java - 以对象列表为键的 HashMap

Question

在 HashMap 中，当我将对象列表作为键传递时，我得到了不同的结果。

List<NewClass> list1 = new ArrayList<>();
List<NewClass> list2 = new ArrayList<>();

NewClass obj1 = new NewClass(1, "ddd", "eee@gmail.com");
NewClass obj2 = new NewClass(2, "ccc", "kkk@gmail.com");

list1.add(obj1);
list1.add(obj2);

list2.add(obj1);
list2.add(obj2);

Map<List<NewClass>, Integer> mapClass = new HashMap<>();
mapClass.put(list1, 1234);
mapClass.put(list2, 4567);

System.out.println(mapClass.size());
System.out.println(mapClass.get(list1));

NewClass obj4 = new NewClass(1, "ddd", "eee@gmail.com");
NewClass obj5 = new NewClass(2, "ccc", "kkk@gmail.com");
List<NewClass> list3 = new ArrayList<>();
list3.add(obj4);
list3.add(obj5);

System.out.println(mapClass.get(list3));

System.out.println(list1.hashCode());
System.out.println(list2.hashCode());
System.out.println(list3.hashCode());

下面是我看到的输出

hashCode() called - Computed hash: -1704251796
hashCode() called - Computed hash: -587009612
hashCode() called - Computed hash: -1704251796
hashCode() called - Computed hash: -587009612
1
hashCode() called - Computed hash: -1704251796
hashCode() called - Computed hash: -587009612
4567
hashCode() called - Computed hash: -1704251796
hashCode() called - Computed hash: -587009612
**null**
hashCode() called - Computed hash: -1704251796
hashCode() called - Computed hash: -587009612
-1879206775
hashCode() called - Computed hash: -1704251796
hashCode() called - Computed hash: -587009612
-1879206775
hashCode() called - Computed hash: -1704251796
hashCode() called - Computed hash: -587009612
-1879206775

尽管所有 3 个列表的哈希码都相同，但 mapClass.get(list3) 正在重新调整为 null。 list3 与 list1/list2 具有相同的对象。为什么会出现这种行为？

Answer 1

来自 map V get(Object key) 文档:

 * ... if this map contains a mapping from a key
 * {@code k} to a value {@code v} such that
 * {@code Objects.equals(key, k)},
 * then this method returns {@code v}; otherwise
 * it returns {@code null}. ...

我不确定你是如何实现 NewClass 的 equals 方法的，但是 NewClass 的以下实现不会返回 null 当调用 System.out.println(mapClass.get(list3));

public class NewClass {
    private int id;
    private String name;
    private String mail;

    NewClass(int id,String name,String mail){
        this.id = id;
        this.name = name;
        this.mail = mail;
    }

    @Override
    public int hashCode() {
        return id * name.hashCode() * mail.hashCode();
    }

    @Override
    public boolean equals(Object o) {

        if (o == this) return true;
        if (!(o instanceof NewClass)) {
            return false;
        }
        NewClass newClass = (NewClass) o;

        return newClass.id == id &&
               newClass.name.equals(name) &&
               newClass.mail.equals(mail);
    }
}

此外，正如评论中提到的可变键不是一个好主意，请检查 this link其中详细说明了原因。