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=NullPointerException with Compartor, help

2 replies on 1 page. Most recent reply: Sep 8, 2003 3:44 AM by David

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giles corey

Posts: 1
Nickname: anonaki
Registered: Sep, 2003

=NullPointerException with Compartor, help Posted: Sep 6, 2003 12:38 PM
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Why does the code below throw a java.lang.NullPointerException?? The two objects being
compared obviously don't become Null references.
any help would be greatly appreciated.

import java.util.*;

class test3
{
public static void main( String [] args )
{
Object obj1 = new Integer( 1 );
Object obj2 = new Integer( 2 );

if ( cmp.compare( obj1 ,obj2 ) == 0 )
System.out.println( "They are Equal" );
}

static Comparator cmp;
}


Senthoorkumaran Punniamoorthy

Posts: 335
Nickname: senthoor
Registered: Mar, 2002

Re: =NullPointerException with Compartor, help Posted: Sep 6, 2003 2:51 PM
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Its throwing an exception since Comparator is a Interface rather than a Class. An interface must be implemented before usage. The Comparator interface has two methods, equals() and compare(). You should refer to the JavaDoc before implementing these interfaces.

In your case you are passing the objects to the interface itself and not to a class which implements the interface.

Check these article for more info

http://www.javaworld.com/javaworld/jw-12-2002/jw-1227-sort_p.html


http://www.onjava.com/lpt/a/3286

David

Posts: 150
Nickname: archangel
Registered: Jul, 2003

Re: =NullPointerException with Compartor, help Posted: Sep 8, 2003 3:44 AM
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Senthoorkumaran is basically right, but I think the point is being missed. You've declared a reference variable ('cmp'), but you never assign anything to it! It's equivalent to doing the following:
  String myString;
  String myNewString = myString.toUpperCase(); // Will throw NullPointerException!

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