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Thread: declaring field variables

  1. #1
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    Default declaring field variables

    Hi,
    I would like to declare a static final class instance, and set one of its field variables in the same line (all outside of a method). Ideally, it'd look something like this:

    public class MyClass {
          public static final MyClass myInstance = new MyClass() { field = value };
    // declaration of field & constructor for MyClass
    }

    but that doesn't work. Is this possible / does anyone have any ideas?


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: declaring field variables

    Put an extra pair of {}s around the assignment statement.
    If you don't understand my answer, don't ignore it, ask a question.

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  4. #3
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    Default Re: declaring field variables

    Alright, I was able to declare the instance, but I wasn't able to edit the fields unless I made the instance non-static. Any solution to that? I'd like to be able to call
    MyClass.myInstance

    in other programs.
    Thanks

  5. #4
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    Default Re: declaring field variables

    Please post the full text of the error message and the code that caused it.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: declaring field variables

    If I have this as the declaration:

    public final MyClass myInstance = new MyClass() {{ field = value }};

    I get this error:

    File: /.../MyOtherClass.java [line: 128]
    Error: non-static variable myInstance cannot be referenced from a static context

    At Line 128:

    function(MyClass.myInstance);

    Or, if I declare myInstance like this:
    public static final MyClass myInstance = new MyClass() {{ field = value }};

    I get this this error:

    File: /.../MyClass.java [line: 11]
    Error: non-static variable field cannot be referenced from a static context

    (where line 11 is the declaration of myInstance)

  7. #6
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    Default Re: declaring field variables

    Please post a small, complete program that I can compile and get these errors from.

    Two lines of code can't be compiled to get the errors.
    If you don't understand my answer, don't ignore it, ask a question.

  8. #7
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    Default Re: declaring field variables

    public class MyClass {
      private int field;
     
      public final MyClass myInstance = new MyClass()      {{ field = 3; }};
     
    }

    public class MyOtherProgram {
     
      public void function(MyClass mc) {
     
      }
     
      public void start() {
        function(MyClass.myInstance);
      }
     
     
      public static void main() {
        MyOtherProgram mop = new MyOtherProgram();
        mop.start();
      }
     
    }

    The start function is there so that we don't get any errors due to the fact that main is a static function, and also to better emulate the actual, complicated programs I'm trying to write.

  9. #8
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    Default Re: declaring field variables

    That code compiles with no errors.

    Where is the problem?
    If you don't understand my answer, don't ignore it, ask a question.

  10. #9
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    Default Re: declaring field variables

    I get this error:

    1 error found:
    File: /.../MyOtherProgram.java [line: 8]
    Error: non-static variable myInstance cannot be referenced from a static context

  11. #10
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    Default Re: declaring field variables

    Whoops, I used the old version of MyClass with a static final class instance.
    The following is the way to reference static variables in a class:
    MyClass.myInstance
    myInstance is not a static variable. You need a reference to an instance of the class.
    aRefToClass.myInstance

    Every instance of the MyClass class will have a myInstance variable.

    What are you trying to do?
    If you don't understand my answer, don't ignore it, ask a question.

  12. #11
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    Default Re: declaring field variables

    I would like it to be a static variable. If I do this:

    public class MyClass {
      private int field;
     
      public static final MyClass myInstance = new MyClass() {{ field = 3; }};
     
    }

    I get this error instead:

    1 error found:
    File: /.../MyClass.java [line: 4]
    Error: non-static variable field cannot be referenced from a static context

  13. #12
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    Default Re: declaring field variables

    If you remove the private for field, it compiles???
    If you don't understand my answer, don't ignore it, ask a question.

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