java8 lambda使用方法

蔚落 2024-04-17 05:34 53阅读 0赞

记录一下，利用java新特性 lambda表达式的使用方法，这里主要列举常用的3个（list转map，分组，过滤）

## 1. list转map ##

//组装list集合
    List<User> userList = Lists.newArrayList();
    userList.add(new User("111", "张三"));
    userList.add(new User("222", "李四"));
    
    //list转map
    Map<String, String> userMap = userList.stream().collect(
            Collectors.toMap(User::getId, User::getName)
    );
    
    //value是用户对象时
    Map<String, String> userMap2 = userList.stream().collect(
            Collectors.toMap(User::getId, a -> a)
    );
    
    System.out.println("userMap === " + userMap);

运行结果

userMap === { 111=张三, 222=李四}

**特殊点：如果key存在相同时**

/** * 注意：如果存在key值相同的，可以用 (k1,k2)->k1 来设置 * (k1,k2)->k1 如果有重复的key,则保留key1,舍弃key2 --- 保留前者 * (k1,k2)->k2 如果有重复的key,则保留key2,舍弃key1 --- 覆盖前者 */
    //组装list集合
    List<User> userList = Lists.newArrayList();
    userList.add(new User("111", "张三"));
    userList.add(new User("222", "李四"));
    userList.add(new User("111", "王五"));
    
    userMap = userList.stream().collect(
            Collectors.toMap(User::getId, User::getName, (k1,k2)->k1)
    );
    System.out.println("userMap k1 === " + userMap);
    
    userMap = userList.stream().collect(
            Collectors.toMap(User::getId, User::getName, (k1,k2)->k2)
    );
    System.out.println("userMap k2 === " + userMap);

运行结果

userMap k1 === { 111=张三, 222=李四}
    userMap k2 === { 111=王五, 222=李四}

## 2. 分组 ##

//组装list集合
    List<User> userList = Lists.newArrayList();
    userList.add(new User("111", "张三"));
    userList.add(new User("222", "李四"));
    userList.add(new User("111", "王五"));
    
    Map<String, List<User>> userGroupMap = userList.stream().collect(
            Collectors.groupingBy(User::getId)
    );
    System.out.println("userGroupMap === " + userGroupMap);

运行结果

userGroupMap === { 111=[User(id=111, name=张三), User(id=111, name=王五)], 222=[User(id=222, name=李四)]}

## 3. 过滤 ##

//组装list集合
    List<User> userList = Lists.newArrayList();
    userList.add(new User("111", "张三"));
    userList.add(new User("222", "李四"));
    userList.add(new User("111", "王五"));
    
    List<User> userFilterList = userList.stream()
            .filter(a -> a.getId().equals("111"))
            .collect(Collectors.toList());
    
    System.out.println("userFilterList === " + userFilterList);

运行结果

userFilterList === [User(id=111, name=张三), User(id=111, name=王五)]

## 4. 完整示例代码 ##

*  **用户对象**

import lombok.Data;
    
    /** * @author yanghao * @version User.java, v 0.1 2019-08-22 17:20 */
    @Data
    public class User { 
    
        /** * 用户编号 */
        private String id;
    
        /** * 用户名称 */
        private String name;
    
        /** * 构造器 * @param id * @param name */
        public User(String id, String name){ 
            this.id = id;
            this.name = name;
        }
        
    }

*  **测试类**

import org.assertj.core.util.Lists;
    import org.junit.Test;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    /** * @author yanghao * @version LambdaTest.java, v 0.1 2019-08-22 17:19 */
    public class LambdaTest { 
    
        @Test
        public void test(){ 
            //组装list集合
            List<User> userList = Lists.newArrayList();
            userList.add(new User("111", "张三"));
            userList.add(new User("222", "李四"));
    
            //1. list转map
            Map<String, String> userMap = userList.stream().collect(
                    Collectors.toMap(User::getId, User::getName)
            );
    
    		//value是用户对象时
    		Map<String, String> userMap2 = userList.stream().collect(
    		        Collectors.toMap(User::getId, a -> a)
    		);
    
            System.out.println("userMap === " + userMap);
            //System.out.println("userMap2 === " + userMap2);
    
            /** * 注意：如果存在key值相同的，可以用 (k1,k2)->k1 来设置 * (k1,k2)->k1 如果有重复的key,则保留key1,舍弃key2 --- 保留前者 * (k1,k2)->k2 如果有重复的key,则保留key2,舍弃key1 --- 覆盖前者 */
            userList.add(new User("111", "王五"));
    
            userMap = userList.stream().collect(
                    Collectors.toMap(User::getId, User::getName, (k1,k2)->k1)
            );
            System.out.println("userMap k1 === " + userMap);
    
            userMap = userList.stream().collect(
                    Collectors.toMap(User::getId, User::getName, (k1,k2)->k2)
            );
            System.out.println("userMap k2 === " + userMap);
    
    
            //2. 分组
            Map<String, List<User>> userGroupMap = userList.stream().collect(
                    Collectors.groupingBy(User::getId)
            );
            System.out.println("userGroupMap === " + userGroupMap);
    
    
            //3.过滤
            List<User> userFilterList = userList.stream()
                    .filter(a -> a.getId().equals("111"))
                    .collect(Collectors.toList());
    
            System.out.println("userFilterList === " + userFilterList);
        }
    }

运行结果

userMap === { 111=张三, 222=李四}
    userMap k1 === { 111=张三, 222=李四}
    userMap k2 === { 111=王五, 222=李四}
    userGroupMap === { 111=[User(id=111, name=张三), User(id=111, name=王五)], 222=[User(id=222, name=李四)]}
    userFilterList === [User(id=111, name=张三), User(id=111, name=王五)]