Code issue

[Tyler Jordan]

I am trying to run the following code in a program. What is wrong with the following. Is there something wrong with doing a conditional on null? Or trying to run a conditional against a null?

String puck = null;
if (puck.equals(null))
{
puck = "puck";
System.out.println("PUCK = " + puck);
}

[Keith Lynn]

To test against null you want to use == and not .equals

[Tyler Jordan]

Oh, is null considered a primative?

[Keith Lynn]

No null is not a primitive.

When you use == between references, it determines if those references refer to the same object.

In the case of null, == will determine if a reference does not point to an actual object.

[marc weber]

Instead of pointing to a memory address, a null reference is a special value (all zeros) indicating that no object is referenced.

[Jesper de Jong]

Originally posted by marc weber:
... a null reference is a special value (all zeros) ...

Ok, this is nitpicking, but: a null value is not necessarily represented by "all zeros". What special value the JVM uses for a null reference is up to the implementation of the JVM.

[Stan James]

The syntax puck.equals(whatever) calls a method on an object. When puck is null there is no object so the JVM can't call the method and it gives the NPE.

[Benjamin Lau]

Test in NetBeans IDE 5.0
Failed:

/*
 * Main.java
 *
 * Created on 2006年8月10日, 下午2:26
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */
 
package first;
 
/**
 *
 * @author Ben
 */
public class Main {
    
    /** Creates a new instance of Main */
    public Main() {
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        String puck=null;
        if (puck.equals(null)){
            puck="puck";
            System.out.println("puck="+puck);
        }
    }
    
}  

error message:java.lang.NullPointerException
Successed:

 
/*
 * Main.java
 *
 * Created on 2006年8月10日, 下午2:26
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */
 
package first;
 
/**
 *
 * @author Ben
 */
public class Main {
    
    /** Creates a new instance of Main */
    public Main() {
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        String puck=null;
        if (puck==null){
            puck="puck";
            System.out.println("puck="+puck);
        }
    }
    
}

[shan sundaram]

Hi,
Otherwise you can also null check using "instanceOf" keyword
Ex:
if(!(puck instanceOf String)) {
.........
}

It will check both null and object reference..

[Jesper de Jong]

First of all, it's instanceof with a small o, not a capital O, and second, although instanceof returns false when you put null on the right hand side, it doesn't check for equality if the right hand side is not null.

Don't use instanceof just for this purpose, that's not what it was meant for.