java - 对象列表中的列总和

Question

我的问题如下:。我有对象列表。我需要添加列 x2 与相同的列 X3 并合并 X1 列。后来把旧的一排去掉了。请建议。

                Input:-

             List<A> list = new ArrayList<>()
            A A1 = new A(X1=111, X2=10, X3= '333')
            A A2 = new A(X1=222, X2=20, X3= '333')
            A A3 = new A(X1=333, X2=2, X3= '444')
            A A4 = new A(X1=444, X2=5, X3= '444')
            A A5 = new A(X1=666, X2=6, X3= '777')

            list.add(A1).addd(A2).add(A3).add(A4).add(A4)

            OutPut should be like below:-
             { 
                Object A1 (list = {111,222}, x2= 30 , x3= '333')
                Object A3 (list = {333,444}, x2= 7 , x3= '444')
                Object A5 (list = {666}, x2= 6 , x3= '777')
            }


public class A {
   private long X1;
   private long X2;
   private String X3;
   private List<Long> list;
 }

Answer 1

尝试这个(java 8+)，将 A 作为可变对象(我不喜欢):

        List<A> objects = new ArrayList<>();

        A A1 = new A(111, 10, "333");
        A A2 = new A(222, 20, "333");
        A A22 = new A(222, 20, "333");
        A A3 = new A(333, 2, "444");
        A A4 = new A(444, 5, "444");
        A A5 = new A(666, 6, "777");

        objects.add(A1);
        objects.add(A2);
        objects.add(A22);
        objects.add(A3);
        objects.add(A4);
        objects.add(A5);


        List<A> output = objects
                .stream()
                .collect(Collectors.collectingAndThen(
                        Collectors.toMap(A::getX3, a -> {
                            a.getList().add(a.getX1());
                            return a;
                        }, (a, b) -> {
                            a.getList().addAll(b.getList());
                            a.setX2(a.getX2() + b.getX2());
                            return a;
                        }),
                        map -> new ArrayList(map.values())
                ));

        System.out.println(output);

输出显示:

[A(X1=111, X2=50, X3=333, list=[111, 222, 222]),A(X1=333, X2=7, X3=444, list=[333, 444]),A(X1=666, X2=6, X3=777, list=[666])]