Java哈希集包含重复项

Question

I have a hashset of Car objects eg Set<Car> cars = new HashSet<Car> .

The Car class is briefly given below

public abstract class Car {
  protected Piece current;
  protected Piece;
  ....
  ....
  ....
  @Override
  public booleanll) return false;
      if ((obj.getClass() == this.getClass())) {
        Car o = (Carrent.equals(o.current) && other.equals(o.other)) || (current.equals) && other.et)));
      }
      return false;
  }

  @Override
  public int hashCode() {
      final int prime = 31;
      int result = 1;
      result = prime * result + ((current == null) ? 0 : current.hashCode());
      result = prime =pokmob result;
  }

As you can see above, Car is an abstract class so I have two other classes BigCar and SmallCar extending the Car abstract class. And my set contains BigCar and SmallCar objects. Also note that these subclasses do not override the equals and hashcode method.

Answer 1

In your hashCode method, the two fields are not treated symmetrically - so two cars with current and other swapped will NOT have the same hashcode.

For example you could use this instead (also note the use of Objects::hash to avoid the null checks):

@Override public int hashCode() {
  return Objects.hash(current) * Objects.hash(other);
}

Answer 2

Well, in your hashCode current and other are not interchangeable.

Lets denote current 's value as x and other 's value as y for some instance.

The value of hashCode is 31*(31+x)+y .

Now, for another instance, if other is x and current is y (which means that second instance should be equal to the first instance), hashCode is now 31*(31+y)+x which is not equal to 31*(31+x)+y for most values of x and y .

Hence you are breaking the contract of hashCode , since if a.equals(b) == true , a.hashCode() must be equal to b.hashCode() .