Java 8 forEach简单例子

1.遍历map

void testMap() {
        Map<String, Integer> map = new HashMap<>();
        map.put("a", 1);
        map.put("b", 1);
        map.put("c", 1);
        map.forEach((k, v)-> System.out.println(k + ":" + v));
    }

输出

a:1
b:1
c:1

2.遍历带lambda条件的map

void testMapLambda() {
        Map<String, Integer> map = new HashMap<>();
        map.put("a", 1);
        map.put("b", 1);
        map.put("c", 1);
        map.forEach((k, v)-> {
            if ("b".equalsIgnoreCase(k)) {
                System.out.println(k + ":" + v);
            }
        });
    }

输出

b:1

3.遍历带lambda条件的list

void testList() {
        List<String> list = new ArrayList<>();
        list.add("abc");
        list.add("def");
        list.add("hij");
        list.forEach(key -> {
                    if ("def".equalsIgnoreCase(key)) {
                        System.out.println(key);
                    }
                }
        );
    }

输出

def

Lambda表达式局部变量

示例

public class LambdaTest {

    int instanceCounter = 0;

    public void method() {
        int localCounter = 0;
        instanceCounter = 5; //Re-assign instance counter so it is no longer effectively final

        Stream.of(1,2,3).forEach(elem -> instanceCounter++); //WHY DOES THE COMPILER NOT COMPLAIN HERE
        Stream.of(1,2,3).forEach(elem -> localCounter++); //Does not compile because localCounter is not effectively final
    }
}

其中instanceCounter是类的成员变量，localCounter是函数的局部变量，在标红的语句报错

Variable used in Lambda expression must be final or effectively final