使用stream将list转成map

jdk1.8 lambda表达式一行便可以将list转成map

其中包括不同的value，以及不同的key及科研重复时解决方案

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

import org.junit.Test;

/**
 * Created by Jackielee on 2017
 * @author: lizhilong
 * @date:   2017-11-24 10:13:57   
 * @Copyright: 2017 www.aliyun.com Inc. All rights reserved.
 */
public class DemoListToMap {
    List list = Arrays.asList(new Student(1, 18, "阿龙", GenderColumn.BOY.getCode()),
                                       new Student(2, 17, "小花", GenderColumn.GIRL.getCode()),
                                       new Student(3, 17, "阿浪", GenderColumn.LADYBOY.getCode()));
    @Test
    public void listToMapByObjectValue(){
        // value 为对象 student -> student jdk1.8返回当前对象
        Map map = list.stream().collect(Collectors.toMap(Student::getId, student -> student));
        // 遍历打印结果
        map.forEach((key, value) -> {
            System.out.println("key: " + key + "    value: " + value);
        });
    }
    @Test
    public void listToMapByNameValue(){
        // value 为对象中的属性
        Map map = list.stream().collect(Collectors.toMap(Student::getId, Student::getName));
        map.forEach((key, value) -> {
            System.out.println("key: " + key + "    value: " + value);
        });
    }
}

value为对象执行结果：

key: 1    value: Student [id=1, age=18, name=阿龙, gender=0]
key: 2    value: Student [id=2, age=17, name=小花, gender=1]
key: 3    value: Student [id=3, age=17, name=阿浪, gender=2]

value为字段执行结果：

key: 1    value: 阿龙
key: 2    value: 小花
key: 3    value: 阿浪

@Test
    public void listToMapByAgeKey(){
        // value 为对象中的属性
        Map map = list.stream().collect(Collectors.toMap(Student::getAge, Student::getName));
    }

执行结果：

java.lang.IllegalStateException: Duplicate key 小花

@Test
    public void listToMapByAgeKey(){
        // value 为对象中的属性
        Map map = list.stream().collect(
            Collectors.toMap(Student::getAge, Student::getName, (key1, key2) -> key1)
        );
        map.forEach((key, value) -> {
            System.out.println("key: " + key + "    value: " + value);
        });
    }

key: 17    value: 小花
key: 18    value: 阿龙