Java8 List、Map、数组互转

CollectionMy.java

 

import constant.Collect;
import entity.User;

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class CollectionMy {


    public static void main(String[] args) {
//        String[] arrays = {"hello"," , ","world"};
//
//        System.out.println(arrayToList1(arrays));
//        System.out.println(arrayToList2(arrays));
//        System.out.println(listToArray1(arrayToList2(arrays)));
//        System.out.println(listToArray2(arrayToList2(arrays)));
//
//        Map<Integer, User> result = listToMap1(Collect.listEntiyChina);
//        System.out.println(result);
//        Map<Integer, User> result = listToMap2(Collect.listEntiyChina);
//        System.out.println(result);
//        sortList(Collect.listEntiyJapan);
//
        Map<Integer, List<User>> result = listToMap4(Collect.listEntiyJapan);
        System.out.println(result);



        Map<String,Integer> mapRepeat = new HashMap<>();

    }

    /**
     * Java8 数组转为List
     * @param arrays
     * @return
     */
    public static List<String> arrayToList1(String[] arrays) {
        List<String> result = Stream.of(arrays).collect(Collectors.toList());

        return result;
    }


    /**
     * Java8 List转为数组
     * @param list
     * @return
     */
    public static String[] listToArray1( List<String> list) {
        String[] result = list.stream().toArray(String[]::new);
        Arrays.stream(result).forEach(str -> System.err.println(str));
        return result;
    }

    /**
     * Java8 List转为Map
     *  ID 必须为唯一性
     * @param list
     * @return
     */
    public static Map<Integer, User> listToMap1(List<User> list) {
        Map<Integer, User> result = list.stream().collect(Collectors.toMap(e -> e.getId(), Function.identity()));
        return result;
    }
    /**
     * Java8 List转为Map
     *  ID 必须为唯一性
     * @param list
     * @return
     */
    public static Map<Integer, User> listToMap2(List<User> list) {
        Map<Integer, User> result = list.stream().collect(Collectors.toMap(User::getId, user->user));
        return result;
    }
    /**
     * Java8 List转为Map
     *  ID重复分组
     * @param list
     * @return
     */
    public static Map<Integer, List<User>> listToMap3(List<User> list) {
        Map<Integer, List<User>> result = list.stream().collect(Collectors.groupingBy(User::getId));
        return result;
    }

    /**
     * Java8 List转为Map
     *  根据年龄排序后，再根据ID重复分组
     * @param list
     * @return
     */
    public static Map<Integer, List<User>> listToMap4(List<User> list) {
        Collections.sort(list, Comparator.comparing(User::getId).thenComparing(User::getAge));
        list.forEach(System.out::println);
        System.out.println("----------------");
        Map<Integer, List<User>> result = list.stream().collect(Collectors.groupingBy(User::getId));
        return result;
    }

    public static void sortList(List<User> list) {
        Collections.sort(list, Comparator.comparing(User::getId).thenComparing(User::getAge));
        list.forEach(System.out::println);
    }

    public static List<String> arrayToList2(String[] arrays) {
        List<String> result = Arrays.asList(arrays);
        return result;
    }


    public static String[] listToArray2( List<String> list) {
        String[] result = list.toArray(new String[list.size()]);
        return result;
    }

 

 

Collect.java

 

import java.util.ArrayList;
import java.util.List;

/**
 * @program: java8
 * @description:
 * @author: bugyun
 * @create: 2018-11-05 10:02
 */
public class Collect {



    public final static List<String> listStr = new ArrayList<>();
    public final static List<Integer> listInt = new ArrayList<>();
    public final static List<User> listEntiyJapan = new ArrayList<>();
    public final static List<User> listEntiyChina = new ArrayList<>();

    static {
        listStr.add("Google");
        listStr.add("Facebook");
        listStr.add("Microsoft");
        listStr.add("IBM");
        listStr.add("Yahoo");

/////////////////////////////////////////////
        listInt.add(25);
        listInt.add(21);
        listInt.add(50);
        listInt.add(21);
        listInt.add(50);
/////////////////////////////////////////////
        User one = new User();
        one.setId(111);
        one.setName("tom");
        one.setPassword("123456");
        one.setAge(111112);

        User two = new User();
        two.setId(122);
        two.setName("jack");
        two.setPassword("123456");
        two.setAge(111111);

        User three = new User();
        three.setId(333);
        three.setName("lisa");
        three.setPassword("123456");
        three.setAge(111114);


        User four = new User();
        four.setId(122);
        four.setName("2j2ack2");
        four.setPassword("123456");
        four.setAge(111113);

        listEntiyJapan.add(four);
        listEntiyJapan.add(three);
        listEntiyJapan.add(one);
        listEntiyJapan.add(two);
////////////////////////////////////////////

        User one1 = new User();
        one1.setId(9000);
        one1.setName("zhangsan");
        one1.setPassword("123456");

        User two1 = new User();
        two1.setId(9001);
        two1.setName("wangwu");
        two1.setPassword("123456");

        User three1 = new User();
        three1.setId(9002);
        three1.setName("zhaoliu");
        three1.setPassword("123456");

        listEntiyChina.add(three1);
        listEntiyChina.add(one1);
        listEntiyChina.add(two1);

    }


}

 

 

User.java

public class User {

    private Integer id ;
    private  String name ;
    private String password ;
    private Integer age;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

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

 

 

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