Thread: 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?

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);
}

So here,

when this code executed
Imports acct1 = new Imports();

It invoked following constructor
public Imports()
{
setNum(0);
}

which set num value as 0.0..

now next when this line of code executed
Imports copy = new Imports(acct1);

As it takes already created object accnt1 (having num 0.00) then copy object will be same as acct1.

Hence both following code will print "Number is 0.0"
System.out.println(acct1);
System.out.println(copy);

Read more about copy Constructor with example program https://www.quizcure.com/java/what-i...n-java-program