com.google.guava/guava/27.1-jre : com/google/common/hash/HashFunction.java

HashFunction
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.
The Apache Software License, Version 2.0
Kevin Bourrillion (Google)
/*
 * Copyright (C) 2011 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.hash;

import com.google.common.annotations.Beta;
import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.Immutable;
import java.nio.ByteBuffer;
import java.nio.charset.Charset;

A hash function is a collision-averse pure function that maps an arbitrary block of data to a
number called a hash code.
Definition
Unpacking this definition:

  block of data: the input for a hash function is always, in concept, an ordered byte array. This hashing API accepts an arbitrary sequence of byte and multibyte values (via Hasher), but this is merely a convenience; these are always translated into raw byte sequences under the covers. 
hash code: each hash function always yields hash codes of the same fixed bit length (given by bits). For example, Hashing.sha1 produces a 160-bit number, while Hashing.murmur3_32() yields only 32 bits. Because a long value is clearly insufficient to hold all hash code values, this API represents a hash code as an instance of HashCode. 
pure function: the value produced must depend only on the input bytes, in the order they appear. Input data is never modified. HashFunction instances should always be stateless, and therefore thread-safe. 
collision-averse: while it can't be helped that a hash function will sometimes
      produce the same hash code for distinct inputs (a "collision"), every hash function strives
      to some degree to make this unlikely. (Without this condition, a function that
      always returns zero could be called a hash function. It is not.)

Summarizing the last two points: "equal yield equal always; unequal yield unequal
often." This is the most important characteristic of all hash functions.
Desirable properties
A high-quality hash function strives for some subset of the following virtues:

  collision-resistant: while the definition above requires making at least some
      token attempt, one measure of the quality of a hash function is how well it succeeds
      at this goal. Important note: it may be easy to achieve the theoretical minimum collision
      rate when using completely random sample input. The true test of a hash function is
      how it performs on representative real-world data, which tends to contain many hidden
      patterns and clumps. The goal of a good hash function is to stamp these patterns out as
      thoroughly as possible.
  
bit-dispersing: masking out any single bit from a hash code should yield only
      the expected twofold increase to all collision rates. Informally, the "information"
      in the hash code should be as evenly "spread out" through the hash code's bits as possible.
      The result is that, for example, when choosing a bucket in a hash table of size 2^8,
      any eight bits could be consistently used.
  
cryptographic: certain hash functions such as Hashing.sha512 are designed to make it as infeasible as possible to reverse-engineer the input that produced a given hash code, or even to discover any two distinct inputs that yield the same result. These
      are called cryptographic hash functions. But, whenever it is learned that either of
      these feats has become computationally feasible, the function is deemed "broken" and should
      no longer be used for secure purposes. (This is the likely eventual fate of all
      cryptographic hashes.)
  
fast: perhaps self-explanatory, but often the most important consideration.

