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Thread: Copy Constructor

  1. #1
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    Default Copy Constructor

    I've written the following code to test a copy constructor to exit a program if there is no input but the program always runs. If there is no input in the brackets then it simply runs with 0.00.

    public class Imports
    {
    	private double num;
     
    	public Imports()
    	{
    		setNum(0);
    	}
     
    	public Imports(double initialNum)
    	{
    		setNum(initialNum);
    	}
     
     
    	public double setNum(double startNum)
    	{
    		num = startNum;
    		return num;
    	}
     
    	public Imports(Imports copy)
    	{
    		if (copy == null)
    		{
    			System.out.println("Fatal Error.");
    			System.exit(0);
    		}
     
    		num = copy.num;
    	}
     
        public String toString()
     	{
     		return (" Number is " + num);
     	}
     
    	public static void main(String[] args)
    	{
    		Imports acct1 = new Imports();
    		Imports copy = new Imports(acct1);
     
     
    		System.out.println(acct1);
    		System.out.println(copy);
     
    	}
    }

    Can someone please tell me why it isn't quitting with an error message when there is nothing in the brackets of acct1?
    Last edited by helloworld922; April 23rd, 2011 at 11:40 PM.


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: Copy Constructor

    There's a difference between passing a null parameter and passing nothing at all.

    // this passes nothing at all, not even a null
    Import acct1 = new Imports();
     
    // this passes a null value
    Import acct1 = new Imports(null);

    Notice how you have a constructor which takes nothing at all? That's what gets called when you pass no arguments to the constructor.

    public Imports() // called when you call Imports with no parameters
    {
        setNum(0);
    }

  3. The Following User Says Thank You to helloworld922 For This Useful Post:

    Rhyssa6 (April 24th, 2011)

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