Override equals and hashCode methods

If you want to override equals method in your class, you should also override hashCode method as well. There are a lot of ways the two methods can be implemented incorrectly.

I found the easiest way is to use the wizard in Eclipse, just running Source | Generate hashCode() and equals()... either from menu or context menu. For example, this is what Eclipse generated for a simple class Scrap with a instance variable number:

   /* (non-Javadoc)
    * @see java.lang.Object#hashCode()
    */
   @Override
   public int hashCode() {
       final int PRIME = 31;
       int result = 1;
       result = PRIME * result + number;
       return result;
   }
  
   /* (non-Javadoc)
    * @see java.lang.Object#equals(java.lang.Object)
    */
   @Override
   public boolean equals(Object obj) {
       if (this == obj)
           return true;
       if (obj == null)
           return false;
       if (getClass() != obj.getClass())
           return false;
       final Scrap other = (Scrap) obj;
       if (number != other.number)
           return false;
       return true;
   }

I don't feel I need to modify anything in the 2 generated methods. I especially like the @Override annotation being used in the generated code. It clearly documents the method overriding and enforces the compile-time check.

Tags: java, eclipse, equals, hashCode