com.google.common.base
Class Functions

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

public final class Functions
extends java.lang.Object

Useful functions.

Author:

Mike Bostock, Vlad Patryshev, Jared Levy

Field Summary

static Function<java.lang.Object,java.lang.String>	TO_STRING See toStringFunction().
static Function<java.lang.String,java.lang.String>	TRIM_STRING See trimString().

Method Summary

static <A,B,C> Function<A,C>	compose(Function<? super B,? extends C> g, Function<? super A,? extends B> f) Returns the composition of two functions.
static <E> Function<java.lang.Object,E>	constant(E value) Returns a Function that returns value for any input.
static <A,B> Function<A,B>	forMap(java.util.Map<? super A,? extends B> map) Returns a function which performs key-to-value lookup on map.
static <A,B> Function<A,B>	forMap(java.util.Map<? super A,? extends B> map, B defaultValue) Returns a function which performs key-to-value lookup on map.
static <T> Function<T,java.lang.Boolean>	forPredicate(Predicate<? super T> predicate) Returns a boolean-valued function that evaluates to the same result as the given predicate.
static <E> Function<E,E>	identity() Returns the identity function.
static Function<java.lang.Object,java.lang.Integer>	toHashCode() Returns a function that determines the Object.hashCode() of its argument.
static <F> Function<F,java.lang.String>	toStringFunction() Returns a function that returns the Object.toString() value of its argument.
static Function<java.lang.String,java.lang.String>	trimString() A function that returns the result of calling String.trim() on its argument.

Methods inherited from class java.lang.Object

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

Field Detail

TO_STRING

public static final Function<java.lang.Object,java.lang.String> TO_STRING

See toStringFunction(). TODO: Consider deprecating this in favor of an accessor method like toStringFunction().

TRIM_STRING

public static final Function<java.lang.String,java.lang.String> TRIM_STRING

See trimString(). TODO: Deprecate this and use trimString().

Method Detail

toStringFunction

public static <F> Function<F,java.lang.String> toStringFunction()

Returns a function that returns the Object.toString() value of its argument. This function is not nullable; invoking apply(null) on it throws a NullPointerException.

toHashCode

public static Function<java.lang.Object,java.lang.Integer> toHashCode()

Returns a function that determines the Object.hashCode() of its argument. For null arguments, this returns 0, as used in hash code calculations by the Java Collections classes.

trimString

public static Function<java.lang.String,java.lang.String> trimString()

A function that returns the result of calling String.trim() on its argument. Note that this function is not @Nullable: it will throw a NullPointerException when applied to null.

identity

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

Returns the identity function.

forMap

public static <A,B> Function<A,B> forMap(java.util.Map<? super A,? extends B> map)

Returns a function which performs key-to-value lookup on map.

The difference between a map and a function is that a map is defined on a set of keys, while a function is defined on a type. The function built by this method returns null for all arguments that do not belong to the map's keyset.

Parameters:

map - Map<A,B> source map

Returns:

Function<A,B> function that returns map.get(a) for each A a

forMap

public static <A,B> Function<A,B> forMap(java.util.Map<? super A,? extends B> map,
                                         @Nullable
                                         B defaultValue)

Returns a function which performs key-to-value lookup on map. The function built by this method returns defaultValue for all its arguments that do not belong to the map's keyset.

Parameters:

map - Map<A,B>

defaultValue - B

Returns:

Function f such that f(a)=map.get(a) if map.containsKey(x), and defaultValue otherwise.

See Also:

forMap(Map)

compose

public static <A,B,C> Function<A,C> compose(Function<? super B,? extends C> g,
                                            Function<? super 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(x) == g(f(x)) for each x.

Parameters:

g - Function<B,C>

f - Function<A,B>

Returns:

Function<A,C> composition of f and g

See Also:

function composition

forPredicate

public static <T> Function<T,java.lang.Boolean> forPredicate(Predicate<? super T> predicate)

Returns a boolean-valued function that evaluates to the same result as the given predicate.

constant

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

Returns 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.