com.google.common.base
Class Functions

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

@GwtCompatible
public final class Functions
extends Object

Static utility methods pertaining to Function instances.

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

See the Guava User Guide article on the use of Function.

Since:

2.0 (imported from Google Collections Library)

Author:

Mike Bostock, 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 <T> Function<Object,T>	forSupplier(Supplier<T> supplier) Returns a function that always returns the result of invoking Supplier.get() on supplier, regardless of its input.
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.

Warning: The returned function may not be consistent with equals (as documented at Function.apply(F)). For example, this function yields different results for the two equal instances ImmutableSet.of(1, 2) and ImmutableSet.of(2, 1).

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.

The returned function is consistent with equals (as documented at Function.apply(F)) if and only if predicate is itself consistent with equals.

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

forSupplier

@Beta
public static <T> Function<Object,T> forSupplier(Supplier<T> supplier)

Returns a function that always returns the result of invoking Supplier.get() on supplier, regardless of its input.

Since:

10.0