【Java8新特性Stream】list转map

用Collectors的toMap方法转换List，一般会遇到两个问题。

• 转换map，key重复问题；
  1.重复时用后面的value 覆盖前面的value
  2.重复时将前面的value 和后面的value拼接起来；
  3.重复时将重复key的数据组成集合
• 另一个是空指针异常，即转为map的value是null

package core.stream;

import org.junit.Before;
import org.junit.Test;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ListToMapTest {

    private List<Student> list ;

    @Before
    public void setUp() {
        list = Arrays.asList(new Student(1, 18, "阿龙"),
                new Student(2, 17, "小花"),
                new Student(3, 17, "阿浪"));
    }

    /**
     * key:Object
     * {1=Student(id=1, age=18, name=阿龙), 2=Student(id=2, age=17, name=小花), 3=Student(id=3, age=17, name=阿浪)}
     */
    @Test
    public void testListToMapByObjectValue() {
        Map<Integer, Student> map = list.stream().collect(Collectors.toMap(Student::getId, student -> student));
        System.out.println(map);
    }

    /**
     * Key:value
     * {1=阿龙, 2=小花, 3=阿浪}
     */
    @Test
    public void testListToMapByNameValue() {
        List<Student> list = Arrays.asList(new Student(1, 18, "阿龙"),
                new Student(2, 17, "小花"),
                new Student(3, 17, "阿浪"));

        Map<Integer, String> map = list.stream().collect(Collectors.toMap(Student::getId, Student::getName));
        System.out.println(map);
    }

    /**
     *重复时将前面的value和后面的value拼接起来
     *例如：按年龄分组统计用户
     *{17=小花,阿浪, 18=阿龙}
     */
    @Test
    public void testListToMapByNameValueConcatenation() {
        Map<Integer,String> map = list.stream().collect(
                Collectors.toMap(Student::getAge,Student::getName,
                        (key1,key2)-> key1+","+key2));

        System.out.println(map);
    }
    /**
     * 重复时将重复的key的数据组成集合
     * 例如：按年龄分组统计用户
     * {17=[小花, 阿浪], 18=[阿龙]}
     */
    @Test
    public void testListToMapByNameList() {
        Map<Integer,List<String>> map = list.stream().collect(
                Collectors.toMap(Student::getAge, p -> {
                    List<String> values = new ArrayList<>();
                    values.add(p.getName());
                    return values;
                },(List<String> value1,List<String> value2)->{
                    value1.addAll(value2);
                    return value1;
                }));
        System.out.println(map);
    }
}

package core.stream;

import lombok.Data;

@Data
public class Student {
 private int id;
 private int age;
 private String name;

    public Student(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }
}