Lambda表达式

对接口使用

接口中只能有一个必须被实现的方法

基础语法

() -> {}

• ()：描述参数列表
• {}：描述方法体

无参无返回值

public interface LambdaNoReturnAndParameter {
    void test();
}

    @Test
    void lambda(){
        LambdaNoReturnAndParameter  lambda = ()->{
            System.out.println("hhh");
        };
        lambda.test();
    }

有参返回值

public interface LambdaTest {
    int test(int a);
}

    @Test
    void lambda(){
        LambdaTest lambdaTest = (a)->{
            return a;
        };

        System.out.println(lambdaTest.test(1));
    }

• 单个参数()可以省略
• 方法体中单行代码，{}可以省略
• 如果单行代码是return，那么{}和return都可以省略

静态方法引用

引用的方法要和接口中的方法一致

public interface LambdaTest1 {
    int test(int a);
}

public class LambdaTest2 {
    public static int test(int a) {
        System.out.println("hhh");
        return a;
    }
}

    @Test
    void lambda1(){
        LambdaTest1 lambdaTest1 = LambdaTest2::test;
        lambdaTest1.test(3);
    }

构造方法引用

public interface LambdaTest {
    Person getPerson();
}

public class Person {
    public Person() {
        System.out.println("无参");
    }
}

    @Test
    void lambda(){
        LambdaTest lambdaTest = Person::new;
        lambdaTest.getPerson();
    }

集合

public class LearnLambda {
    public static void main(String[] args) {
        List<User> list = new ArrayList<>();
        list.add(new User("zhangsan3",12));
        list.add(new User("zhangsan1",10));
        list.add(new User("zhangsan2",11));
        list.add(new User("zhangsan5",14));
        list.add(new User("zhangsan4",13));
        //降序
        list.sort((o1,o2) ->o2.getAge() - o1.getAge());
        System.out.println(list);
        //升序
        list.sort(Comparator.comparingInt(User::getAge));
        System.out.println(list);
        //删除年龄大于12的
        list.removeIf(user -> user.getAge() > 12);
        System.out.println(list);
    }
}

class User{
    private String name;
    private Integer age;

    public User(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public Integer getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

List转List

List<User> list = .......
List<Integer> collect = list.stream().map(user -> user .getId()).collect(Collectors.toList());

List转Map

List<User> list = .......
 Map<Integer, User> map= list .stream().collect(Collectors.toMap(User::getId, user-> user));
 Map<Integer, List<User>> groupBy = list .stream().collect(Collectors.groupingBy(User::getId));

Map转List

        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"zhangsan");
        map.put(2,"lisi");
        map.put(3,"wangwu");
        map.put(4,"xiaoming");
        List<String> collect = map.entrySet().stream().map(e -> e.getValue()).collect(Collectors.toList());