapter.write(out, value.get(i));
        }
        out.endArray();
      }
      @Override public AtomicLongArray read(JsonReader in) throws IOException {
        List<Long> list = new ArrayList<Long>();
        in.beginArray();
        while (in.hasNext()) {
            long value = longAdapter.read(in).longValue();
            list.add(value);
        }
        in.endArray();
        int length = list.size();
        AtomicLongArray array = new AtomicLongArray(length);
        for (int i = 0; i < length; ++i) {
          array.set(i, list.get(i));
        }
        return array;
      }
    }.nullSafe();
  }

  Returns the type adapter for  type. 
Throws: IllegalArgumentException – if this GSON cannot serialize and deserialize type./**
   * Returns the type adapter for {@code} type.
   *
   * @throws IllegalArgumentException if this GSON cannot serialize and
   *     deserialize {@code type}.
   */
  @SuppressWarnings("unchecked")
  public <T> TypeAdapter<T> getAdapter(TypeToken<T> type) {
    TypeAdapter<?> cached = typeTokenCache.get(type == null ? NULL_KEY_SURROGATE : type);
    if (cached != null) {
      return (TypeAdapter<T>) cached;
    }

    Map<TypeToken<?>, FutureTypeAdapter<?>> threadCalls = calls.get();
    boolean requiresThreadLocalCleanup = false;
    if (threadCalls == null) {
      threadCalls = new HashMap<TypeToken<?>, FutureTypeAdapter<?>>();
      calls.set(threadCalls);
      requiresThreadLocalCleanup = true;
    }

    // the key and value type parameters always agree
    FutureTypeAdapter<T> ongoingCall = (FutureTypeAdapter<T>) threadCalls.get(type);
    if (ongoingCall != null) {
      return ongoingCall;
    }

    try {
      FutureTypeAdapter<T> call = new FutureTypeAdapter<T>();
      threadCalls.put(type, call);

      for (TypeAdapterFactory factory : factories) {
        TypeAdapter<T> candidate = factory.create(this, type);
        if (candidate != null) {
          call.setDelegate(candidate);
          typeTokenCache.put(type, candidate);
          return candidate;
        }
      }
      throw new IllegalArgumentException("GSON (" + GsonBuildConfig.VERSION + ") cannot handle " + type);
    } finally {
      threadCalls.remove(type);

      if (requiresThreadLocalCleanup) {
        calls.remove();
      }
    }
  }

  This method is used to get an alternate type adapter for the specified type. This is used to access a type adapter that is overridden by a TypeAdapterFactory that you may have registered. This features is typically used when you want to register a type adapter that does a little bit of work but then delegates further processing to the Gson default type adapter. Here is an example: Let's say we want to write a type adapter that counts the number of objects being read
 from or written to JSON. We can achieve this by writing a type adapter factory that uses
 the getDelegateAdapter method:
 
 
 class StatsTypeAdapterFactory implements TypeAdapterFactory {
   public int numReads = 0;
   public int numWrites = 0;
   public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
     final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
     return new TypeAdapter<T>() {
       public void write(JsonWriter out, T value) throws IOException {
         ++numWrites;
         delegate.write(out, value);
       }
       public T read(JsonReader in) throws IOException {
         ++numReads;
         return delegate.read(in);
       }
     };
   }
 }
  
 This factory can now be used like this:
  
 StatsTypeAdapterFactory stats = new StatsTypeAdapterFactory();
 Gson gson = new GsonBuilder().registerTypeAdapterFactory(stats).create();
 // Call gson.toJson() and fromJson methods on objects
 System.out.println("Num JSON reads" + stats.numReads);
 System.out.println("Num JSON writes" + stats.numWrites);
 
