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Thread: Exception in thread "main" java.lang.NullPointerException

  1. #1
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    Exclamation Exception in thread "main" java.lang.NullPointerException

    Hi! I'm getting
    Exception in thread "main" java.lang.NullPointerException
    and I really don't understand why it's happening.
    Last edited by theandroidgirl; July 19th, 2014 at 01:45 AM.


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    Default Re: Exception in thread "main" java.lang.NullPointerException

    Please post the entire error message. Furthermore, do you know what a NullPointerException is and what it means if you get one?
    Perhaps google the term "NullPointerMessage" and read up on some tutorials about it.

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    Default Re: Exception in thread "main" java.lang.NullPointerException

    Yes exactly. A NullPointerException happens when you try to call a method on an object, but the object does not exist.
    For example, compare these two bits of code:
    String s = "Hello World";
    String[] arr = s.split(" ");
    String s = null;
    String[] arr = s.split(" ");

    The first one will work and return an array of "Hello" and "World".
    The second piece of code will throw a NullPointerException. The reason is, that we have a variable s and we try to call a method on the object that is being referenced by s.
    But the problem is, that there is no object being referenced by s. In our example s is a NullPointer, its a variable referencing the null value.

    Somewhere in your code you call a method on the reference of a variable but the variable is a NullPointer. This happens in line 187 as your compiler tells you.
    You should now go to line 187 and try to find out which of these variables used there are your NullPointer (perhaps there is only 1 variable used). You can do this by adding some System.out.println() statements to your code to print the references of your variables.

    If you found the problem, think where you should initialize the variable with a non-null value. Come back here if you have additional questions.

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