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Doubt regarding "null" in java

Mansukhdeep Thind
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Joined: Jul 27, 2010
Posts: 1157

Hi

Please have a look at the following code snippet:


When we invoke the method isEmpty with null as argument, why does it not return boolean value "true" for s==null. Isn't null==null true?


~ Mansukh
Seetharaman Venkatasamy
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Joined: Jan 28, 2008
Posts: 5575

Mansukhdeep Thind wrote:Isn't null==null true?

Let's test : System.out.println(null == null);

If you get exception in runtime. you have to analyze why you are getting that
Mansukhdeep Thind
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Joined: Jul 27, 2010
Posts: 1157

I followed your instruction. printing null==null does in fact return a Boolean true. Then why does throw null pointer exception? Or is it simply the act of passing a null to the method that spoils the show?
Mansukhdeep Thind
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Joined: Jul 27, 2010
Posts: 1157

I just realized I am thinking on wrong lines. It is the bitwise OR operator that is causing harm. How does this "|" operator function. What s meant by bitwise OR?
Henry Wong
author
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Joined: Sep 28, 2004
Posts: 18101
    
  39

Mansukhdeep Thind wrote:I just realized I am thinking on wrong lines. It is the bitwise OR operator that is causing harm. How does this "|" operator function. What s meant by bitwise OR?


As you figured out, a bitwise OR doesn't short circuit. It is used to OR all the bits of the two operands. From your code, the logical OR is what you need.

Henry


Books: Java Threads, 3rd Edition, Jini in a Nutshell, and Java Gems (contributor)
Mansukhdeep Thind
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Joined: Jul 27, 2010
Posts: 1157

So when the JVM tries to evaluate s.length, it hits a brick wall as String reference "s" is null(pointing to nothing). Hence, the stack explodes throwing a NullPointerException. Correct Henry?
 
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