Function.java revision 2278037aa1653a32014df55f325879b4fe75fc27 
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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 */
40@FunctionalInterface
41public interface Function<T, R> {
42
43    /**
44     * Applies this function to the given argument.
45     *
46     * @param t the function argument
47     * @return the function result
48     */
49    R apply(T t);
50
51    /**
52     * Returns a composed function that first applies the {@code before}
53     * function to its input, and then applies this function to the result.
54     * If evaluation of either function throws an exception, it is relayed to
55     * the caller of the composed function.
56     *
57     * @param <V> the type of input to the {@code before} function, and to the
58     *           composed function
59     * @param before the function to apply before this function is applied
60     * @return a composed function that first applies the {@code before}
61     * function and then applies this function
62     * @throws NullPointerException if before is null
63     *
64     * @see #andThen(Function)
65     */
66    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
67        Objects.requireNonNull(before);
68        return (V v) -> apply(before.apply(v));
69    }
70
71    /**
72     * Returns a composed function that first applies this function to
73     * its input, and then applies the {@code after} function to the result.
74     * If evaluation of either function throws an exception, it is relayed to
75     * the caller of the composed function.
76     *
77     * @param <V> the type of output of the {@code after} function, and of the
78     *           composed function
79     * @param after the function to apply after this function is applied
80     * @return a composed function that first applies this function and then
81     * applies the {@code after} function
82     * @throws NullPointerException if after is null
83     *
84     * @see #compose(Function)
85     */
86    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
87        Objects.requireNonNull(after);
88        return (T t) -> after.apply(apply(t));
89    }
90
91    /**
92     * Returns a function that always returns its input argument.
93     *
94     * @param <T> the type of the input and output objects to the function
95     * @return a function that always returns its input argument
96     */
97    static <T> Function<T, T> identity() {
98        return t -> t;
99    }
100}
101