 Note that this call will skip all factories registered before skipPast. In case of multiple TypeAdapterFactories registered it is up to the caller of this function to insure that the order of registration does not prevent this method from reaching a factory they would expect to reply from this call. Note that since you can not override type adapter factories for String and Java primitive types, our stats factory will not count the number of String or primitives that will be read or written. 
Params: skipPast – The type adapter factory that needs to be skipped while searching for
  a matching type adapter. In most cases, you should just pass this (the type adapter factory from where getDelegateAdapter method is being invoked).
type – Type for which the delegate adapter is being searched for.
Since: 2.2/**
   * This method is used to get an alternate type adapter for the specified type. This is used
   * to access a type adapter that is overridden by a {@link TypeAdapterFactory} that you
   * may have registered. This features is typically used when you want to register a type
   * adapter that does a little bit of work but then delegates further processing to the Gson
   * default type adapter. Here is an example:
   * <p>Let's say we want to write a type adapter that counts the number of objects being read
   *  from or written to JSON. We can achieve this by writing a type adapter factory that uses
   *  the <code>getDelegateAdapter</code> method:
   *  <pre> {@code
   *  class StatsTypeAdapterFactory implements TypeAdapterFactory {
   *    public int numReads = 0;
   *    public int numWrites = 0;
   *    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
   *      final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
   *      return new TypeAdapter<T>() {
   *        public void write(JsonWriter out, T value) throws IOException {
   *          ++numWrites;
   *          delegate.write(out, value);
   *        }
   *        public T read(JsonReader in) throws IOException {
   *          ++numReads;
   *          return delegate.read(in);
   *        }
   *      };
   *    }
   *  }
   *  } </pre>
   *  This factory can now be used like this:
   *  <pre> {@code
   *  StatsTypeAdapterFactory stats = new StatsTypeAdapterFactory();
   *  Gson gson = new GsonBuilder().registerTypeAdapterFactory(stats).create();
   *  // Call gson.toJson() and fromJson methods on objects
   *  System.out.println("Num JSON reads" + stats.numReads);
   *  System.out.println("Num JSON writes" + stats.numWrites);
   *  }</pre>
   *  Note that this call will skip all factories registered before {@code skipPast}. In case of
   *  multiple TypeAdapterFactories registered it is up to the caller of this function to insure
   *  that the order of registration does not prevent this method from reaching a factory they
   *  would expect to reply from this call.
   *  Note that since you can not override type adapter factories for String and Java primitive
   *  types, our stats factory will not count the number of String or primitives that will be
   *  read or written.
   * @param skipPast The type adapter factory that needs to be skipped while searching for
   *   a matching type adapter. In most cases, you should just pass <i>this</i> (the type adapter
   *   factory from where {@link #getDelegateAdapter} method is being invoked).
   * @param type Type for which the delegate adapter is being searched for.
   *
   * @since 2.2
   */
  public <T> TypeAdapter<T> getDelegateAdapter(TypeAdapterFactory skipPast, TypeToken<T> type) {
    // Hack. If the skipPast factory isn't registered, assume the factory is being requested via
    // our @JsonAdapter annotation.
    if (!factories.contains(skipPast)) {
      skipPast = jsonAdapterFactory;
    }

    boolean skipPastFound = false;
    for (TypeAdapterFactory factory : factories) {
      if (!skipPastFound) {
        if (factory == skipPast) {
          skipPastFound = true;
        }
        continue;
      }

      TypeAdapter<T> candidate = factory.create(this, type);
      if (candidate != null) {
        return candidate;
      }
    }
    throw new IllegalArgumentException("GSON cannot serialize " + type);
  }

  Returns the type adapter for  type. 
Throws: IllegalArgumentException – if this GSON cannot serialize and deserialize type./**
   * Returns the type adapter for {@code} type.
   *
   * @throws IllegalArgumentException if this GSON cannot serialize and
   *     deserialize {@code type}.
   */
  public <T> TypeAdapter<T> getAdapter(Class<T> type) {
    return getAdapter(TypeToken.get(type));
  }

  This method serializes the specified object into its equivalent representation as a tree of JsonElements. This method should be used when the specified object is not a generic type. This method uses Object.getClass() to get the type for the specified object, but the getClass() loses the generic type information because of the Type Erasure feature of Java. Note that this method works fine if the any of the object fields are of generic type, just the object itself should not be of a generic type. If the object is of generic type, use toJsonTree(Object, Type) instead. 
Params: src – the object for which Json representation is to be created setting for Gson
Returns: Json representation of src.
Since: 1.4/**
   * This method serializes the specified object into its equivalent representation as a tree of
   * {@link JsonElement}s. This method should be used when the specified object is not a generic
   * type. This method uses {@link Class#getClass()} to get the type for the specified object, but
   * the {@code getClass()} loses the generic type information because of the Type Erasure feature
   * of Java. Note that this method works fine if the any of the object fields are of generic type,
   * just the object itself should not be of a generic type. If the object is of generic type, use
   * {@link #toJsonTree(Object, Type)} instead.
   *
   * @param src the object for which Json representation is to be created setting for Gson
   * @return Json representation of {@code src}.
   * @since 1.4
   */
  public JsonElement toJsonTree(Object src) {
    if (src == null) {
      return JsonNull.INSTANCE;
    }
    return toJsonTree(src, src.getClass());
  }

