【Java8新特性Stream】list轉map

用Collectors的toMap方法轉換List，一般會遇到兩個問題。

• 轉換map，key重復問題；
  1.重復時用后面的value 覆蓋前面的value
  2.重復時將前面的value 和后面的value拼接起來；
  3.重復時將重復key的數據組成集合
• 另一個是空指針異常，即轉為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, "阿龍"),
                new Student(2, 17, "小花"),
                new Student(3, 17, "阿浪"));
    }

    /**
     * key:Object
     * {1=Student(id=1, age=18, name=阿龍), 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=阿龍, 2=小花, 3=阿浪}
     */
    @Test
    public void testListToMapByNameValue() {
        List<Student> list = Arrays.asList(new Student(1, 18, "阿龍"),
                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);
    }

    /**
     *重復時將前面的value和后面的value拼接起來
     *例如：按年齡分組統計用戶
     *{17=小花,阿浪, 18=阿龍}
     */
    @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);
    }
    /**
     * 重復時將重復的key的數據組成集合
     * 例如：按年齡分組統計用戶
     * {17=[小花, 阿浪], 18=[阿龍]}
     */
    @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;
    }
}