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null pointer exception
Joined: Oct 26, 2008
Oct 30, 2008 14:46:00
Without line 6 this code throws nullpointer exception.
but with it ,prints null.
How come? please explain
public static void main(String args)
s = s + ""; //line 6
String s1 = s.toLowerCase();
SCJP 5 , SCWCD 5
Joined: Oct 27, 2005
Oct 30, 2008 14:53:00
Every static or instance field gets an initial value. For numbers that's 0, for booleans false, and for objects
Since line 6 is the only assignment to static field s, without that line s will remain null. When you then try to call a method on it (line 8) - BOOM!
Now on line 6 you assign something to s. You are using the old value of s for that though, so the result will be
null + ""
Because of this concatenation, the null is converted to string literal "null" and then concatenated with the empty string - leading to the string "null".
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Joined: Aug 31, 2004
Oct 30, 2008 15:10:00
For details (and future reference), see
JLS - 15.18.1 String Concatenation Operator +
If an operand of type String is null, then the string "null" is used instead of that operand.
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It is sorta covered in the
JavaRanch Style Guide
subject: null pointer exception
ask help about gc!!!
working with null...
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