  This method serializes the specified object, including those of generic types, into its equivalent representation as a tree of JsonElements. This method must be used if the specified object is a generic type. For non-generic objects, use toJsonTree(Object) instead. 
Params: src – the object for which JSON representation is to be created
typeOfSrc – The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use: Type typeOfSrc = new TypeToken<Collection<Foo>>(){}.getType();
Returns: Json representation of src
Since: 1.4/**
   * This method serializes the specified object, including those of generic types, into its
   * equivalent representation as a tree of {@link JsonElement}s. This method must be used if the
   * specified object is a generic type. For non-generic objects, use {@link #toJsonTree(Object)}
   * instead.
   *
   * @param src the object for which JSON representation is to be created
   * @param typeOfSrc The specific genericized type of src. You can obtain
   * this type by using the {@link com.google.gson.reflect.TypeToken} class. For example,
   * to get the type for {@code Collection<Foo>}, you should use:
   * <pre>
   * Type typeOfSrc = new TypeToken&lt;Collection&lt;Foo&gt;&gt;(){}.getType();
   * </pre>
   * @return Json representation of {@code src}
   * @since 1.4
   */
  public JsonElement toJsonTree(Object src, Type typeOfSrc) {
    JsonTreeWriter writer = new JsonTreeWriter();
    toJson(src, typeOfSrc, writer);
    return writer.get();
  }

  This method serializes the specified object into its equivalent Json representation. This method should be used when the specified object is not a generic type. This method uses Object.getClass() to get the type for the specified object, but the getClass() loses the generic type information because of the Type Erasure feature of Java. Note that this method works fine if the any of the object fields are of generic type, just the object itself should not be of a generic type. If the object is of generic type, use toJson(Object, Type) instead. If you want to write out the object to a Writer, use toJson(Object, Appendable) instead. 
Params: src – the object for which Json representation is to be created setting for Gson
Returns: Json representation of src./**
   * This method serializes the specified object into its equivalent Json representation.
   * This method should be used when the specified object is not a generic type. This method uses
   * {@link Class#getClass()} to get the type for the specified object, but the
   * {@code getClass()} loses the generic type information because of the Type Erasure feature
   * of Java. Note that this method works fine if the any of the object fields are of generic type,
   * just the object itself should not be of a generic type. If the object is of generic type, use
   * {@link #toJson(Object, Type)} instead. If you want to write out the object to a
   * {@link Writer}, use {@link #toJson(Object, Appendable)} instead.
   *
   * @param src the object for which Json representation is to be created setting for Gson
   * @return Json representation of {@code src}.
   */
  public String toJson(Object src) {
    if (src == null) {
      return toJson(JsonNull.INSTANCE);
    }
    return toJson(src, src.getClass());
  }

  This method serializes the specified object, including those of generic types, into its equivalent Json representation. This method must be used if the specified object is a generic type. For non-generic objects, use toJson(Object) instead. If you want to write out the object to a Appendable, use toJson(Object, Type, Appendable) instead. 
Params: src – the object for which JSON representation is to be created
typeOfSrc – The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use: Type typeOfSrc = new TypeToken<Collection<Foo>>(){}.getType();
Returns: Json representation of src/**
   * This method serializes the specified object, including those of generic types, into its
   * equivalent Json representation. This method must be used if the specified object is a generic
   * type. For non-generic objects, use {@link #toJson(Object)} instead. If you want to write out
   * the object to a {@link Appendable}, use {@link #toJson(Object, Type, Appendable)} instead.
   *
   * @param src the object for which JSON representation is to be created
   * @param typeOfSrc The specific genericized type of src. You can obtain
   * this type by using the {@link com.google.gson.reflect.TypeToken} class. For example,
   * to get the type for {@code Collection<Foo>}, you should use:
   * <pre>
   * Type typeOfSrc = new TypeToken&lt;Collection&lt;Foo&gt;&gt;(){}.getType();
   * </pre>
   * @return Json representation of {@code src}
   */
  public String toJson(Object src, Type typeOfSrc) {
    StringWriter writer = new StringWriter();
    toJson(src, typeOfSrc, writer);
    return writer.toString();
  }

  This method serializes the specified object into its equivalent Json representation. This method should be used when the specified object is not a generic type. This method uses Object.getClass() to get the type for the specified object, but the getClass() loses the generic type information because of the Type Erasure feature of Java. Note that this method works fine if the any of the object fields are of generic type, just the object itself should not be of a generic type. If the object is of generic type, use toJson(Object, Type, Appendable) instead. 
Params: src – the object for which Json representation is to be created setting for Gson
writer – Writer to which the Json representation needs to be written
Throws: JsonIOException – if there was a problem writing to the writer
Since: 1.2/**
   * This method serializes the specified object into its equivalent Json representation.
   * This method should be used when the specified object is not a generic type. This method uses
   * {@link Class#getClass()} to get the type for the specified object, but the
   * {@code getClass()} loses the generic type information because of the Type Erasure feature
   * of Java. Note that this method works fine if the any of the object fields are of generic type,
   * just the object itself should not be of a generic type. If the object is of generic type, use
   * {@link #toJson(Object, Type, Appendable)} instead.
   *
   * @param src the object for which Json representation is to be created setting for Gson
   * @param writer Writer to which the Json representation needs to be written
   * @throws JsonIOException if there was a problem writing to the writer
   * @since 1.2
   */
  public void toJson(Object src, Appendable writer) throws JsonIOException {
    if (src != null) {
      toJson(src, src.getClass(), writer);
    } else {
      toJson(JsonNull.INSTANCE, writer);
    }
  }

