https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2009 The Guava Authors
 *
 * 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.common.reflect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static java.util.Arrays.asList;

import com.google.common.annotations.Beta;
import com.google.common.base.Joiner;
import com.google.common.base.Objects;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.Maps;
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.lang.reflect.WildcardType;
import java.util.Arrays;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.concurrent.atomic.AtomicInteger;
import org.checkerframework.checker.nullness.qual.Nullable;


An object of this class encapsulates type mappings from type variables. Mappings are established with where and types are resolved using resolveType. 
Note that usually type mappings are already implied by the static type hierarchy (for example, the E type variable declared by class List naturally maps to String in the context of class MyStringList implements List<String>. In such case, prefer to use TypeToken.resolveType since it's simpler and more type safe. This class should only be used when the type mapping isn't implied by the static type hierarchy, but provided through other means such as an annotation or external configuration file. 
Author:Ben Yu
Since:15.0
/**
 * An object of this class encapsulates type mappings from type variables. Mappings are established
 * with {@link #where} and types are resolved using {@link #resolveType}.
 *
 * <p>Note that usually type mappings are already implied by the static type hierarchy (for example,
 * the {@code E} type variable declared by class {@code List} naturally maps to {@code String} in
 * the context of {@code class MyStringList implements List<String>}. In such case, prefer to use
 * {@link TypeToken#resolveType} since it's simpler and more type safe. This class should only be
 * used when the type mapping isn't implied by the static type hierarchy, but provided through other
 * means such as an annotation or external configuration file.
 *
 * @author Ben Yu
 * @since 15.0
 */
@Beta
public final class TypeResolver {

  private final TypeTable typeTable;

  public TypeResolver() {
    this.typeTable = new TypeTable();
  }

  private TypeResolver(TypeTable typeTable) {
    this.typeTable = typeTable;
  }

  
Returns a resolver that resolves types "covariantly".

For example, when resolving List<T> in the context of ArrayList<?>, 
<T> is covariantly resolved to <?> such that return type of List::get is <?>. 
/**
   * Returns a resolver that resolves types "covariantly".
   *
   * <p>For example, when resolving {@code List<T>} in the context of {@code ArrayList<?>}, {@code
   * <T>} is covariantly resolved to {@code <?>} such that return type of {@code List::get} is
   * {@code <?>}.
   */
  static TypeResolver covariantly(Type contextType) {
    return new TypeResolver().where(TypeMappingIntrospector.getTypeMappings(contextType));
  }

  
Returns a resolver that resolves types "invariantly".

For example, when resolving List<T> in the context of ArrayList<?>, 
<T> cannot be invariantly resolved to <?> because otherwise the parameter type of List::set will be <?> and it'll falsely say any object can be passed into ArrayList<?>::set. 
Instead, <?> will be resolved to a capture in the form of a type variable 
<capture-of-? extends Object>, effectively preventing set from accepting any type. 
/**
   * Returns a resolver that resolves types "invariantly".
   *
   * <p>For example, when resolving {@code List<T>} in the context of {@code ArrayList<?>}, {@code
   * <T>} cannot be invariantly resolved to {@code <?>} because otherwise the parameter type of
   * {@code List::set} will be {@code <?>} and it'll falsely say any object can be passed into
   * {@code ArrayList<?>::set}.
   *
   * <p>Instead, {@code <?>} will be resolved to a capture in the form of a type variable {@code
   * <capture-of-? extends Object>}, effectively preventing {@code set} from accepting any type.
   */
  static TypeResolver invariantly(Type contextType) {
    Type invariantContext = WildcardCapturer.INSTANCE.capture(contextType);
    return new TypeResolver().where(TypeMappingIntrospector.getTypeMappings(invariantContext));
  }

