Link: Effective Java - Tips & tricks - What about equals()?

Effective Java: "Joshua Bloch's book Effective Java Programming Language Guide" is about tips and tricks about java.

This book i really recommend.

The only thing is, that the implementation of equals should be made different.

In stead of:

public boolean equals(Object o) {

    if (! (o instanceof MyObject)) {

      return false;

    }

    MyObject other = (MyObject) o;

    return (value == null ? other.value == null : 

        value.equals(other.value));

}

Use:

public boolean equals(Object o) {

    if ((o == null) || (this.getClass() != o.getClass())) {

        return false;

    }

    MyObject other = (MyObject) o;

    return (value == null ? other.value == null :

        value.equals(other.value));

}

The reason to use getClass() instead of instanceof, is the problem when you have inheritance.
The equals must be symmetric, which means if a.equals(b) then b.equals(c) must be true.

Example:

... Class A...

  public boolean equalsFalse(Object o) {

    if (! (o instanceof A)) {

      return false;

    }

    ...

  }

}





... Class B extends A...

  public boolean equalsFalse(Object o) {

    if (! (o instanceof B)) {

      return false;

    }

    ...

  }

}

In this case this would happen:

	A a = new A();

	B b = new B();

	

	a.equals(b); <- would return true

	b.equals(a); <- would return false

If you use getClass() you will instead get:

	a.equals(b); <- would return false

	b.equals(a); <- would return false. 

        Which means the symmetric isn't broke!

I've made some tests that shows that getClass() takes about 7% longer time to evaluate than instanceof.
But this is a price I would like to pay to be sure that I don't have any problems with inheritance.