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Thread: Help With Null Pointer Exception

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    Default Help With Null Pointer Exception

    Null Pointer Exception fixed.
    Last edited by kendraheartt; August 16th, 2012 at 10:36 AM.


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    Default Re: Help With Null Pointer Exception

    In you error message, the first line should say something like java.lang.NullPointerException.
    The next line should tell you which variable has not been initialized and the line number of your code where the error seems to be.
    You have only shown parts of your code, so it is not easy to pick out the problem without the error message and/or the rest of the code.

    Try finding the problem using the information provided by the error message you get. If you can not find it, post again with the relevant code and error message.

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    Default Re: Help With Null Pointer Exception

    Look at the line where the NPE occurred and find the variable with the null value. If you can't see what variable is null, add a println just before the line with the NPE and print out the values of all the variables used on the line with the error.
    When you find the variable, backtrack in the code to see why that variable does not have a valid value.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default

    First thing I notice is that you are not allocating "theMissile"..need to do a new like you did for "the Bomb".

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    Default Re: Help With Null Pointer Exception

    Okay, so I just did the same thing for the missile as I did the bomb, and I'm still getting the null pointer exception. Im pretty sure theres an error with theShield variable, but I cant seem to figure out what...

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    Default Re: Help With Null Pointer Exception

    I'm still getting the null pointer exception.
    Please copy and paste here the full text of the error message, It says where the error happens.

    If theShield variable has a null value, where in the code is it supposed to get a valid value? Is that code being executed?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help With Null Pointer Exception

    So far you have had 2 NullPointerExceptions, which you have correctd. Now a third one. I seriously suggest you try reading the error message and figuring it out. The message tells you the name of the variable which has not been initialized and the line number where there is the first attempt to use it. First the missile and the bomb, now you suspect the shield....

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    Default Re: Help With Null Pointer Exception

    Quote Originally Posted by Norm View Post
    Please copy and paste here the full text of the error message, It says where the error happens.

    If theShield variable has a null value, where in the code is it supposed to get a valid value? Is that code being executed?
    It says Exception in thread "Thread-37" java.lang.NullPointerException
    at objectdraw.Drawbale2D.overlaps (Drawable2D.java:123)
    at Shield.isHit(Shield.java:103)
    at Bomb.run(Bomb.java:87)
    whenever a bomb is shot off by the aliens

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    Default Re: Help With Null Pointer Exception

    Exception in thread "Thread-37" java.lang.NullPointerException
    at objectdraw.Drawbale2D.overlaps (Drawable2D.java:123)
    at Shield.isHit(Shield.java:103)
    at Bomb.run(Bomb.java:87)
    In Drawable2D at line 123 (called from Shield line 103) there is a variable with a null value. Look at line 123, find the variable with the null value and then backtrack in the code to find out why that variable does not have a valid value.
    If you can't tell which variable has the null value, add a println before line 123 that prints out the values of all the variables on line 123 so you can see.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help With Null Pointer Exception

    It works only if I isolate each variable (the bomb works if I don't include the || (!shield[row][col].isHidden () && shield[row][col].overlaps (missile)) portion and vice versa for the missile... is there maybe something wrong with the way I'm writing my if statemnt in the isHit method?

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    Default Re: Help With Null Pointer Exception

    something wrong with the way I'm writing my if statemnt
    If you know that a variable can have a null value, you should check for that first before using the variable in the rest of the if statement.

    Do you know which variable has the null value?
    You need to find the variable with the null value.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help With Null Pointer Exception

    when I include both it comes back as both of them as null if i include both of them in the if statement (both the missile and the bomb), but if i only include one in the if statement it doesn't give a null pointer exception... so I'm confused as to why they work individually but not together...

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    Default Re: Help With Null Pointer Exception

    comes back as both of them as null
    Sorry, what you said makes no sense.
    What prints out when you print the values of all the variables on line 123 where the NPE happens?

    Can you post the code for line 123 and the println statement(s) that prints the variables on that line and also what is printed out?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help With Null Pointer Exception

    if ((!shield[row][col].isHidden () && (shield[row][col].overlaps (missile)) || shield[row][col].overlaps (bomb))) is the line where i keep getting the NPE... Sorry but I don't quite understand what you want/how you want me to print out the variables

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    Default Re: Help With Null Pointer Exception

    Print out the values of: shield[row][col], row and col
    also print out the contents of the shield array:
    System.out.println("shield="+Arrays.toString(shiel d));
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help With Null Pointer Exception

    Row: 1, 2, 3... 25
    Col: 1, 2, 3.... 25
    shield[row][col] didn't show up when i put in the println statements and
    added what you told me to print i got an error saying the Variable Arrays could not be found... is there something i have to import for that to work?

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    Default Re: Help With Null Pointer Exception

    shield[row][col] didn't show up
    Did the location of the NPE change?
    Why didn't the value of shield[row][col] print?

    When posting what is printed on the console by the program including the error message, you should copy and paste it here, not type it in. There are probably important details you are missing. By not giving full details on what the code is doing, you are making it very hard to find the problem.

    The Arrays class is in the java util package. You should look in the API doc for that detail.

    To copy the contents of the command prompt window:
    Click on Icon in upper left corner
    Select Edit
    Select 'Select All' - The selection will show
    Click in upper left again
    Select Edit and click 'Copy'

    Paste here.

    I'm done for tonight. Back tomorrow.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help With Null Pointer Exception

    Quote Originally Posted by kendraheartt View Post
    ... i got an error saying the Variable Arrays could not be found...
    Paste the error message as it appears and the line of code the error was found on. It sounds like a tweaked line according to how you worded the error message.

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