  This method serializes the specified object, including those of generic types, into its equivalent Json representation. This method must be used if the specified object is a generic type. For non-generic objects, use toJson(Object, Appendable) instead. 
Params: src – the object for which JSON representation is to be created
typeOfSrc – The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use: Type typeOfSrc = new TypeToken<Collection<Foo>>(){}.getType();
writer – Writer to which the Json representation of src needs to be written.
Throws: JsonIOException – if there was a problem writing to the writer
Since: 1.2/**
   * This method serializes the specified object, including those of generic types, into its
   * equivalent Json representation. This method must be used if the specified object is a generic
   * type. For non-generic objects, use {@link #toJson(Object, Appendable)} instead.
   *
   * @param src the object for which JSON representation is to be created
   * @param typeOfSrc The specific genericized type of src. You can obtain
   * this type by using the {@link com.google.gson.reflect.TypeToken} class. For example,
   * to get the type for {@code Collection<Foo>}, you should use:
   * <pre>
   * Type typeOfSrc = new TypeToken&lt;Collection&lt;Foo&gt;&gt;(){}.getType();
   * </pre>
   * @param writer Writer to which the Json representation of src needs to be written.
   * @throws JsonIOException if there was a problem writing to the writer
   * @since 1.2
   */
  public void toJson(Object src, Type typeOfSrc, Appendable writer) throws JsonIOException {
    try {
      JsonWriter jsonWriter = newJsonWriter(Streams.writerForAppendable(writer));
      toJson(src, typeOfSrc, jsonWriter);
    } catch (IOException e) {
      throw new JsonIOException(e);
    }
  }

  Writes the JSON representation of src of type typeOfSrc to writer. 
Throws: JsonIOException – if there was a problem writing to the writer/**
   * Writes the JSON representation of {@code src} of type {@code typeOfSrc} to
   * {@code writer}.
   * @throws JsonIOException if there was a problem writing to the writer
   */
  @SuppressWarnings("unchecked")
  public void toJson(Object src, Type typeOfSrc, JsonWriter writer) throws JsonIOException {
    TypeAdapter<?> adapter = getAdapter(TypeToken.get(typeOfSrc));
    boolean oldLenient = writer.isLenient();
    writer.setLenient(true);
    boolean oldHtmlSafe = writer.isHtmlSafe();
    writer.setHtmlSafe(htmlSafe);
    boolean oldSerializeNulls = writer.getSerializeNulls();
    writer.setSerializeNulls(serializeNulls);
    try {
      ((TypeAdapter<Object>) adapter).write(writer, src);
    } catch (IOException e) {
      throw new JsonIOException(e);
    } catch (AssertionError e) {
      throw new AssertionError("AssertionError (GSON " + GsonBuildConfig.VERSION + "): " + e.getMessage(), e);
    } finally {
      writer.setLenient(oldLenient);
      writer.setHtmlSafe(oldHtmlSafe);
      writer.setSerializeNulls(oldSerializeNulls);
    }
  }

  Converts a tree of JsonElements into its equivalent JSON representation. 
Params: jsonElement – root of a tree of JsonElements
Returns: JSON String representation of the tree
Since: 1.4/**
   * Converts a tree of {@link JsonElement}s into its equivalent JSON representation.
   *
   * @param jsonElement root of a tree of {@link JsonElement}s
   * @return JSON String representation of the tree
   * @since 1.4
   */
  public String toJson(JsonElement jsonElement) {
    StringWriter writer = new StringWriter();
    toJson(jsonElement, writer);
    return writer.toString();
  }

  Writes out the equivalent JSON for a tree of JsonElements. 
Params: jsonElement – root of a tree of JsonElements
writer – Writer to which the Json representation needs to be written
Throws: JsonIOException – if there was a problem writing to the writer
Since: 1.4/**
   * Writes out the equivalent JSON for a tree of {@link JsonElement}s.
   *
   * @param jsonElement root of a tree of {@link JsonElement}s
   * @param writer Writer to which the Json representation needs to be written
   * @throws JsonIOException if there was a problem writing to the writer
   * @since 1.4
   */
  public void toJson(JsonElement jsonElement, Appendable writer) throws JsonIOException {
    try {
      JsonWriter jsonWriter = newJsonWriter(Streams.writerForAppendable(writer));
      toJson(jsonElement, jsonWriter);
    } catch (IOException e) {
      throw new JsonIOException(e);
    }
  }

