Function.java revision d052fb54474c6e3d99da861b02e4b49ac98790cc 
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25package java.util.function;
26
27import java.util.Objects;
28
29/**
30 * Represents a function that accepts one argument and produces a result.
31 *
32 * <p>This is a <a href="package-summary.html">functional interface</a>
33 * whose functional method is {@link #apply(Object)}.
34 *
35 * @param <T> the type of the input to the function
36 * @param <R> the type of the result of the function
37 *
38 * @since 1.8
39 * @hide 1.8
40 */
41@FunctionalInterface
42public interface Function<T, R> {
43
44    /**
45     * Applies this function to the given argument.
46     *
47     * @param t the function argument
48     * @return the function result
49     */
50    R apply(T t);
51
52    /**
53     * Returns a composed function that first applies the {@code before}
54     * function to its input, and then applies this function to the result.
55     * If evaluation of either function throws an exception, it is relayed to
56     * the caller of the composed function.
57     *
58     * @param <V> the type of input to the {@code before} function, and to the
59     *           composed function
60     * @param before the function to apply before this function is applied
61     * @return a composed function that first applies the {@code before}
62     * function and then applies this function
63     * @throws NullPointerException if before is null
64     *
65     * @see #andThen(Function)
66     */
67    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
68        Objects.requireNonNull(before);
69        return (V v) -> apply(before.apply(v));
70    }
71
72    /**
73     * Returns a composed function that first applies this function to
74     * its input, and then applies the {@code after} function to the result.
75     * If evaluation of either function throws an exception, it is relayed to
76     * the caller of the composed function.
77     *
78     * @param <V> the type of output of the {@code after} function, and of the
79     *           composed function
80     * @param after the function to apply after this function is applied
81     * @return a composed function that first applies this function and then
82     * applies the {@code after} function
83     * @throws NullPointerException if after is null
84     *
85     * @see #compose(Function)
86     */
87    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
88        Objects.requireNonNull(after);
89        return (T t) -> after.apply(apply(t));
90    }
91
92    /**
93     * Returns a function that always returns its input argument.
94     *
95     * @param <T> the type of the input and output objects to the function
96     * @return a function that always returns its input argument
97     */
98    static <T> Function<T, T> identity() {
99        return t -> t;
100    }
101}
102