Package com.google.common.base

Class Functions

  • @GwtCompatible
public final class Functions
extends java.lang.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)

    • Method Summary

      All Methods Static Methods Concrete Methods 
      Modifier and Type Method Description
      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<java.lang.Object,​E> constant​(E value)
      Creates a function that returns value for any input.
      static <K,​V>
      Function<K,​V>      		 forMap​(java.util.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​(java.util.Map<K,​V> map)
      Returns a function which performs a map lookup.
      static <T> Function<T,​java.lang.Boolean> forPredicate​(Predicate<T> predicate)
      Creates a function that returns the same boolean output as the given predicate for all inputs.
      static <T> Function<java.lang.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<java.lang.Object,​java.lang.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<java.lang.Object,​java.lang.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​(java.util.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. See also forMap(Map, Object), which returns a default value in this case.

        Note: if map is a BiMap (or can be one), you can use Maps.asConverter instead to get a function that also supports reverse conversion.

      • forMap

        public static <K,​V> Function<K,​V> forMap​(java.util.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. See also forMap(Map), which throws an exception in this case.
        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,​java.lang.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<java.lang.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<java.lang.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