  Returns a new JSON writer configured for the settings on this Gson instance.
/**
   * Returns a new JSON writer configured for the settings on this Gson instance.
   */
  public JsonWriter newJsonWriter(Writer writer) throws IOException {
    if (generateNonExecutableJson) {
      writer.write(JSON_NON_EXECUTABLE_PREFIX);
    }
    JsonWriter jsonWriter = new JsonWriter(writer);
    if (prettyPrinting) {
      jsonWriter.setIndent("  ");
    }
    jsonWriter.setSerializeNulls(serializeNulls);
    return jsonWriter;
  }

  Returns a new JSON reader configured for the settings on this Gson instance.
/**
   * Returns a new JSON reader configured for the settings on this Gson instance.
   */
  public JsonReader newJsonReader(Reader reader) {
    JsonReader jsonReader = new JsonReader(reader);
    jsonReader.setLenient(lenient);
    return jsonReader;
  }

  Writes the JSON for jsonElement to writer. 
Throws: JsonIOException – if there was a problem writing to the writer/**
   * Writes the JSON for {@code jsonElement} to {@code writer}.
   * @throws JsonIOException if there was a problem writing to the writer
   */
  public void toJson(JsonElement jsonElement, JsonWriter writer) throws JsonIOException {
    boolean oldLenient = writer.isLenient();
    writer.setLenient(true);
    boolean oldHtmlSafe = writer.isHtmlSafe();
    writer.setHtmlSafe(htmlSafe);
    boolean oldSerializeNulls = writer.getSerializeNulls();
    writer.setSerializeNulls(serializeNulls);
    try {
      Streams.write(jsonElement, writer);
    } catch (IOException e) {
      throw new JsonIOException(e);
    } catch (AssertionError e) {
      throw new AssertionError("AssertionError (GSON " + GsonBuildConfig.VERSION + "): " + e.getMessage(), e);
    } finally {
      writer.setLenient(oldLenient);
      writer.setHtmlSafe(oldHtmlSafe);
      writer.setSerializeNulls(oldSerializeNulls);
    }
  }

  This method deserializes the specified Json into an object of the specified class. It is not suitable to use if the specified class is a generic type since it will not have the generic type information because of the Type Erasure feature of Java. Therefore, this method should not be used if the desired type is a generic type. Note that this method works fine if the any of the fields of the specified object are generics, just the object itself should not be a generic type. For the cases when the object is of generic type, invoke fromJson(String, Type). If you have the Json in a Reader instead of a String, use fromJson(Reader, Class<Object>) instead. 
Params: json – the string from which the object is to be deserialized
classOfT – the class of T
Type parameters: <T> – the type of the desired object
Throws: JsonSyntaxException – if json is not a valid representation for an object of type
classOfT
Returns: an object of type T from the string. Returns null if json is null or if json is empty./**
   * This method deserializes the specified Json into an object of the specified class. It is not
   * suitable to use if the specified class is a generic type since it will not have the generic
   * type information because of the Type Erasure feature of Java. Therefore, this method should not
   * be used if the desired type is a generic type. Note that this method works fine if the any of
   * the fields of the specified object are generics, just the object itself should not be a
   * generic type. For the cases when the object is of generic type, invoke
   * {@link #fromJson(String, Type)}. If you have the Json in a {@link Reader} instead of
   * a String, use {@link #fromJson(Reader, Class)} instead.
   *
   * @param <T> the type of the desired object
   * @param json the string from which the object is to be deserialized
   * @param classOfT the class of T
   * @return an object of type T from the string. Returns {@code null} if {@code json} is {@code null}
   * or if {@code json} is empty.
   * @throws JsonSyntaxException if json is not a valid representation for an object of type
   * classOfT
   */
  public <T> T fromJson(String json, Class<T> classOfT) throws JsonSyntaxException {
    Object object = fromJson(json, (Type) classOfT);
    return Primitives.wrap(classOfT).cast(object);
  }

  This method deserializes the specified Json into an object of the specified type. This method is useful if the specified object is a generic type. For non-generic objects, use fromJson(String, Class<Object>) instead. If you have the Json in a Reader instead of a String, use fromJson(Reader, Type) instead. 
Params: json – the string from which the object is to be deserialized
typeOfT – The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use: Type typeOfT = new TypeToken<Collection<Foo>>(){}.getType();
Type parameters: <T> – the type of the desired object
Throws: JsonParseException – if json is not a valid representation for an object of type typeOfT
JsonSyntaxException – if json is not a valid representation for an object of type
Returns: an object of type T from the string. Returns null if json is null./**
   * This method deserializes the specified Json into an object of the specified type. This method
   * is useful if the specified object is a generic type. For non-generic objects, use
   * {@link #fromJson(String, Class)} instead. If you have the Json in a {@link Reader} instead of
   * a String, use {@link #fromJson(Reader, Type)} instead.
   *
   * @param <T> the type of the desired object
   * @param json the string from which the object is to be deserialized
   * @param typeOfT The specific genericized type of src. You can obtain this type by using the
   * {@link com.google.gson.reflect.TypeToken} class. For example, to get the type for
   * {@code Collection<Foo>}, you should use:
   * <pre>
   * Type typeOfT = new TypeToken&lt;Collection&lt;Foo&gt;&gt;(){}.getType();
   * </pre>
   * @return an object of type T from the string. Returns {@code null} if {@code json} is {@code null}.
   * @throws JsonParseException if json is not a valid representation for an object of type typeOfT
   * @throws JsonSyntaxException if json is not a valid representation for an object of type
   */
  @SuppressWarnings("unchecked")
  public <T> T fromJson(String json, Type typeOfT) throws JsonSyntaxException {
    if (json == null) {
      return null;
    }
    StringReader reader = new StringReader(json);
    T target = (T) fromJson(reader, typeOfT);
    return target;
  }

