使用可变对象作为Java Map的key,会带来潜在风险的一个例子

package hashMap;
import java.util.HashMap;
import java.util.Map;
/* 
 * 可变对象是指创建后自身状态能改变的对象。换句话说，可变对象是该对象在创建后它的哈希值可能被改变。
在下面的代码中，对象MutableKey的键在创建时变量 i=10 j=20，哈希值是1291。
然后我们改变实例的变量值，该对象的键 i 和 j 从10和20分别改变成30和40。现在Key的哈希值已经变成1931。
显然，这个对象的键在创建后发生了改变。所以类MutableKey是可变的。
 */
class MutableKey {
  private int i;
  private int j;
  public MutableKey(int i, int j) {
    this.i = i;
    this.j = j;
  }
  public final int getI() {
    return i;
  }
  public final void setI(int i) {
    this.i = i;
  }
  public final int getJ() {
    return j;
  }
  public final void setJ(int j) {
    this.j = j;
  }
  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + i;
    result = prime * result + j;
    return result;
  }
  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null) {
      return false;
    }
    if (!(obj instanceof MutableKey)) {
      return false;
    }
    MutableKey other = (MutableKey) obj;
    if (i != other.i) {
      return false;
    }
    if (j != other.j) {
      return false;
    }
    return true;
  }
}
public class objectAsKey {
  private static void simulateObjectLost() {
    // HashMap
    Map<MutableKey, String> map = new HashMap<>();
    // Object created
    MutableKey key = new MutableKey(10, 20);
    // Insert entry.
    map.put(key, "Robin");
    // This line will print 'Robin'
    System.out.println(map.get(key));
    // Object State is changed after object creation.
    // i.e. Object hash code will be changed.
    key.setI(30);
    // This line will print null as Map would be unable to retrieve the entry.
    System.out.println(map.get(key));
  }
  public static void main(String[] args) {
    // Object created
    MutableKey key = new MutableKey(10, 20);
    System.out.println("Hash code: " + key.hashCode());
    // Object State is changed after object creation.
    key.setI(30);
    key.setJ(40);
    System.out.println("Hash code: " + key.hashCode());
    simulateObjectLost();
  }
}