如何在java中使用group By作为映射？

Question

我有一个包含地图列表的列表

    Map<String, Object> hm = new HashMap<>();
    hm.put(id, 1);
    hm.put(amount, 25000);
    list.add(hm);

列表元素的索引=1,2,3,4,5,6

id=1, 2, 3 ,4 ,5, 4值

金额的值=10000、450000、25000、45000、35000、75000

列表应该排序如下（列表的索引）=6,4（id很大），2,5,3,1

Answer 1

有很多可能性，这一个修改了原始列表：

Comparator<Map<String, Object>> sortByAmount = Comparator.comparing(m -> (int) m.get("amount"));
Comparator<Map<String, Object>> sortById = Comparator.comparing(m -> (int) m.get("id"));

list.sort(sortByAmount.reversed().thenComparing(sortById));

如果要保留原始列表，可以使用流api：

list.stream().sorted(sortByAmount.reversed().thenComparing(sortById));

Answer 2

我认为使用CustomObject而不是Map将帮助您以更简单的方式解决这个问题。

自定义对象

//fell free to change data type of fields as per your requirement
public class CustomObject {
    private Integer id;
    private Integer amount;

    public CustomObject(Integer id, Integer amount) {
        this.id = id;
        this.amount = amount;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getAmount() {
        return amount;
    }

    public void setAmount(Integer amount) {
        this.amount = amount;
    }
}

现在列表将包含对象而不是地图，这将使排序更加容易。

List<CustomObject> list=new ArrayList<>();
        CustomObject customObject1=new CustomObject(1,10000);
        CustomObject customObject2=new CustomObject(2,45000);
        CustomObject customObject3=new CustomObject(3,25000);
        CustomObject customObject4=new CustomObject(4,45000);
        CustomObject customObject5=new CustomObject(5,35000);
        CustomObject customObject6=new CustomObject(6,75000);

        list.add(customObject1);
        list.add(customObject2);
        list.add(customObject3);
        list.add(customObject4);
        list.add(customObject5);
        list.add(customObject6);

        // You can use thenComparing() as much as you required incase you want to compare with multiple field of your custom object
        // for your problem it has only two field
        Collections.sort(list, Comparator.comparing(CustomObject::getAmount)
                .thenComparing(CustomObject::getId).reversed());

        list.forEach(x->{

            System.out.println(x.getId()+" -->" +x.getAmount());
        });

输出：

6 -->75000
4 -->45000
2 -->45000
5 -->35000
3 -->25000
1 -->10000