java 8 分组

package com.demo;

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

import com.alibaba.fastjson.JSONObject;

public class Test7 {

	public static void main(String[] args) {
		List<Student> stuList = new ArrayList<Student>();
		Student st1 = new Student("123","aaa");
		Student st2 = new Student("234","bbb");
		Student st3 = new Student("345","ccc");
		Student st4 = new Student("345","ccc");
		Student st5 = new Student("123","ddd");
		stuList.add(st1);
		stuList.add(st2);
		stuList.add(st3);
		stuList.add(st4);
		stuList.add(st5);
		//1.提取出list对象中的一个属性
		Map<String, List<Student>> s = stuList.stream().collect(  
	               Collectors.groupingBy(Student::getId));
		s.forEach((k, v)->{  
		    v.forEach(System.out::println);
		});
		//输出1：
		/*Student [id=123	aaa]
		  Student [id=123	ddd]
		  Student [id=234	bbb]
		  Student [id=345	ccc]
		  Student [id=345	ccc]
		*/
		Object js = JSONObject.toJSON(s);
		System.out.println(js);
		//输出2：
		/*{
			"345":[{"name":"ccc","id":"345"},{"name":"ccc","id":"345"}],
			"123":[{"name":"aaa","id":"123"},{"name":"ddd","id":"123"}],
			"234":[{"name":"bbb","id":"234"}]
		  }
		*/
	}
}