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Thread: Exercise 98: how to write a main method with what I'm given

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    Cool Exercise 98: how to write a main method with what I'm given

    Here are the instructions:

     
    The program below includes the expanded version of the above definition of the APPoint class. In the space provided, complete the definition of the main method — replacing the two comments by appropriate code — so that two APPoint objects are created. The first should be assigned to the variable a and should represent the point with coordinate (25.36.8). The second should be assigned to the variable b and should represent the point with coordinate (–5,16.8).
     
      public class APPoint 
      { 
        private double myX; 
        private double myY; 
     
        public APPoint( double x, double y ) 
        { 
          myX = x; 
          myY = y; 
        } 
     
        public double getX()  
        { 
          return myX; 
        } 
     
        public double getY() 
        { 
          return myY; 
        } 
    public static void main( String[] args )
    {
      APPoint a = //...
      //...
     
      System.out.println( "a is (" + a.getX() + "," + a.getY() + ")" );
      System.out.println( "b is (" + b.getX() + "," + b.getY() + ")" );
    }
      }

    I have to replace the two commented lines with code with the definition of the "main" method. As always, I'm grateful for all help.

    Thanks, ghostheadx

    P.S. I just need a start. I could have the answer any time I wanted at the click of a button. Please remember that I have a "show solution" button, that I am purposely NOT clicking. That is because I want to be guided, as opposed to being fed the solution unless I take to long and fall behind. Nevertheless, all help appreciated.


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    Default Re: Exercise 98: how to write a main method with what I'm given

    Are you asking how to code the call to a class's constructor?
    See the tutorial: Passing Information to a Method or a Constructor (The Java™ Tutorials > Learning the Java Language > Classes and Objects)
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Exercise 98: how to write a main method with what I'm given

    I think yes, but I read the article you gave me and can't quite understand based on that what a constructor IS? Is it in that article? I don't see the term "constructor" in there, but thanks. Could you help explain what a constructor is and then I'll be glad to use that information? Thanks for helping.

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    Default Re: Exercise 98: how to write a main method with what I'm given

    If you move around in the tutorial a bit you can find other articles:
    Providing Constructors for Your Classes (The Java™ Tutorials > Learning the Java Language > Classes and Objects)

    A constructor is the "method" that is called when an instance of a class is created with a new statement.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Exercise 98: how to write a main method with what I'm given

    So it's the method that creates the instance of a class? It's sort of like a special method, JUST for that class?

    --- Update ---

    What does this line in the article mean

    " If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor."

    I get what a superclass is. But what is an IMPLICIT superclass? Lol, I feel so stupid for asking that question. I just think knowing that word might help my understanding a little bit.

    --- Update ---

    I made it work. Here is my code:

     
    public static void main( String[] args )
    {
      APPoint a = new APPoint (25.3, -6.8);
      APPoint b = new APPoint (-5, 16.8);
     
      System.out.println( "a is (" + a.getX() + "," + a.getY() + ")" );
      System.out.println( "b is (" + b.getX() + "," + b.getY() + ")" );
    }

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    Default Re: Exercise 98: how to write a main method with what I'm given

    It's sort of like a special method, JUST for that class?
    Yes.

    The Object class is the super class for all classes.

    I think the word implicit is used in its normal English language way.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Exercise 98: how to write a main method with what I'm given

    So, it's like a method JUST FOR creating ONE type of instance. It's the blueprints for a specific object, as opposed to one in the general category, i.e. the superclass.

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    Default Re: Exercise 98: how to write a main method with what I'm given

    A specific constructor receives args to define an instance of the class.
    A class can have many constructors.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Exercise 98: how to write a main method with what I'm given

    Alright, so a constructor automatically makes a specific type of instance of a class?

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    Default Re: Exercise 98: how to write a main method with what I'm given

    A constructor "constructs" an instance of a class.
    a constructor automatically makes a specific type of instance of a class?
    I'm not sure what "automatically" and "special type" mean. Leaving those out says this which is about correct:
    a constructor makes an instance of a class
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Exercise 98: how to write a main method with what I'm given

    Rephrasing and rearranging the words ghostheadx used, I will simply put: A constructor is automatically called when a new instance of its class is created.

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    Default Re: Exercise 98: how to write a main method with what I'm given

    //Creating Object To The Class APPoint using new() poerator which dynamically allocates space in heap
    APPoint a = new APPoint (25.3, -6.8);
    //when an object is created to the class then constructor declared in the class APPoint is invoked and perform its functionality i.e a constructor is automatically invoked when an object is created to the class.Also a constructor can be invoked even multiple objects created to the class and once for each object.
    APPoint b = new APPoint (-5, 16.8);

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