Providing input to a hash function
The primary way to provide the data that your hash function should act on is via a Hasher. Obtain a new hasher from the hash function using newHasher, "push" the relevant data into it using methods like Hasher.putBytes(byte[]), and finally ask for the 
HashCode when finished using Hasher.hash. (See an example of this.) 
If all you want to hash is a single byte array, string or long value, there are convenient shortcut methods defined directly on HashFunction to make this easier. 
Hasher accepts primitive data types, but can also accept any Object of type T provided that you implement a Funnel<T> to specify how to "feed" data from that object into the function. (See an example of this.) 
Compatibility note: Throughout this API, multibyte values are always interpreted in
little-endian order. That is, hashing the byte array {0x01, 0x02, 0x03, 0x04} is equivalent to hashing the int value 0x04030201. If this isn't what you need, methods such as Integer.reverseBytes and Ints.toByteArray will help. 
Relationship to Object.hashCode
Java's baked-in concept of hash codes is constrained to 32 bits, and provides no separation between hash algorithms and the data they act on, so alternate hash algorithms can't be easily substituted. Also, implementations of hashCode tend to be poor-quality, in part because they end up depending on other existing poor-quality hashCode implementations, including those in many JDK classes. 
Object.hashCode implementations tend to be very fast, but have weak collision prevention and no expectation of bit dispersion. This leaves them perfectly suitable for use in hash tables, because extra collisions cause only a slight performance hit, while poor bit dispersion is easily corrected using a secondary hash function (which all reasonable hash table implementations in Java use). For the many uses of hash functions beyond data structures, however, Object.hashCode almost always falls short -- hence this library. 
Author: Kevin Bourrillion
Since: 11.0/**
 * A hash function is a collision-averse pure function that maps an arbitrary block of data to a
 * number called a <i>hash code</i>.
 *
 * <h3>Definition</h3>
 *
 * <p>Unpacking this definition:
 *
 * <ul>
 *   <li><b>block of data:</b> the input for a hash function is always, in concept, an ordered byte
 *       array. This hashing API accepts an arbitrary sequence of byte and multibyte values (via
 *       {@link Hasher}), but this is merely a convenience; these are always translated into raw
 *       byte sequences under the covers.
 *   <li><b>hash code:</b> each hash function always yields hash codes of the same fixed bit length
 *       (given by {@link #bits}). For example, {@link Hashing#sha1} produces a 160-bit number,
 *       while {@link Hashing#murmur3_32()} yields only 32 bits. Because a {@code long} value is
 *       clearly insufficient to hold all hash code values, this API represents a hash code as an
 *       instance of {@link HashCode}.
 *   <li><b>pure function:</b> the value produced must depend only on the input bytes, in the order
 *       they appear. Input data is never modified. {@link HashFunction} instances should always be
 *       stateless, and therefore thread-safe.
 *   <li><b>collision-averse:</b> while it can't be helped that a hash function will sometimes
 *       produce the same hash code for distinct inputs (a "collision"), every hash function strives
 *       to <i>some</i> degree to make this unlikely. (Without this condition, a function that
 *       always returns zero could be called a hash function. It is not.)
 * </ul>
 *
 * <p>Summarizing the last two points: "equal yield equal <i>always</i>; unequal yield unequal
 * <i>often</i>." This is the most important characteristic of all hash functions.
 *
 * <h3>Desirable properties</h3>
 *
 * <p>A high-quality hash function strives for some subset of the following virtues:
 *
 * <ul>
 *   <li><b>collision-resistant:</b> while the definition above requires making at least <i>some</i>
 *       token attempt, one measure of the quality of a hash function is <i>how well</i> it succeeds
 *       at this goal. Important note: it may be easy to achieve the theoretical minimum collision
 *       rate when using completely <i>random</i> sample input. The true test of a hash function is
 *       how it performs on representative real-world data, which tends to contain many hidden
 *       patterns and clumps. The goal of a good hash function is to stamp these patterns out as
 *       thoroughly as possible.
 *   <li><b>bit-dispersing:</b> masking out any <i>single bit</i> from a hash code should yield only
 *       the expected <i>twofold</i> increase to all collision rates. Informally, the "information"
 *       in the hash code should be as evenly "spread out" through the hash code's bits as possible.
 *       The result is that, for example, when choosing a bucket in a hash table of size 2^8,
 *       <i>any</i> eight bits could be consistently used.
 *   <li><b>cryptographic:</b> certain hash functions such as {@link Hashing#sha512} are designed to
 *       make it as infeasible as possible to reverse-engineer the input that produced a given hash
 *       code, or even to discover <i>any</i> two distinct inputs that yield the same result. These
 *       are called <i>cryptographic hash functions</i>. But, whenever it is learned that either of
 *       these feats has become computationally feasible, the function is deemed "broken" and should
 *       no longer be used for secure purposes. (This is the likely eventual fate of <i>all</i>
 *       cryptographic hashes.)
 *   <li><b>fast:</b> perhaps self-explanatory, but often the most important consideration.
 * </ul>
 *
 * <h3>Providing input to a hash function</h3>
 *
 * <p>The primary way to provide the data that your hash function should act on is via a {@link
 * Hasher}. Obtain a new hasher from the hash function using {@link #newHasher}, "push" the relevant
 * data into it using methods like {@link Hasher#putBytes(byte[])}, and finally ask for the {@code
 * HashCode} when finished using {@link Hasher#hash}. (See an {@linkplain #newHasher example} of
 * this.)
 *
 * <p>If all you want to hash is a single byte array, string or {@code long} value, there are
 * convenient shortcut methods defined directly on {@link HashFunction} to make this easier.
 *
 * <p>Hasher accepts primitive data types, but can also accept any Object of type {@code T} provided
 * that you implement a {@link Funnel}{@code <T>} to specify how to "feed" data from that object
 * into the function. (See {@linkplain Hasher#putObject an example} of this.)
 *
 * <p><b>Compatibility note:</b> Throughout this API, multibyte values are always interpreted in
 * <i>little-endian</i> order. That is, hashing the byte array {@code {0x01, 0x02, 0x03, 0x04}} is
 * equivalent to hashing the {@code int} value {@code 0x04030201}. If this isn't what you need,
 * methods such as {@link Integer#reverseBytes} and {@link Ints#toByteArray} will help.
 *
 * <h3>Relationship to {@link Object#hashCode}</h3>
 *
 * <p>Java's baked-in concept of hash codes is constrained to 32 bits, and provides no separation
 * between hash algorithms and the data they act on, so alternate hash algorithms can't be easily
 * substituted. Also, implementations of {@code hashCode} tend to be poor-quality, in part because
 * they end up depending on <i>other</i> existing poor-quality {@code hashCode} implementations,
 * including those in many JDK classes.
 *
 * <p>{@code Object.hashCode} implementations tend to be very fast, but have weak collision
 * prevention and <i>no</i> expectation of bit dispersion. This leaves them perfectly suitable for
 * use in hash tables, because extra collisions cause only a slight performance hit, while poor bit
 * dispersion is easily corrected using a secondary hash function (which all reasonable hash table
 * implementations in Java use). For the many uses of hash functions beyond data structures,
 * however, {@code Object.hashCode} almost always falls short -- hence this library.
 *
 * @author Kevin Bourrillion
 * @since 11.0
 */
@Beta
@Immutable
public interface HashFunction {
  Begins a new hash code computation by returning an initialized, stateful Hasher instance that is ready to receive data. Example: 
HashFunction hf = Hashing.md5();
HashCode hc = hf.newHasher()
    .putLong(id)
    .putBoolean(isActive)
    .hash();

