哈希表和可比较对象中使用的 Java Null 对象

Question

我有一个代表 UNKNOWN 值或“空对象”的对象。

在 SQL 中，这个对象永远不应该等于任何东西，包括另一个 UNKNOWN，所以 (UNKNOWN == UNKNOWN) -> false。

然而，该对象在哈希表中使用并且类型是Comparable，所以我创建了一个类如下:

public class Tag implements Comparable {    
  final static UNKNOWN = new Tag("UNKNOWN");
  String id;

  public Tag(String id) { 
    this.id = id; 
  }

  public int hashCode(){
    return id.hashCode();
  }

  public String toString(){
    return id;
  }

  public boolean equals(Object o){    
    if (this == UNKNOWN || o == UNKNOWN || o == null || !(o instanceof Tag))
      return false;

    return this.id.equals(((Tag)o).id);
  }

  public int compareTo(Tag o){
    if (this == UNKNOWN)
      return -1;
    if (o == UNKNOWN || o == null)
      return 1;

    return this.id.compareTo(o.id);
  }
}

但是现在 compareTo() 似乎“不一致”？

有没有更好的方法来实现compareTo()？

Answer 1

compareTo 的文档提到这种情况:

It is strongly recommended, but not strictly required that

(x.compareTo(y)==0) == (x.equals(y))

Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is "Note: this class has a natural ordering that is inconsistent with equals."

因此，如果您希望您的对象是Comparable，但仍然不允许通过equals 方法使两个UNKNOWN 对象相等，你必须让你的 compareTo “与 equals 不一致。”

一个合适的实现是:

public int compareTo(Tag t) {
    return this.id.compareTo(t.id);
}

否则，您可以明确指出UNKNOWN 值尤其不是Comparable:

public static boolean isUnknown(Tag t) {
    return t == UNKNOWN || (t != null && "UNKNOWN".equals(t.id));
}

public int compareTo(Tag t) {
    if (isUnknown(this) || isUnknown(t)) {
        throw new IllegalStateException("UNKNOWN is not Comparable");
    }
    return this.id.compareTo(t.id);
}