com.google.common.base
Class Functions

java.lang.Object
  com.google.common.base.Functions

public final class Functions
extends Object

Useful functions.

All methods returns serializable functions as long as they're given serializable parameters.

Since:

2010.01.04 stable (imported from Google Collections Library)

Author:

Mike Bostock, Vlad Patryshev, Jared Levy

Method Summary

static <A,B,C> Function<A,C>	compose(Function<B,C> g, Function<A,? extends B> f) Returns the composition of two functions.
static <E> Function<Object,E>	constant(E value) Creates a function that returns value for any input.
static <K,V> Function<K,V>	forMap(Map<K,? extends V> map, V defaultValue) Returns a function which performs a map lookup with a default value.
static <K,V> Function<K,V>	forMap(Map<K,V> map) Returns a function which performs a map lookup.
static <T> Function<T,Boolean>	forPredicate(Predicate<T> predicate) Creates a function that returns the same boolean output as the given predicate for all inputs.
static <E> Function<E,E>	identity() Returns the identity function.
static Function<Object,String>	toStringFunction() Returns a function that calls toString() on its argument.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

toStringFunction

public static Function<Object,String> toStringFunction()

Returns a function that calls toString() on its argument. The function does not accept nulls; it will throw a NullPointerException when applied to null.

identity

public static <E> Function<E,E> identity()

Returns the identity function.

forMap

public static <K,V> Function<K,V> forMap(Map<K,V> map)

Returns a function which performs a map lookup. The returned function throws an IllegalArgumentException if given a key that does not exist in the map.

forMap

public static <K,V> Function<K,V> forMap(Map<K,? extends V> map,
                                         @Nullable
                                         V defaultValue)

Returns a function which performs a map lookup with a default value. The function created by this method returns defaultValue for all inputs that do not belong to the map's key set.

Parameters:

map - source map that determines the function behavior

defaultValue - the value to return for inputs that aren't map keys

Returns:

function that returns map.get(a) when a is a key, or defaultValue otherwise

compose

public static <A,B,C> Function<A,C> compose(Function<B,C> g,
                                            Function<A,? extends B> f)

Returns the composition of two functions. For f: A->B and g: B->C, composition is defined as the function h such that h(a) == g(f(a)) for each a.

Parameters:

g - the second function to apply

f - the first function to apply

Returns:

the composition of f and g

See Also:

function composition

forPredicate

public static <T> Function<T,Boolean> forPredicate(Predicate<T> predicate)

Creates a function that returns the same boolean output as the given predicate for all inputs.

constant

public static <E> Function<Object,E> constant(@Nullable
                                              E value)

Creates a function that returns value for any input.

Parameters:

value - the constant value for the function to return

Returns:

a function that always returns value