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25 package java.util.function;
26 
27 import 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
41 public 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      */
apply(T t)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      */
compose(Function<? super V, ? extends T> before)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      */
andThen(Function<? super R, ? extends V> after)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      */
identity()97     static <T> Function<T, T> identity() {
98         return t -> t;
99     }
100 }
101