  
Returns a new TypeResolver with type variables in formal mapping to types in actual. 
For example, if formal is a TypeVariable T, and actual is 
String.class, then new TypeResolver().where(formal, actual) will resolve ParameterizedType List<T> to List<String>, and resolve Map<T, Something> to Map<String, Something> etc. Similarly, formal and actual can be Map<K, V> and Map<String, Integer> respectively, or they can be E[] and String[] respectively, or even any arbitrary combination thereof. 
Params:
  • formal – The type whose type variables or itself is mapped to other type(s). It's almost always a bug if formal isn't a type variable and contains no type variable. Make sure you are passing the two parameters in the right order.
  • actual – The type that the formal type variable(s) are mapped to. It can be or contain yet other type variables, in which case these type variables will be further resolved if corresponding mappings exist in the current TypeResolver instance.
/**
   * Returns a new {@code TypeResolver} with type variables in {@code formal} mapping to types in
   * {@code actual}.
   *
   * <p>For example, if {@code formal} is a {@code TypeVariable T}, and {@code actual} is {@code
   * String.class}, then {@code new TypeResolver().where(formal, actual)} will {@linkplain
   * #resolveType resolve} {@code ParameterizedType List<T>} to {@code List<String>}, and resolve
   * {@code Map<T, Something>} to {@code Map<String, Something>} etc. Similarly, {@code formal} and
   * {@code actual} can be {@code Map<K, V>} and {@code Map<String, Integer>} respectively, or they
   * can be {@code E[]} and {@code String[]} respectively, or even any arbitrary combination
   * thereof.
   *
   * @param formal The type whose type variables or itself is mapped to other type(s). It's almost
   *     always a bug if {@code formal} isn't a type variable and contains no type variable. Make
   *     sure you are passing the two parameters in the right order.
   * @param actual The type that the formal type variable(s) are mapped to. It can be or contain yet
   *     other type variables, in which case these type variables will be further resolved if
   *     corresponding mappings exist in the current {@code TypeResolver} instance.
   */
  public TypeResolver where(Type formal, Type actual) {
    Map<TypeVariableKey, Type> mappings = Maps.newHashMap();
    populateTypeMappings(mappings, checkNotNull(formal), checkNotNull(actual));
    return where(mappings);
  }

  
Returns a new TypeResolver with variable mapping to type. 
/** Returns a new {@code TypeResolver} with {@code variable} mapping to {@code type}. */
  TypeResolver where(Map<TypeVariableKey, ? extends Type> mappings) {
    return new TypeResolver(typeTable.where(mappings));
  }

  private static void populateTypeMappings(
      final Map<TypeVariableKey, Type> mappings, final Type from, final Type to) {
    if (from.equals(to)) {
      return;
    }
    new TypeVisitor() {
      @Override
      void visitTypeVariable(TypeVariable<?> typeVariable) {
        mappings.put(new TypeVariableKey(typeVariable), to);
      }

      @Override
      void visitWildcardType(WildcardType fromWildcardType) {
        if (!(to instanceof WildcardType)) {
          return; // okay to say <?> is anything
        }
        WildcardType toWildcardType = (WildcardType) to;
        Type[] fromUpperBounds = fromWildcardType.getUpperBounds();
        Type[] toUpperBounds = toWildcardType.getUpperBounds();
        Type[] fromLowerBounds = fromWildcardType.getLowerBounds();
        Type[] toLowerBounds = toWildcardType.getLowerBounds();
        checkArgument(
            fromUpperBounds.length == toUpperBounds.length
                && fromLowerBounds.length == toLowerBounds.length,
            "Incompatible type: %s vs. %s",
            fromWildcardType,
            to);
        for (int i = 0; i < fromUpperBounds.length; i++) {
          populateTypeMappings(mappings, fromUpperBounds[i], toUpperBounds[i]);
        }
        for (int i = 0; i < fromLowerBounds.length; i++) {
          populateTypeMappings(mappings, fromLowerBounds[i], toLowerBounds[i]);
        }
      }

      @Override
      void visitParameterizedType(ParameterizedType fromParameterizedType) {
        if (to instanceof WildcardType) {
          return; // Okay to say Foo<A> is <?>
        }
        ParameterizedType toParameterizedType = expectArgument(ParameterizedType.class, to);
        if (fromParameterizedType.getOwnerType() != null
            && toParameterizedType.getOwnerType() != null) {
          populateTypeMappings(
              mappings, fromParameterizedType.getOwnerType(), toParameterizedType.getOwnerType());
        }
        checkArgument(
            fromParameterizedType.getRawType().equals(toParameterizedType.getRawType()),
            "Inconsistent raw type: %s vs. %s",
            fromParameterizedType,
            to);
        Type[] fromArgs = fromParameterizedType.getActualTypeArguments();
        Type[] toArgs = toParameterizedType.getActualTypeArguments();
        checkArgument(
            fromArgs.length == toArgs.length,
            "%s not compatible with %s",
            fromParameterizedType,
            toParameterizedType);
        for (int i = 0; i < fromArgs.length; i++) {
          populateTypeMappings(mappings, fromArgs[i], toArgs[i]);
        }
      }

