The functionality of the || operator

Question

I have created an equals method. I believe I can do it in two ways but java doesn't seem to agree. Here is the first example which I know works.

    public class HelloWorld {

     public boolean equals(Object rhs){
        if(this==rhs){
            return true;
        }else if( rhs==null || this.getClass() != rhs.getClass()){
            return false;
        }else{

            HelloWorld tmp =(HelloWorld)rhs;
            return true;
        }
     }

     public static void main(String[] args){
        HelloWorld c = new HelloWorld();
        System.out.println(c.equals(null));

     }  
}

And here is the second example which doesn't work. Instead of returning false I get a nullpointerexception.

    public class HelloWorld {

     public boolean equals(Object rhs){
        if(this==rhs){
            return true;
        }else if(this.getClass() != rhs.getClass() || rhs==null){
            return false;
        }else{

            HelloWorld tmp =(HelloWorld)rhs;
            return true;
        }
     }

     public static void main(String[] args){
        HelloWorld c = new HelloWorld();
        System.out.println(c.equals(null));

     }  
}

My question then... The only difference between the two pieces of code is that in the first piece I've written

    rhs ==null ||...

And in the second piece it's on the other side of the OR-operator so

    ...|| rhs==null

Why would the first case not give me a Nullpointer but the second case does? Why should it matter on which side of the OR operator i write the boolean statement?

Answer 1

It matters because ||, being the conditional "OR" operator, won't evaluate the right expression if the left expression is true, because the result can already be determined. This is known as "short-circuit" behavior.

Section 15.24 of the JLS covers this:

The conditional-or operator || operator is like | (§15.22.2), but evaluates its right-hand operand only if the value of its left-hand operand is false.

The right hand expression is not evaluated, so the NullPointerException is avoided.

(The && conditional "AND" operator, also "short circuits" and will only evaluate the right side if the left side is true.)