Overriding the equals ( ) method

[Deon Smith]

Hello,
Could someone possibly explain this issue? I’ve read that whenever creating classes that do not have a version of the equals( ) method, you should override the method provided by Object. Is the code below the proper way to do this? Thanks.
class K {
public boolean equals ( ) {
}
public static void main (String [] args) {
K k1 = new K( );
K k2 = new K( );
if (k1.equals(k2)) {
System.out.println (“They equal” ;
}
else {
System.out.println (“Not Equal” ;
}
}
}

[Wilfried LAURENT]

No it is not the right way.
You must conform to the exact signature of the method you are overriding:

public class K {
 int value=0;
  public boolean equals(Object obj) {
    //reflexive
    if (this == obj)
      return true;
    // Check that obj is a K
    if (!(anObject instanceof K))
      return false;
    // Safe cast
    K k=(K)obj;
    if (value==k.value)
return true;
    else
        return false;
    }
}

But don't forget that according to the "hashcode" contract, you also have to override it:

If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result

W.
[ October 09, 2002: Message edited by: Wilfried LAURENT ]

[Jamie Robertson]

fresh from the JavaRanch e-press:
Equals and Hash Code - Introduction

[Colin Kenworthy]

Rather than write "if <a condition> is true then return true, otherwise return false" why not just return <a condition> ? As in:

public class K {
   int value=0;
   public boolean equals(Object obj) {
      //reflexive
      if (this == obj)
         return true;
      // Check that obj is a K
      if (!(obj instanceof K))
         return false;
      // Safe cast
      K k = (K)obj;
      return (value == k.value);
   }
}

or even just

      // Safe cast
      return (value == ((K)obj).value);

[John Dale]

It is important to know how to override equals, hashCode, and how to write compareTo. It is also important to know when to override equals and when not to override equals.
Often the behavior of Object.equals() is exactly what is needed. Override Object.equals() when and only when whether two objects are to be considered equal based on information contained in the objects, or in objects to which they refer. Use Object.equals() when and only each instance of a class is to be considered not equal to any other instance of that class, no matter what information it contains or refers to.
The sticky problem with equals arises when you are subclassing a class that already has an equals method. But you already know what to do about that.

[Wilfried LAURENT]

To Colin:

and of course if value weren't an int but any Object, use equals instead of == .
W.