Creating Hash Codes for your Classes

• Your hashCode method must be compatible with the equals method
  • if x.equals(y) then x.hashCode() == y.hashCode()
• You get into trouble if your class defines an equals method but not a hashCode method
  • If we forget to define hashCode method for Coin it inherits the method from Object superclass
  • That method computes a hash code from the memory location of the object
  • Effect: any two objects are very likely to have a different hash code

    Coin coin1 = new Coin(0.25, "quarter");
    Coin coin2 = new Coin(0.25, "quarter");

• In general, define either both hashCode and equals methods or neither