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Why the code throws Exception?

 
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Which of these 'if' will throw NullPointerException?
String s = "hello";
s = null;
if((s != null) && (s.length() > 0))
if((s != null) & (s.length() > 0))
if((s != null) | | (s.length() > 0))
if((s != null) | (s.length() > 0))
If have any explanations on this topic, I'd appretiate this.
Thank a lot.
 
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Aull,
In answer to your question, the first if statement is the only one which will NOT throw a NullPointerException.

The reason has to do with what expressions are evaluated with certain operators.
With the && operator, if the first expression is false, the second expression is never evaluated. That is why the first if statement does not throw an exception.
With the & operator, both expressions are evaluated. Since s IS null, we get the NullPointerException when we try to evaluate s.length();
With the | | operator, if one operand is true, the result is true without regard to the other operand. In the case of the third if statement, however, the first operand is false, so the second operand is evaluated and throws an exception.
Hope this helps.
Stephanie
 
Aull Sahar
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Originally posted by Stephanie Grasson:
[B]Aull,
In answer to your question, the first if statement is the only one which will NOT throw a NullPointerException.

The reason has to do with what expressions are evaluated with certain operators.
With the && operator, if the first expression is false, the second expression is never evaluated. That is why the first if statement does not throw an exception.
With the & operator, both expressions are evaluated. Since s IS null, we get the NullPointerException when we try to evaluate s.length();
With the | | operator, if one operand is true, the result is true without regard to the other operand. In the case of the third if statement, however, the first operand is false, so the second operand is evaluated and throws an exception.
Hope this helps.
Stephanie[/B][/Q
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