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Thread: Beginner Problem

  1. #1
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    Default Beginner Problem

    Hi, I'm a little confused with an assignment. I've try to figure it out for a while but I'm stuck.

    The instructions are as follows:

    Make 2 classes WrapperShallow and WrapperDeep.

    Each class is simply a wrapper class to hold a private array variable. int [] a;
    The default constructor for each class should initialize a.
    Each class should have a toString() and equals().
    Each class should have a setArray method that allows you to set the a variable.

    WrapperShallow should have an invalid copy constructor.

    public WrapperShallow(WrapperShallow ws){
    a = ws.a;
    }

    WrapperDeep should have a properly functioning copy constructor.

    public WrapperDeep(WrapperDeep ws){
    a = new int[3];
    for(int i = 0; i < 3; i++)
    a[i]=ws.a[i];
    }

    Think about why shallow is wrong and deep is correct! What happens to the old a in the WrapperDeep copy constructor? (think garbage collection)

    Example Output:

    --------------------Configuration: <Default>--------------------

    **** TESTING SHALLOW OBJECTS ****

    inital shallow object contains
    7 17 77
    copy shallow object contains
    7 17 77
    inital shallow object changed to
    13 14 15
    copy shallow object not changed contains
    13 14 15
    WOOPS! ws.equals(ws2) is true

    **** TESTING DEEP OBJECTS ****

    inital deep object contains
    2 3 4
    copy deep object contains
    2 3 4
    inital deep object changed to
    7 6 -5
    copy deep object not changed contains
    2 3 4
    RIGHT! wd.equals(wd2) is false

    Process completed.


    So far I have this:
    class WrapperShallow 
    {
    	private int[] a = new int[2];
     
     
     
    	public void setArray()
    	{
    		a[0] = 7;
    		a[1] = 17;
    		a[2] = 77;
    		for(int i = 0; i < 3; i++)
    		{
            	System.out.println("Inital shallow object contain the numbers:\n");
            	System.out.print(a[i] + " ");
            }
     
    	}
     
    	public WrapperShallow (WrapperShallow ws)
    	{
    		a = ws.a;	
    	}
     
    	public String toString()
    	{
    		return Integer.toString(a[0]); 	
    	}
     
    	public boolean equal(WrapperShallow ws2)
    	{
    		return a.equals(ws2);
    	}
     
     
    }
     
     
     
    class WrapperDeep
    {
     
    	private int[] a = new int[2];
     
    	public void setArray()
    	{
    		a[0] = 7;
            a[1] = 17;
            a[2] = 77;
     
    		for(int i = 0; i < 3; i++)
    		{
    			System.out.println("Inital shallow object contain the numbers:\n");
    			System.out.print(a[i] + " ");
    		}
    	}
    	public WrapperDeep(WrapperDeep wd)
    	{
    		a = new int[3];
     
    		for (int i = 0; i < 3; i++)
    			a[i] = wd.a[i];
    	}
     
    	public String toString()
     
    	{
    		return Integer.toString(a[0]); 
    	}
     
    	public boolean equal(WrapperDeep wd2)
    	{
    		return a.equals(wd2);
    	}
     
     
    }
     
     
     
     
     
    public class WrapperDemo {
     
     
    	public static void main(String[] args) 
    	{
    		WrapperShallow ws = new WrapperShallow();		
    		ws.setArray();
     
     
    	}
     
     
    }

    Just ignore the toString and Equals. I'll change those later.
    The error I'm trying to understand is in this line "WrapperShallow ws = new WrapperShallow();"
    I think I'm supposed to pass an array in the parentheses however I get an error every time I try to. I know I'm supposed to send something because it is supposed to receive something. Am I approaching this problem in the correct way? Any help would be greatly appreciated.

    Thank you!


  2. #2
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    Default Re: Beginner Problem

    Ok, in the WrapperShallow class, you have declared the following to be the constructor:
    public WrapperShallow (WrapperShallow ws)
        {
            a = ws.a;  
        }

    Since that is the only constructor that has been declared, that is the only one that you can use.

    So when you make the call:
    WrapperShallow ws = new WrapperShallow();

    That is not allowed because there is no constructor that looks like this:
    public WrapperShallow ()
    {
            ... 
    }

    Realistically, I can't see the above code really doing anything with the specific constructor that is now there. Since you need to send each WrapperShallow a WrapperShallow object in its constructor and there are no alternate constructors. Essentially, you would have to either endlessely create WrapperShallow objects for each WrapperShallow object, or you would have to send it null, which would defeat the entire purpose of the constructor.
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  3. #3
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    Default Re: Beginner Problem

    thanks for the reply.

    Yeah that's what I am so confused about.

    The instructions say to include this:
    public WrapperShallow (WrapperShallow ws)
        {
            a = ws.a;  
        }

    But like you said it's expecting a WrapperShallow to be sent to it. Do you think the instructor made a mistake? I'm not really sure what to do at this point. I might just send him an email and see if he can help me.

    Thanks again.

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