内置函数式接口-Function

向右看齐 2024-03-22 20:12 100阅读 0赞

## Function（功能性） ##

### 接口定义 ###

@FunctionalInterface
    public interface Function<T, R> {
        
    
        R apply(T t);
    
        default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        
            Objects.requireNonNull(before);
            return (V v) -> apply(before.apply(v));
        }
    
        default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        
            Objects.requireNonNull(after);
            return (T t) -> after.apply(apply(t));
        }
    
        static <T> Function<T, T> identity() {
        
            return t -> t;
        }

### 接口使用 ###

#### 传统方式 ####

Function<String, Integer> function = new Function<String, Integer>() {
        
        @Override
        public Integer apply(String s) {
        
            return s.length();
        }
    };
    
    List<Integer> list = Stream.of("aaa", "bbbbb", "ccccccv").map(function).toList();

#### lambda表达式 ####

Function<String, Integer> function1 = (s)-> s.length();
    List<Integer> list2 = Stream.of("aaa", "bbbbb", "ccccccv").map(function1).toList();
    
    
    List<Integer> list2 = Stream.of("aaa", "bbbbb", "ccccccv").map((s)-> s.length()).toList();

#### compose ####

Function<Integer, Integer> multiplyBy2 = i -> i * 2; // 扩大两倍
    Function<Integer, Integer> squared = i -> i * i; // 求平方
    
    //32 先平方再乘2 → 先4×4然后16×2
    System.out.println(multiplyBy2.compose(squared).apply(4));//32

#### andThen ####

Function<Integer, Integer> times2 = i -> i * 2; // 扩大两倍
    Function<Integer, Integer> squared = i -> i * i; // 求平方
    
    //64 先乘2再平方 → 先4×2然后8×8
    System.out.println(times2.andThen(squared).apply(4));

#### identity ####

Map<String, Integer> map = Stream.of("This", "is", "a", "test")
    .collect(Collectors.toMap(Function.identity(), String::length));
    // {a=1, test=4, This=4, is=2}