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Thread: NullPointerException

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    Default NullPointerException

    Hello.
    Why is the NullPointerException generated in the following code snippet at run-time?
      		class Info{			
    			public String name;
    			public String version;
    			public String arch;
    			double CPUSpeed;
    		};
    		Info info =  new Info();
    		Info[] queue = new Info[100];
     
    		int pos = 0;
     
    				queue[pos].name = System.getProperty("os.name");
    				System.out.println( queue[pos].name);
    (the last two lines are problematic)


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    Default Re: NullPointerException

    Define 'problematic' - in Java terms. Post your errors in their entirety, copied from exactly as they appear at your end.

    Java statements apart from variable declarations and initializations must occur within a method. I don't see a method in what you've posted. And there are other syntactic problems with what you've posted.

    What are you trying to do?

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    Senior Member PhHein's Avatar
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    Default Re: NullPointerException

    Info[] queue = new Info[100]; // Array that CAN hold 100 Info references
    int pos = 0;
    queue[pos].name = System.getProperty("os.name"); // First slot of the array was never filled with an Info reference

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    Default Re: NullPointerException

    Quote Originally Posted by PhHein View Post
    Info[] queue = new Info[100]; // Array that CAN hold 100 Info references
    int pos = 0;
    queue[pos].name = System.getProperty("os.name"); // First slot of the array was never filled with an Info reference
    What shall I do?

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    Member Ada Lovelace's Avatar
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    Default Re: NullPointerException

    Just a bit of info in-case your are unsure of what the array-pointer thing is

    There is a very close connection between pointers and arrays. In fact they
    are more or less one and the same thing. When you declare an array as:

    int[]a;
    You in fact declaring a pointer a to the first element in the array. That is, a is exactly the same as &a[0].
    The only difference between a and a pointer variable is that the array name is a constant pointer
    - you cannot change the location it points at. When you write an expression such as a[i]
    this is converted into a pointer expression that gives the value of the appropriate element.

    Wishes Ada xx
    If to Err is human - then programmers are most human of us all.
    "The Analytical Engine offers a new, a vast, and a powerful language . . .
    for the purposes of mankind
    ."
    Augusta Ada Byron, Lady Lovelace (1851)

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    Default Re: NullPointerException

    Quote Originally Posted by Ada Lovelace View Post
    Just a bit of info in-case your are unsure of what the array-pointer thing is

    There is a very close connection between pointers and arrays. In fact they
    are more or less one and the same thing. When you declare an array as:



    You in fact declaring a pointer a to the first element in the array. That is, a is exactly the same as &a[0].
    The only difference between a and a pointer variable is that the array name is a constant pointer
    - you cannot change the location it points at. When you write an expression such as a[i]
    this is converted into a pointer expression that gives the value of the appropriate element.

    Wishes Ada xx
    But I wrote
    Info[] queue = new Info[100]
    madam.
    Doesn't that create an anonymous pointer and assign queue to refer to it?

    Do I instead write Info queue[] = new Info[100]?

  7. #7
    Member Ada Lovelace's Avatar
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    Default Re: NullPointerException

    Do I instead write Info queue[] = new Info[100]?
    no, because arrays in Java are not declared like that. I was just pointing out to
    you what the difference between a pointer and an array was under the hood. The syntax
    I showed was using the C language - which is not a managed language like Java is.

    In this stance, because Java handles all memory allocation for you - it might look like this:

    info[]queue = new info[100];

    new will create an instance of info and assign the result to queue.
    So, queue - because it has the instance of info - would act as a pointer
    to the first element of the array info.

    *queue->info[0] // this is suggested - not correct syntax

    Please also note that what I am trying to explain is how the memory is allocated
    using arrays and pointers. This is not an answer to your original question, but
    instead to give you a bit of understanding on how things work under the hood.

    Wishes Ada xx
    If to Err is human - then programmers are most human of us all.
    "The Analytical Engine offers a new, a vast, and a powerful language . . .
    for the purposes of mankind
    ."
    Augusta Ada Byron, Lady Lovelace (1851)

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    Default Re: NullPointerException

    Thanks
    I just needed to new each element separately

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