java8stream测试

stream测试

/**
 * @Author: liuxuewei
 * @Date: 2022/6/28 19:22
 * @Description:
 */
public class Test20220628 {
    public static void main(String[] args) {
        List<Demo> list = new ArrayList<>();
        list.add(new Demo(1,"lxw"));
        list.add(new Demo(2,"ydd"));
        list.add(new Demo(3,"lyy"));
        list.add(new Demo(3,"test"));
        System.out.println("list=>" + list);
        //list=>[Test20220628.Demo(pid=1, name=lxw), Test20220628.Demo(pid=2, name=ydd), Test20220628.Demo(pid=3, name=lyy), Test20220628.Demo(pid=3, name=test)]
        //现在有个需求，需要把list中的所有pid取出来，单独形成一个list列表
        //原先做法
        List<Integer> pids1 = new ArrayList<>();
        for (Demo demo : list) {
            pids1.add(demo.getPid());
        }
        System.out.println(pids1);
        //使用stream做法
        List<Integer> pids2 = list.stream().map(r -> r.getPid()).distinct().collect(Collectors.toList()); //取出对象列表中的某数据并去重得到列表
        System.out.println("pids2=>" + pids2);
        //pids=>[1, 2, 3]


        list.stream().collect(Collectors.groupingBy(r -> r.getPid())).forEach((k,v)->{
            System.out.println("K=>" + k + ";v=>" + v);//k代表group的k值,这里就是pid,v代表k对应的对象列表[所以这里取值要用v.get(0)],这里就是一个具体的Demo对象
            System.out.println(v.get(0));
        });

        for (Demo demo : list) {
            System.out.println(demo);
        }
    }

    @Data
    @AllArgsConstructor
    public static class Demo{
        Integer pid;
        String name;
    }
}