/**
   * Begins a new hash code computation by returning an initialized, stateful {@code Hasher}
   * instance that is ready to receive data. Example:
   *
   * <pre>{@code
   * HashFunction hf = Hashing.md5();
   * HashCode hc = hf.newHasher()
   *     .putLong(id)
   *     .putBoolean(isActive)
   *     .hash();
   * }</pre>
   */
  Hasher newHasher();

  Begins a new hash code computation as newHasher(), but provides a hint of the expected size of the input (in bytes). This is only important for non-streaming hash functions (hash functions that need to buffer their whole input before processing any of it). /**
   * Begins a new hash code computation as {@link #newHasher()}, but provides a hint of the expected
   * size of the input (in bytes). This is only important for non-streaming hash functions (hash
   * functions that need to buffer their whole input before processing any of it).
   */
  Hasher newHasher(int expectedInputSize);

  Shortcut for newHasher().putInt(input).hash(); returns the hash code for the given int value, interpreted in little-endian byte order. The implementation might
perform better than its longhand equivalent, but should not perform worse.
Since: 12.0/**
   * Shortcut for {@code newHasher().putInt(input).hash()}; returns the hash code for the given
   * {@code int} value, interpreted in little-endian byte order. The implementation <i>might</i>
   * perform better than its longhand equivalent, but should not perform worse.
   *
   * @since 12.0
   */
  HashCode hashInt(int input);

  Shortcut for newHasher().putLong(input).hash(); returns the hash code for the given long value, interpreted in little-endian byte order. The implementation might
perform better than its longhand equivalent, but should not perform worse.
/**
   * Shortcut for {@code newHasher().putLong(input).hash()}; returns the hash code for the given
   * {@code long} value, interpreted in little-endian byte order. The implementation <i>might</i>
   * perform better than its longhand equivalent, but should not perform worse.
   */
  HashCode hashLong(long input);

  Shortcut for newHasher().putBytes(input).hash(). The implementation might
perform better than its longhand equivalent, but should not perform worse.
/**
   * Shortcut for {@code newHasher().putBytes(input).hash()}. The implementation <i>might</i>
   * perform better than its longhand equivalent, but should not perform worse.
   */
  HashCode hashBytes(byte[] input);