      @Override
      void visitGenericArrayType(GenericArrayType fromArrayType) {
        if (to instanceof WildcardType) {
          return; // Okay to say A[] is <?>
        }
        Type componentType = Types.getComponentType(to);
        checkArgument(componentType != null, "%s is not an array type.", to);
        populateTypeMappings(mappings, fromArrayType.getGenericComponentType(), componentType);
      }

      @Override
      void visitClass(Class<?> fromClass) {
        if (to instanceof WildcardType) {
          return; // Okay to say Foo is <?>
        }
        // Can't map from a raw class to anything other than itself or a wildcard.
        // You can't say "assuming String is Integer".
        // And we don't support "assuming String is T"; user has to say "assuming T is String".
        throw new IllegalArgumentException("No type mapping from " + fromClass + " to " + to);
      }
    }.visit(from);
  }

  
Resolves all type variables in type and all downstream types and returns a corresponding type with type variables resolved. 
/**
   * Resolves all type variables in {@code type} and all downstream types and returns a
   * corresponding type with type variables resolved.
   */
  public Type resolveType(Type type) {
    checkNotNull(type);
    if (type instanceof TypeVariable) {
      return typeTable.resolve((TypeVariable<?>) type);
    } else if (type instanceof ParameterizedType) {
      return resolveParameterizedType((ParameterizedType) type);
    } else if (type instanceof GenericArrayType) {
      return resolveGenericArrayType((GenericArrayType) type);
    } else if (type instanceof WildcardType) {
      return resolveWildcardType((WildcardType) type);
    } else {
      // if Class<?>, no resolution needed, we are done.
      return type;
    }
  }

  Type[] resolveTypesInPlace(Type[] types) {
    for (int i = 0; i < types.length; i++) {
      types[i] = resolveType(types[i]);
    }
    return types;
  }

  private Type[] resolveTypes(Type[] types) {
    Type[] result = new Type[types.length];
    for (int i = 0; i < types.length; i++) {
      result[i] = resolveType(types[i]);
    }
    return result;
  }

  private WildcardType resolveWildcardType(WildcardType type) {
    Type[] lowerBounds = type.getLowerBounds();
    Type[] upperBounds = type.getUpperBounds();
    return new Types.WildcardTypeImpl(resolveTypes(lowerBounds), resolveTypes(upperBounds));
  }

  private Type resolveGenericArrayType(GenericArrayType type) {
    Type componentType = type.getGenericComponentType();
    Type resolvedComponentType = resolveType(componentType);
    return Types.newArrayType(resolvedComponentType);
  }

  private ParameterizedType resolveParameterizedType(ParameterizedType type) {
    Type owner = type.getOwnerType();
    Type resolvedOwner = (owner == null) ? null : resolveType(owner);
    Type resolvedRawType = resolveType(type.getRawType());

    Type[] args = type.getActualTypeArguments();
    Type[] resolvedArgs = resolveTypes(args);
    return Types.newParameterizedTypeWithOwner(
        resolvedOwner, (Class<?>) resolvedRawType, resolvedArgs);
  }

  private static <T> T expectArgument(Class<T> type, Object arg) {
    try {
      return type.cast(arg);
    } catch (ClassCastException e) {
      throw new IllegalArgumentException(arg + " is not a " + type.getSimpleName());
    }
  }

  
A TypeTable maintains mapping from TypeVariable to types. 
/** A TypeTable maintains mapping from {@link TypeVariable} to types. */
  private static class TypeTable {
    private final ImmutableMap<TypeVariableKey, Type> map;

    TypeTable() {
      this.map = ImmutableMap.of();
    }

    private TypeTable(ImmutableMap<TypeVariableKey, Type> map) {
      this.map = map;
    }