  This method deserializes the Json read from the specified reader into an object of the specified class. It is not suitable to use if the specified class is a generic type since it will not have the generic type information because of the Type Erasure feature of Java. Therefore, this method should not be used if the desired type is a generic type. Note that this method works fine if the any of the fields of the specified object are generics, just the object itself should not be a generic type. For the cases when the object is of generic type, invoke fromJson(Reader, Type). If you have the Json in a String form instead of a Reader, use fromJson(String, Class<Object>) instead. 
Params: json – the reader producing the Json from which the object is to be deserialized.
classOfT – the class of T
Type parameters: <T> – the type of the desired object
Throws: JsonIOException – if there was a problem reading from the Reader
JsonSyntaxException – if json is not a valid representation for an object of type
Returns: an object of type T from the string. Returns null if json is at EOF.
Since: 1.2/**
   * This method deserializes the Json read from the specified reader into an object of the
   * specified class. It is not suitable to use if the specified class is a generic type since it
   * will not have the generic type information because of the Type Erasure feature of Java.
   * Therefore, this method should not be used if the desired type is a generic type. Note that
   * this method works fine if the any of the fields of the specified object are generics, just the
   * object itself should not be a generic type. For the cases when the object is of generic type,
   * invoke {@link #fromJson(Reader, Type)}. If you have the Json in a String form instead of a
   * {@link Reader}, use {@link #fromJson(String, Class)} instead.
   *
   * @param <T> the type of the desired object
   * @param json the reader producing the Json from which the object is to be deserialized.
   * @param classOfT the class of T
   * @return an object of type T from the string. Returns {@code null} if {@code json} is at EOF.
   * @throws JsonIOException if there was a problem reading from the Reader
   * @throws JsonSyntaxException if json is not a valid representation for an object of type
   * @since 1.2
   */
  public <T> T fromJson(Reader json, Class<T> classOfT) throws JsonSyntaxException, JsonIOException {
    JsonReader jsonReader = newJsonReader(json);
    Object object = fromJson(jsonReader, classOfT);
    assertFullConsumption(object, jsonReader);
    return Primitives.wrap(classOfT).cast(object);
  }

  This method deserializes the Json read from the specified reader into an object of the specified type. This method is useful if the specified object is a generic type. For non-generic objects, use fromJson(Reader, Class<Object>) instead. If you have the Json in a String form instead of a Reader, use fromJson(String, Type) instead. 
Params: json – the reader producing Json from which the object is to be deserialized
typeOfT – The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use: Type typeOfT = new TypeToken<Collection<Foo>>(){}.getType();
Type parameters: <T> – the type of the desired object
Throws: JsonIOException – if there was a problem reading from the Reader
JsonSyntaxException – if json is not a valid representation for an object of type
Returns: an object of type T from the json. Returns null if json is at EOF.
Since: 1.2/**
   * This method deserializes the Json read from the specified reader into an object of the
   * specified type. This method is useful if the specified object is a generic type. For
   * non-generic objects, use {@link #fromJson(Reader, Class)} instead. If you have the Json in a
   * String form instead of a {@link Reader}, use {@link #fromJson(String, Type)} instead.
   *
   * @param <T> the type of the desired object
   * @param json the reader producing Json from which the object is to be deserialized
   * @param typeOfT The specific genericized type of src. You can obtain this type by using the
   * {@link com.google.gson.reflect.TypeToken} class. For example, to get the type for
   * {@code Collection<Foo>}, you should use:
   * <pre>
   * Type typeOfT = new TypeToken&lt;Collection&lt;Foo&gt;&gt;(){}.getType();
   * </pre>
   * @return an object of type T from the json. Returns {@code null} if {@code json} is at EOF.
   * @throws JsonIOException if there was a problem reading from the Reader
   * @throws JsonSyntaxException if json is not a valid representation for an object of type
   * @since 1.2
   */
  @SuppressWarnings("unchecked")
  public <T> T fromJson(Reader json, Type typeOfT) throws JsonIOException, JsonSyntaxException {
    JsonReader jsonReader = newJsonReader(json);
    T object = (T) fromJson(jsonReader, typeOfT);
    assertFullConsumption(object, jsonReader);
    return object;
  }

