@FunctionalInterface

1.此注解表明该接口是一个函数式接口，所谓的函数式接口，是指“有且只有一个抽象方法”
2.接口中的静态方法，默认方法（default修饰），以及java.lang.Object类中的方法都不算抽象方法。
3.如果接口符合函数式接口的定义，则此注解加不加无所谓，加了会方便编译器检查。如果不符合函数式接口定义，则此注解会报错。

先来看下stream的函数接口

@FunctionalInterface
public interface Consumer<T> {

    /**
     * Performs this operation on the given argument.
     *
     * @param t the input argument
     */
    void accept(T t);

    /**
     * Returns a composed {@code Consumer} that performs, in sequence, this
     * operation followed by the {@code after} operation. If performing either
     * operation throws an exception, it is relayed to the caller of the
     * composed operation.  If performing this operation throws an exception,
     * the {@code after} operation will not be performed.
     *
     * @param after the operation to perform after this operation
     * @return a composed {@code Consumer} that performs in sequence this
     * operation followed by the {@code after} operation
     * @throws NullPointerException if {@code after} is null
     */
    default Consumer<T> andThen(Consumer<? super T> after) {
        Objects.requireNonNull(after);
        return (T t) -> { accept(t); after.accept(t); };
    }
}

//List
@Override
    public void forEach(Consumer<? super E> action) {　　　　　　　　//函数接口
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        @SuppressWarnings("unchecked")
        final E[] elementData = (E[]) this.elementData;
        final int size = this.size;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            action.accept(elementData[i]);　　　　　　　　　　　　　　//调用具体实现
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }


list.forEach(s -> System.out.println(s))

 

示例二

//注解用于检查是否是函数式接口，default、static除外，有且只有一个抽象方法
@FunctionalInterface
public interface FunctionTest {
    void saveMessage(String message);

    default void saveMessage2(){
        System.out.println(" defaultMethod.....");
    };

    static void aaa(){
        System.out.println("static method .....");
    }
}

public class Test1 {
    public static void main(String[] args) {
        FunctionTest functionTest = new FunctionTest() {
            @Override
            public void saveMessage(String message) {
                System.out.println("111111");
            }
        };

        FunctionTest functionTest2 = s -> System.out.println(s);

        functionTest2.saveMessage("saveMessage........");

        Consumer<Integer> consumer = new Consumer<Integer>() {
            @Override
            public void accept(Integer o) {
                System.out.println(o);
            }
        };

        Consumer<Integer> consumer1 = s -> System.out.println(s);

        consumer1.accept(11111);

    }
}

 

接口定义了抽象的方法，我们先使用impl或者lambda实现了函数式接口，后面抽象方法调用的时候（如consumer.accept）,就是调用具体的实例方法。

java.util.function jdk还提供了其他的接口供使用，很方便，可以不需要自己定义接口，也不用写具体的impl文件。