源码如下

@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); };
    }
}

这个接口的accept方法，就是接收一个泛型，然后有一个没有返回值的函数。类似的我们可以有以下使用例子。

import java.util.function.Consumer;

public class Main {

    public static void main(String[] args) {
        testConsumer();
        testAndThen();
    }
    public static void testConsumer(){
        Consumer<Integer> consumer = integer -> System.out.println("print square =" +integer * integer);
        consumer.accept(3);
    }
    public static void testAndThen(){
        Consumer<Integer> consumer1 = integer -> System.out.println("first =" + integer);
        Consumer<Integer> consumer2 = integer -> System.out.println("second =" +  integer);
        Consumer<Integer> consumer3 = x -> {
            throw new NullPointerException("test Exception");};
        consumer1.andThen(consumer2).andThen(consumer3).accept(2);
    }

}

输出结果如下：

print square =9
first =2
second =2
Exception in thread "main" java.lang.NullPointerException: test Exception
    at Main.lambda$testAndThen$3(Main.java:17)
    at java.util.function.Consumer.lambda$andThen$0(Consumer.java:65)
    at Main.testAndThen(Main.java:19)
    at Main.main(Main.java:7)

Process finished with exit code 1

假如把

consumer1.andThen(consumer2).andThen(consumer3).accept(2);

换成

consumer1.andThen(consumer2.andThen(consumer3)).accept(2);

输出结果会变成

print square =9
Exception in thread "main" java.lang.NullPointerException: test Exception
first =2
	at Main.lambda$testAndThen$3(Main.java:17)
second =2
	at java.util.function.Consumer.lambda$andThen$0(Consumer.java:65)
	at java.util.function.Consumer.lambda$andThen$0(Consumer.java:65)
	at Main.testAndThen(Main.java:19)
	at Main.main(Main.java:7)

Process finished with exit code 1

因为方法的调用会使得consumer3的accept先调用，所以先把异常抛出了。

 

最后对Consuerm这个接口自己的总结。总的来说，只要比较了解lambda表达式还是比较好理解这个接口的意思的。

理解源码

default Consumer<T> andThen(Consumer<? super T> after) {
    Objects.requireNonNull(after);
    return (T t) -> { accept(t); after.accept(t); };
}

其意思就是返回一个，同时具有当前类跟输入参数的accept方法，如同方法名字一样andThen,类似一种相加的功能。