    
Returns a new TypeResolver with variable mapping to type. 
/** Returns a new {@code TypeResolver} with {@code variable} mapping to {@code type}. */
    final TypeTable where(Map<TypeVariableKey, ? extends Type> mappings) {
      ImmutableMap.Builder<TypeVariableKey, Type> builder = ImmutableMap.builder();
      builder.putAll(map);
      for (Entry<TypeVariableKey, ? extends Type> mapping : mappings.entrySet()) {
        TypeVariableKey variable = mapping.getKey();
        Type type = mapping.getValue();
        checkArgument(!variable.equalsType(type), "Type variable %s bound to itself", variable);
        builder.put(variable, type);
      }
      return new TypeTable(builder.build());
    }

    final Type resolve(final TypeVariable<?> var) {
      final TypeTable unguarded = this;
      TypeTable guarded =
          new TypeTable() {
            @Override
            public Type resolveInternal(TypeVariable<?> intermediateVar, TypeTable forDependent) {
              if (intermediateVar.getGenericDeclaration().equals(var.getGenericDeclaration())) {
                return intermediateVar;
              }
              return unguarded.resolveInternal(intermediateVar, forDependent);
            }
          };
      return resolveInternal(var, guarded);
    }

    
Resolves var using the encapsulated type mapping. If it maps to yet another non-reified type or has bounds, forDependants is used to do further resolution, which doesn't try to resolve any type variable on generic declarations that are already being resolved. 
Should only be called and overridden by resolve(TypeVariable<?>). 
/**
     * Resolves {@code var} using the encapsulated type mapping. If it maps to yet another
     * non-reified type or has bounds, {@code forDependants} is used to do further resolution, which
     * doesn't try to resolve any type variable on generic declarations that are already being
     * resolved.
     *
     * <p>Should only be called and overridden by {@link #resolve(TypeVariable)}.
     */
    Type resolveInternal(TypeVariable<?> var, TypeTable forDependants) {
      Type type = map.get(new TypeVariableKey(var));
      if (type == null) {
        Type[] bounds = var.getBounds();
        if (bounds.length == 0) {
          return var;
        }
        Type[] resolvedBounds = new TypeResolver(forDependants).resolveTypes(bounds);
        /*
         * We'd like to simply create our own TypeVariable with the newly resolved bounds. There's
         * just one problem: Starting with JDK 7u51, the JDK TypeVariable's equals() method doesn't
         * recognize instances of our TypeVariable implementation. This is a problem because users
         * compare TypeVariables from the JDK against TypeVariables returned by TypeResolver. To
         * work with all JDK versions, TypeResolver must return the appropriate TypeVariable
         * implementation in each of the three possible cases:
         *
         * 1. Prior to JDK 7u51, the JDK TypeVariable implementation interoperates with ours.
         * Therefore, we can always create our own TypeVariable.
         *
         * 2. Starting with JDK 7u51, the JDK TypeVariable implementations does not interoperate
         * with ours. Therefore, we have to be careful about whether we create our own TypeVariable:
         *
         * 2a. If the resolved types are identical to the original types, then we can return the
         * original, identical JDK TypeVariable. By doing so, we sidestep the problem entirely.
         *
         * 2b. If the resolved types are different from the original types, things are trickier. The
         * only way to get a TypeVariable instance for the resolved types is to create our own. The
         * created TypeVariable will not interoperate with any JDK TypeVariable. But this is OK: We
         * don't _want_ our new TypeVariable to be equal to the JDK TypeVariable because it has
         * _different bounds_ than the JDK TypeVariable. And it wouldn't make sense for our new
         * TypeVariable to be equal to any _other_ JDK TypeVariable, either, because any other JDK
         * TypeVariable must have a different declaration or name. The only TypeVariable that our
         * new TypeVariable _will_ be equal to is an equivalent TypeVariable that was also created
         * by us. And that equality is guaranteed to hold because it doesn't involve the JDK
         * TypeVariable implementation at all.
         */
        if (Types.NativeTypeVariableEquals.NATIVE_TYPE_VARIABLE_ONLY
            && Arrays.equals(bounds, resolvedBounds)) {
          return var;
        }
        return Types.newArtificialTypeVariable(
            var.getGenericDeclaration(), var.getName(), resolvedBounds);
      }
      // in case the type is yet another type variable.
      return new TypeResolver(forDependants).resolveType(type);
    }
  }

  private static final class TypeMappingIntrospector extends TypeVisitor {

    private final Map<TypeVariableKey, Type> mappings = Maps.newHashMap();