  private static void assertFullConsumption(Object obj, JsonReader reader) {
    try {
      if (obj != null && reader.peek() != JsonToken.END_DOCUMENT) {
        throw new JsonIOException("JSON document was not fully consumed.");
      }
    } catch (MalformedJsonException e) {
      throw new JsonSyntaxException(e);
    } catch (IOException e) {
      throw new JsonIOException(e);
    }
  }

  Reads the next JSON value from reader and convert it to an object of type typeOfT. Returns null, if the reader is at EOF. Since Type is not parameterized by T, this method is type unsafe and should be used carefully 
Throws: JsonIOException – if there was a problem writing to the Reader
JsonSyntaxException – if json is not a valid representation for an object of type/**
   * Reads the next JSON value from {@code reader} and convert it to an object
   * of type {@code typeOfT}. Returns {@code null}, if the {@code reader} is at EOF.
   * Since Type is not parameterized by T, this method is type unsafe and should be used carefully
   *
   * @throws JsonIOException if there was a problem writing to the Reader
   * @throws JsonSyntaxException if json is not a valid representation for an object of type
   */
  @SuppressWarnings("unchecked")
  public <T> T fromJson(JsonReader reader, Type typeOfT) throws JsonIOException, JsonSyntaxException {
    boolean isEmpty = true;
    boolean oldLenient = reader.isLenient();
    reader.setLenient(true);
    try {
      reader.peek();
      isEmpty = false;
      TypeToken<T> typeToken = (TypeToken<T>) TypeToken.get(typeOfT);
      TypeAdapter<T> typeAdapter = getAdapter(typeToken);
      T object = typeAdapter.read(reader);
      return object;
    } catch (EOFException e) {
      /*
       * For compatibility with JSON 1.5 and earlier, we return null for empty
       * documents instead of throwing.
       */
      if (isEmpty) {
        return null;
      }
      throw new JsonSyntaxException(e);
    } catch (IllegalStateException e) {
      throw new JsonSyntaxException(e);
    } catch (IOException e) {
      // TODO(inder): Figure out whether it is indeed right to rethrow this as JsonSyntaxException
      throw new JsonSyntaxException(e);
    } catch (AssertionError e) {
      throw new AssertionError("AssertionError (GSON " + GsonBuildConfig.VERSION + "): " + e.getMessage(), e);
    } finally {
      reader.setLenient(oldLenient);
    }
  }

  This method deserializes the Json read from the specified parse tree into an object of the specified type. It is not suitable to use if the specified class is a generic type since it will not have the generic type information because of the Type Erasure feature of Java. Therefore, this method should not be used if the desired type is a generic type. Note that this method works fine if the any of the fields of the specified object are generics, just the object itself should not be a generic type. For the cases when the object is of generic type, invoke fromJson(JsonElement, Type). 
Params: json – the root of the parse tree of JsonElements from which the object is to be deserialized
classOfT – The class of T
Type parameters: <T> – the type of the desired object
Throws: JsonSyntaxException – if json is not a valid representation for an object of type typeOfT
Returns: an object of type T from the json. Returns null if json is null.
Since: 1.3/**
   * This method deserializes the Json read from the specified parse tree into an object of the
   * specified type. It is not suitable to use if the specified class is a generic type since it
   * will not have the generic type information because of the Type Erasure feature of Java.
   * Therefore, this method should not be used if the desired type is a generic type. Note that
   * this method works fine if the any of the fields of the specified object are generics, just the
   * object itself should not be a generic type. For the cases when the object is of generic type,
   * invoke {@link #fromJson(JsonElement, Type)}.
   * @param <T> the type of the desired object
   * @param json the root of the parse tree of {@link JsonElement}s from which the object is to
   * be deserialized
   * @param classOfT The class of T
   * @return an object of type T from the json. Returns {@code null} if {@code json} is {@code null}.
   * @throws JsonSyntaxException if json is not a valid representation for an object of type typeOfT
   * @since 1.3
   */
  public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonSyntaxException {
    Object object = fromJson(json, (Type) classOfT);
    return Primitives.wrap(classOfT).cast(object);
  }

