Unpacking this definition: * *
Summarizing the last two points: "equal yield equal always; unequal yield unequal * often." This is the most important characteristic of all hash functions. * *
A high-quality hash function strives for some subset of the following virtues: * *
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.) * *
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. * *
Hasher accepts primitive data types, but can also accept any Object of type {@code T} provided
* that you implement a {@link Funnel}{@code Compatibility note: Throughout this API, multibyte values are always interpreted in
* little-endian 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.
*
* 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 other existing poor-quality {@code hashCode} implementations,
* including those in many JDK classes.
*
* {@code 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, {@code Object.hashCode} almost always falls short -- hence this library.
*
* @author Kevin Bourrillion
* @since 11.0
*/
@Immutable
@ElementTypesAreNonnullByDefault
public interface HashFunction {
/**
* Begins a new hash code computation by returning an initialized, stateful {@code Hasher}
* instance that is ready to receive data. Example:
*
* 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
* {@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 {@code newHasher().putString(input, charset).hash()}. Characters are encoded using
* the given {@link 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 {@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 {@code newHasher().putObject(instance, funnel).hash()}. The implementation
* might perform better than its longhand equivalent, but should not perform worse.
*
* @since 14.0
*/
Relationship to {@link Object#hashCode}
*
* {@code
* HashFunction hf = Hashing.md5();
* HashCode hc = hf.newHasher()
* .putLong(id)
* .putBoolean(isActive)
* .hash();
* }
*/
Hasher newHasher();
/**
* 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 {@code newHasher().putInt(input).hash()}; returns the hash code for the given
* {@code 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
*/
HashCode hashInt(int input);
/**
* 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 might
* perform better than its longhand equivalent, but should not perform worse.
*/
HashCode hashLong(long input);
/**
* Shortcut for {@code newHasher().putBytes(input).hash()}. The implementation might
* perform better than its longhand equivalent, but should not perform worse.
*/
HashCode hashBytes(byte[] input);
/**
* Shortcut for {@code newHasher().putBytes(input, off, len).hash()}. The implementation
* might 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 {@code newHasher().putBytes(input).hash()}. The implementation might
* perform better than its longhand equivalent, but should not perform worse.
*
* @since 23.0
*/
HashCode hashBytes(ByteBuffer input);
/**
* Shortcut for {@code 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 {@code char} are
* hashed directly (in that order).
*
*