  Shortcut for newHasher().putBytes(input, off, len).hash(). The implementation might perform better than its longhand equivalent, but should not perform worse.
Throws: IndexOutOfBoundsException – if off < 0 or off + len > bytes.length or len < 0/**
   * Shortcut for {@code newHasher().putBytes(input, off, len).hash()}. The implementation
   * <i>might</i> perform better than its longhand equivalent, but should not perform worse.
   *
   * @throws IndexOutOfBoundsException if {@code off < 0} or {@code off + len > bytes.length} or
   *     {@code len < 0}
   */
  HashCode hashBytes(byte[] input, int off, int len);

  Shortcut for newHasher().putBytes(input).hash(). The implementation might
perform better than its longhand equivalent, but should not perform worse.
Since: 23.0/**
   * Shortcut for {@code newHasher().putBytes(input).hash()}. The implementation <i>might</i>
   * perform better than its longhand equivalent, but should not perform worse.
   *
   * @since 23.0
   */
  HashCode hashBytes(ByteBuffer input);

  Shortcut for newHasher().putUnencodedChars(input).hash(). The implementation might perform better than its longhand equivalent, but should not perform worse. Note that no character encoding is performed; the low byte and high byte of each char are hashed directly (in that order). Warning: This method will produce different output than most other languages do when running the same hash function on the equivalent input. For cross-language compatibility, use hashString, usually with a charset of UTF-8. For other use cases, use 
hashUnencodedChars. 
Since: 15.0 (since 11.0 as hashString(CharSequence))./**
   * Shortcut for {@code newHasher().putUnencodedChars(input).hash()}. The implementation
   * <i>might</i> perform better than its longhand equivalent, but should not perform worse. Note
   * that no character encoding is performed; the low byte and high byte of each {@code char} are
   * hashed directly (in that order).
   *
   * <p><b>Warning:</b> This method will produce different output than most other languages do when
   * running the same hash function on the equivalent input. For cross-language compatibility, use
   * {@link #hashString}, usually with a charset of UTF-8. For other use cases, use {@code
   * hashUnencodedChars}.
   *
   * @since 15.0 (since 11.0 as hashString(CharSequence)).
   */
  HashCode hashUnencodedChars(CharSequence input);

  Shortcut for newHasher().putString(input, charset).hash(). Characters are encoded using the given Charset. The implementation might perform better than its longhand
equivalent, but should not perform worse.
Warning: This method, which reencodes the input before hashing it, is useful only for cross-language compatibility. For other use cases, prefer hashUnencodedChars, which is faster, produces the same output across Java releases, and hashes every char in the input, even if some are invalid. /**
   * Shortcut for {@code newHasher().putString(input, charset).hash()}. Characters are encoded using
   * the given {@link Charset}. The implementation <i>might</i> perform better than its longhand
   * equivalent, but should not perform worse.
   *
   * <p><b>Warning:</b> This method, which reencodes the input before hashing it, is useful only for
   * cross-language compatibility. For other use cases, prefer {@link #hashUnencodedChars}, which is
   * faster, produces the same output across Java releases, and hashes every {@code char} in the
   * input, even if some are invalid.
   */
  HashCode hashString(CharSequence input, Charset charset);

  Shortcut for newHasher().putObject(instance, funnel).hash(). The implementation might perform better than its longhand equivalent, but should not perform worse.
Since: 14.0/**
   * Shortcut for {@code newHasher().putObject(instance, funnel).hash()}. The implementation
   * <i>might</i> perform better than its longhand equivalent, but should not perform worse.
   *
   * @since 14.0
   */
  <T> HashCode hashObject(T instance, Funnel<? super T> funnel);

  Returns the number of bits (a multiple of 32) that each hash code produced by this hash
function has.
/**
   * Returns the number of bits (a multiple of 32) that each hash code produced by this hash
   * function has.
   */
  int bits();
}
/

com.google.guava/ guava/ 27.1-jre/ com/google/common/hash/HashFunction.java

Definition

Desirable properties

Providing input to a hash function

Relationship to `Object.hashCode`