    
Returns type mappings using type parameters and type arguments found in the generic superclass and the super interfaces of contextClass. 
/**
     * Returns type mappings using type parameters and type arguments found in the generic
     * superclass and the super interfaces of {@code contextClass}.
     */
    static ImmutableMap<TypeVariableKey, Type> getTypeMappings(Type contextType) {
      checkNotNull(contextType);
      TypeMappingIntrospector introspector = new TypeMappingIntrospector();
      introspector.visit(contextType);
      return ImmutableMap.copyOf(introspector.mappings);
    }

    @Override
    void visitClass(Class<?> clazz) {
      visit(clazz.getGenericSuperclass());
      visit(clazz.getGenericInterfaces());
    }

    @Override
    void visitParameterizedType(ParameterizedType parameterizedType) {
      Class<?> rawClass = (Class<?>) parameterizedType.getRawType();
      TypeVariable<?>[] vars = rawClass.getTypeParameters();
      Type[] typeArgs = parameterizedType.getActualTypeArguments();
      checkState(vars.length == typeArgs.length);
      for (int i = 0; i < vars.length; i++) {
        map(new TypeVariableKey(vars[i]), typeArgs[i]);
      }
      visit(rawClass);
      visit(parameterizedType.getOwnerType());
    }

    @Override
    void visitTypeVariable(TypeVariable<?> t) {
      visit(t.getBounds());
    }

    @Override
    void visitWildcardType(WildcardType t) {
      visit(t.getUpperBounds());
    }

    private void map(final TypeVariableKey var, final Type arg) {
      if (mappings.containsKey(var)) {
        // Mapping already established
        // This is possible when following both superClass -> enclosingClass
        // and enclosingclass -> superClass paths.
        // Since we follow the path of superclass first, enclosing second,
        // superclass mapping should take precedence.
        return;
      }
      // First, check whether var -> arg forms a cycle
      for (Type t = arg; t != null; t = mappings.get(TypeVariableKey.forLookup(t))) {
        if (var.equalsType(t)) {
          // cycle detected, remove the entire cycle from the mapping so that
          // each type variable resolves deterministically to itself.
          // Otherwise, a F -> T cycle will end up resolving both F and T
          // nondeterministically to either F or T.
          for (Type x = arg; x != null; x = mappings.remove(TypeVariableKey.forLookup(x))) {}
          return;
        }
      }
      mappings.put(var, arg);
    }
  }

  // This is needed when resolving types against a context with wildcards
  // For example:
  // class Holder<T> {
  //   void set(T data) {...}
  // }
  // Holder<List<?>> should *not* resolve the set() method to set(List<?> data).
  // Instead, it should create a capture of the wildcard so that set() rejects any List<T>.
  private static class WildcardCapturer {

    static final WildcardCapturer INSTANCE = new WildcardCapturer();

    private final AtomicInteger id;

    private WildcardCapturer() {
      this(new AtomicInteger());
    }

    private WildcardCapturer(AtomicInteger id) {
      this.id = id;
    }

    final Type capture(Type type) {
      checkNotNull(type);
      if (type instanceof Class) {
        return type;
      }
      if (type instanceof TypeVariable) {
        return type;
      }
      if (type instanceof GenericArrayType) {
        GenericArrayType arrayType = (GenericArrayType) type;
        return Types.newArrayType(
            notForTypeVariable().capture(arrayType.getGenericComponentType()));
      }
      if (type instanceof ParameterizedType) {
        ParameterizedType parameterizedType = (ParameterizedType) type;
        Class<?> rawType = (Class<?>) parameterizedType.getRawType();
        TypeVariable<?>[] typeVars = rawType.getTypeParameters();
        Type[] typeArgs = parameterizedType.getActualTypeArguments();
        for (int i = 0; i < typeArgs.length; i++) {
          typeArgs[i] = forTypeVariable(typeVars[i]).capture(typeArgs[i]);
        }
        return Types.newParameterizedTypeWithOwner(
            notForTypeVariable().captureNullable(parameterizedType.getOwnerType()),
            rawType,
            typeArgs);
      }
      if (type instanceof WildcardType) {
        WildcardType wildcardType = (WildcardType) type;
        Type[] lowerBounds = wildcardType.getLowerBounds();
        if (lowerBounds.length == 0) { // ? extends something changes to capture-of
          return captureAsTypeVariable(wildcardType.getUpperBounds());
        } else {
          // TODO(benyu): handle ? super T somehow.
          return type;
        }
      }
      throw new AssertionError("must have been one of the known types");
    }

