POJO 类型的 Java Set 无法添加元素

Question

我有一个数据对象类:

public class MyDataObject {
  private String value;
  private String text;
  private Set<MyDataObject> child;

  // Getter & setters

  // Constructor
  public MyDataObject(final String value, final String text) {
    this.value = value;
    this.text = text;
  }

  @Override
  public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    MyDataObject that = (MyDataObject) o;
    return value.equals(that.value) &&
            text.equals(that.text) &&
            Objects.equals(child, that.child);
  }

  @Override
  public int hashCode() {
    return Objects.hash(value, text, child);
  }
}

我正在我的 MyDataObject 对象之一中添加 2 个子元素。

说:

MyDataObject myDataObject = new MyDataObject("USA", "United States");
Set<MyDataObject> childSet = new TreeSet<>(Comparator.comparing(MyDataObject::getText));
childSet.add(new MyDataObject("NY", "Bronx"));      // This is added
childSet.add(new MyDataObject("NY", "Manhattan"));  // This is not being added, returing false.
myDataObject.setChild(childSet);

我还重写了 hashcode 和 equals 方法来考虑子元素。

我错过了什么？

Answer 1

您提到的问题可以通过Comparator.comparing(MyDataObject::getValue)重现。然而，比较器 Comparator.comparing(MyDataObject::getText) 和 Comparator.comparing(MyDataObject::getValue) 与 equals 不一致。请参阅 TreeSet 的 javadoc哪里提到了这一点。

    final Comparator<MyDataObject> myDataObjectComparator = Comparator.comparing(MyDataObject::getValue);
    final MyDataObject myDataObject1 = new MyDataObject("NY", "Bronx");
    final MyDataObject myDataObject2 = new MyDataObject("NY", "Manhattan");
    System.out.println(myDataObject1.equals(myDataObject2));
    System.out.println(myDataObjectComparator.compare(myDataObject1, myDataObject2));

请注意，equals 表示对象不相等，而 compareTo 给出 0。对于相同的两个对象。