  This method deserializes the Json read from the specified parse tree into an object of the specified type. This method is useful if the specified object is a generic type. For non-generic objects, use fromJson(JsonElement, Class<Object>) instead. 
Params: json – the root of the parse tree of JsonElements from which the object is to be deserialized
typeOfT – The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use: Type typeOfT = new TypeToken<Collection<Foo>>(){}.getType();
Type parameters: <T> – the type of the desired object
Throws: JsonSyntaxException – if json is not a valid representation for an object of type typeOfT
Returns: an object of type T from the json. Returns null if json is null.
Since: 1.3/**
   * This method deserializes the Json read from the specified parse tree into an object of the
   * specified type. This method is useful if the specified object is a generic type. For
   * non-generic objects, use {@link #fromJson(JsonElement, Class)} instead.
   *
   * @param <T> the type of the desired object
   * @param json the root of the parse tree of {@link JsonElement}s from which the object is to
   * be deserialized
   * @param typeOfT The specific genericized type of src. You can obtain this type by using the
   * {@link com.google.gson.reflect.TypeToken} class. For example, to get the type for
   * {@code Collection<Foo>}, you should use:
   * <pre>
   * Type typeOfT = new TypeToken&lt;Collection&lt;Foo&gt;&gt;(){}.getType();
   * </pre>
   * @return an object of type T from the json. Returns {@code null} if {@code json} is {@code null}.
   * @throws JsonSyntaxException if json is not a valid representation for an object of type typeOfT
   * @since 1.3
   */
  @SuppressWarnings("unchecked")
  public <T> T fromJson(JsonElement json, Type typeOfT) throws JsonSyntaxException {
    if (json == null) {
      return null;
    }
    return (T) fromJson(new JsonTreeReader(json), typeOfT);
  }

  static class FutureTypeAdapter<T> extends TypeAdapter<T> {
    private TypeAdapter<T> delegate;

    public void setDelegate(TypeAdapter<T> typeAdapter) {
      if (delegate != null) {
        throw new AssertionError();
      }
      delegate = typeAdapter;
    }

    @Override public T read(JsonReader in) throws IOException {
      if (delegate == null) {
        throw new IllegalStateException();
      }
      return delegate.read(in);
    }

    @Override public void write(JsonWriter out, T value) throws IOException {
      if (delegate == null) {
        throw new IllegalStateException();
      }
      delegate.write(out, value);
    }
  }

  @Override
  public String toString() {
    return new StringBuilder("{serializeNulls:")
        .append(serializeNulls)
        .append(",factories:").append(factories)
        .append(",instanceCreators:").append(constructorConstructor)
        .append("}")
        .toString();
  }
}
Author:	Inderjeet Singh, Joel Leitch, Jesse Wilson
See Also:	TypeToken
Params:	skipPast – The type adapter factory that needs to be skipped while searching for a matching type adapter. In most cases, you should just pass this (the type adapter factory from where `getDelegateAdapter` method is being invoked). type – Type for which the delegate adapter is being searched for.
Since:	2.2
Params:	src – the object for which Json representation is to be created setting for Gson
Returns:	Json representation of `src`.
Since:	1.4
Params:	src – the object for which Json representation is to be created setting for Gson writer – Writer to which the Json representation needs to be written
Throws:	JsonIOException – if there was a problem writing to the writer
Since:	1.2
Params:	jsonElement – root of a tree of `JsonElement`s
Returns:	JSON String representation of the tree
Since:	1.4
Params:	json – the string from which the object is to be deserialized classOfT – the class of T
Type parameters:	<T> – the type of the desired object
Throws:	JsonSyntaxException – if json is not a valid representation for an object of type classOfT
Returns:	an object of type T from the string. Returns `null` if `json` is `null` or if `json` is empty.
Params:	json – the reader producing the Json from which the object is to be deserialized. classOfT – the class of T
Type parameters:	<T> – the type of the desired object
Throws:	JsonIOException – if there was a problem reading from the Reader JsonSyntaxException – if json is not a valid representation for an object of type
Returns:	an object of type T from the string. Returns `null` if `json` is at EOF.
Since:	1.2
Params:	json – the reader producing Json from which the object is to be deserialized typeOfT – The specific genericized type of src. You can obtain this type by using the `TypeToken` class. For example, to get the type for `Collection<Foo>`, you should use: Type typeOfT = new TypeToken<Collection<Foo>>(){}.getType();
Type parameters:	<T> – the type of the desired object
Throws:	JsonIOException – if there was a problem reading from the Reader JsonSyntaxException – if json is not a valid representation for an object of type
Returns:	an object of type T from the json. Returns `null` if `json` is at EOF.
Since:	1.2
Params:	json – the root of the parse tree of `JsonElement`s from which the object is to be deserialized classOfT – The class of T
Type parameters:	<T> – the type of the desired object
Throws:	JsonSyntaxException – if json is not a valid representation for an object of type typeOfT
Returns:	an object of type T from the json. Returns `null` if `json` is `null`.
Since:	1.3
/

com.google.code.gson/ gson/ 2.8.5/ com/google/gson/Gson.java