    TypeVariable<?> captureAsTypeVariable(Type[] upperBounds) {
      String name =
          "capture#" + id.incrementAndGet() + "-of ? extends " + Joiner.on('&').join(upperBounds);
      return Types.newArtificialTypeVariable(WildcardCapturer.class, name, upperBounds);
    }

    private WildcardCapturer forTypeVariable(final TypeVariable<?> typeParam) {
      return new WildcardCapturer(id) {
        @Override
        TypeVariable<?> captureAsTypeVariable(Type[] upperBounds) {
          Set<Type> combined = new LinkedHashSet<>(asList(upperBounds));
          // Since this is an artifically generated type variable, we don't bother checking
          // subtyping between declared type bound and actual type bound. So it's possible that we
          // may generate something like <capture#1-of ? extends Foo&SubFoo>.
          // Checking subtype between declared and actual type bounds
          // adds recursive isSubtypeOf() call and feels complicated.
          // There is no contract one way or another as long as isSubtypeOf() works as expected.
          combined.addAll(asList(typeParam.getBounds()));
          if (combined.size() > 1) { // Object is implicit and only useful if it's the only bound.
            combined.remove(Object.class);
          }
          return super.captureAsTypeVariable(combined.toArray(new Type[0]));
        }
      };
    }

    private WildcardCapturer notForTypeVariable() {
      return new WildcardCapturer(id);
    }

    private Type captureNullable(@Nullable Type type) {
      if (type == null) {
        return null;
      }
      return capture(type);
    }
  }

  
Wraps around TypeVariable<?> to ensure that any two type variables are equal as long as they are declared by the same GenericDeclaration and have the same name, even if their bounds differ. 
While resolving a type variable from a var -> type map, we don't care whether the type variable's bound has been partially resolved. As long as the type variable "identity" matches. 
On the other hand, if for example we are resolving List<A extends B> to 
List<A extends String>, we need to compare that <A extends B> is unequal to <A
extends String> in order to decide to use the transformed type instead of the original type. 
/**
   * Wraps around {@code TypeVariable<?>} to ensure that any two type variables are equal as long as
   * they are declared by the same {@link java.lang.reflect.GenericDeclaration} and have the same
   * name, even if their bounds differ.
   *
   * <p>While resolving a type variable from a {@code var -> type} map, we don't care whether the
   * type variable's bound has been partially resolved. As long as the type variable "identity"
   * matches.
   *
   * <p>On the other hand, if for example we are resolving {@code List<A extends B>} to {@code
   * List<A extends String>}, we need to compare that {@code <A extends B>} is unequal to {@code <A
   * extends String>} in order to decide to use the transformed type instead of the original type.
   */
  static final class TypeVariableKey {
    private final TypeVariable<?> var;

    TypeVariableKey(TypeVariable<?> var) {
      this.var = checkNotNull(var);
    }

    @Override
    public int hashCode() {
      return Objects.hashCode(var.getGenericDeclaration(), var.getName());
    }

    @Override
    public boolean equals(Object obj) {
      if (obj instanceof TypeVariableKey) {
        TypeVariableKey that = (TypeVariableKey) obj;
        return equalsTypeVariable(that.var);
      } else {
        return false;
      }
    }

    @Override
    public String toString() {
      return var.toString();
    }

    
Wraps t in a TypeVariableKey if it's a type variable. 
/** Wraps {@code t} in a {@code TypeVariableKey} if it's a type variable. */
    static TypeVariableKey forLookup(Type t) {
      if (t instanceof TypeVariable) {
        return new TypeVariableKey((TypeVariable<?>) t);
      } else {
        return null;
      }
    }

    
Returns true if type is a TypeVariable with the same name and declared by the same GenericDeclaration. 
/**
     * Returns true if {@code type} is a {@code TypeVariable} with the same name and declared by the
     * same {@code GenericDeclaration}.
     */
    boolean equalsType(Type type) {
      if (type instanceof TypeVariable) {
        return equalsTypeVariable((TypeVariable<?>) type);
      } else {
        return false;
      }
    }

    private boolean equalsTypeVariable(TypeVariable<?> that) {
      return var.getGenericDeclaration().equals(that.getGenericDeclaration())
          && var.getName().equals(that.getName());
    }
  }
}