Question

I need to implement the equals method in some class A. Class A has an orderer collection of Enum type, and the behaviour I want to achive is that equals returns true for two instances of Class A that have exactly the same Enum values in the collection (in exactly the same positions of the collection).

As I’m new to java, I’m having problems with this, and I dont know how to properly implement equals or the hashcode methods, so any help would be good 🙂

Answer 1

If you’re using eclipse (netbeans has similar features, as do most java IDEs), you can simply got to the “Source” menu, and choose “Generate hashcode() and equals()”. Then you select the fields you want to be considered (in your case the list of enum values.

That being said, assuming you already have the enum, here’s the code that eclipse generated for me. Not that hashcode usually involves a prime number, as well as multiplication and addition. This tends to give you somewhat decent distribution of values.

public class Foo {
   private List<FooEnum> enumValues;

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

   @Override
   public boolean equals(Object obj) {
       if (this == obj)
           return true;
       if (obj == null)
           return false;
       if (getClass() != obj.getClass())
           return false;
       Foo other = (Foo) obj;
       if (enumValues == null) {
           if (other.enumValues != null)
               return false;
       }
       else if (!enumValues.equals(other.enumValues))
           return false;
       return true;
   }


}