jdk8 Stream 流中将集合转成 map，重复 key 处理，统计最大值，获取某个属性集合等 10 种最常用方法

//1、list转map，指定key-value，key,value是对象中的某个属性值.Map<String,String> userMap1 = userList.stream().collect(Collectors.toMap(User::getId,User::getName));System.out.println("1->"+userMap1);
//2、list转map 指定key-value，key为属性值，value是对象本身Map<String,User> userMap2 = userList.stream().collect(Collectors.toMap(User::getId,User->User));System.out.println("2->"+userMap2);
//3、list转map 指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身Map<String,User> userMap3 = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));System.out.println("3->"+userMap3);
//4、指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身，遇到key 冲突的解决办法，选择第二个key的值value2覆盖第一个key的值value1。Map<String,User> userMap4 = userList.stream().collect(Collectors.toMap(User::getName, Function.identity(),(value1,value2)->value2));System.out.println("4->"+userMap4);
//5、指定key-value，按照name进行分组 value是对象集合Map<String, List<User>> userMap5 = userList.stream().collect(Collectors.groupingBy(User::getName));System.out.println("5->"+userMap5);
//6.统计所有ageInteger totalAge = userList.stream().map(User::getAge).reduce(0, Integer::sum);System.out.println("6总年龄->"+totalAge);
//7.只对名称为zhangsan的age  进行统计汇总Integer totalAgeFilter = userList.stream().filter(t -> t.getName().equals("zhangsan")).map(User::getAge).reduce(0, Integer::sum);System.out.println("7总年龄filter->"+totalAgeFilter);
//8.获取最大ageInteger maxAge = userList.stream().map(User::getAge).max((x, y) -> x - y).get();System.out.println("8最大年龄->"+maxAge);
//9.循环打印userList.stream().forEach(user -> System.out.println("9->"+user));
//10.获取名称集合List<String> nameList = userList.stream().map(User::getName).collect(Collectors.toList());System.out.println("10名称集合->"+ JSON.toJSONString(nameList));

/** * @author gzh -》javaNice * 用户类 */public class User {
    /**     * id     */    private String id;
    /**     * 名称     */    private String name;
    /**     * 年龄     */    private Integer age;
    public String getId() {        return id;    }
    public void setId(String id) {        this.id = id;    }
    public String getName() {        return name;    }
    public void setName(String name) {        this.name = name;    }
    public Integer getAge() {        return age;    }
    public void setAge(Integer age) {        this.age = age;    }
    @Override    public String toString() {        return "User{" +                "id='" + id + '\'' +                ", name='" + name + '\'' +                ", age=" + age +                '}';    }
    public User() {    }
    public User(String id, String name, Integer age) {        this.id = id;        this.name = name;        this.age = age;    }}

import com.alibaba.fastjson.JSON;import org.assertj.core.util.Lists;
import java.util.List;import java.util.Map;import java.util.function.Function;import java.util.stream.Collectors;
/** * @author gzh -》javaNice * * */public class StreamDemo {    public static void main(String[] args) {        User user1=new User("1","zhangsan",1);        User user2=new User("2","zhangsan",2);        User user3=new User("3","lisi",2);
        List<User> userList= Lists.newArrayList();        userList.add(user1);        userList.add(user2);        userList.add(user3);

        //1、list转map，指定key-value，key,value是对象中的某个属性值.        Map<String,String> userMap1 = userList.stream().collect(Collectors.toMap(User::getId,User::getName));        System.out.println("1->"+userMap1);
        //2、list转map 指定key-value，key为属性值，value是对象本身        Map<String,User> userMap2 = userList.stream().collect(Collectors.toMap(User::getId,User->User));        System.out.println("2->"+userMap2);
        //3、list转map 指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身        Map<String,User> userMap3 = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));        System.out.println("3->"+userMap3);
        //4、指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身，遇到key 冲突的解决办法，选择第二个key的值value2覆盖第一个key的值value1。        Map<String,User> userMap4 = userList.stream().collect(Collectors.toMap(User::getName, Function.identity(),(value1,value2)->value2));        System.out.println("4->"+userMap4);
        //5、指定key-value，按照name进行分组 value是对象集合        Map<String, List<User>> userMap5 = userList.stream().collect(Collectors.groupingBy(User::getName));        System.out.println("5->"+userMap5);
        //6.统计所有age        Integer totalAge = userList.stream().map(User::getAge).reduce(0, Integer::sum);        System.out.println("6总年龄->"+totalAge);
        //7.只对名称为zhangsan的age  进行统计汇总        Integer totalAgeFilter = userList.stream().filter(t -> t.getName().equals("zhangsan")).map(User::getAge).reduce(0, Integer::sum);        System.out.println("7总年龄filter->"+totalAgeFilter);
        //8.获取最大age        Integer maxAge = userList.stream().map(User::getAge).max((x, y) -> x - y).get();        System.out.println("8最大年龄->"+maxAge);
        //9.循环打印        userList.stream().forEach(user -> System.out.println("9->"+user));
        //10.获取名称集合        List<String> nameList = userList.stream().map(User::getName).collect(Collectors.toList());        System.out.println("10名称集合->"+ JSON.toJSONString(nameList));


    }}