Help with some code I can not understand

[Ioanna Katsanou]

Hello

One of the questions in Enthuware was what does the following code print:

public class Holder {

	int value = 1;
	Holder link;
	
	public Holder ( int val){
		this.value = val;
	}
	
	
	public static void main(String[] args) {
		final Holder a = new Holder(5);    //1
		Holder b = new Holder(10);     //2
		a.link = b;                            //3
		b.link = setIt(a, b);               //4
		System.out.println(a.link.value + " "+ b.link.value);    //5
	}
	
	
	public static Holder setIt(final Holder x, final Holder y){
		x.link = y.link;    //6
		return x;        //7
		
	}

}

The answer is that in line //5 it throws a null pointer exception.

I really cannot understand at all this question.
Could anyone help??

[Paul Anilprem]

This thread might be helpful:
http://enthuware.com/forum/viewtopic.php?f=2&t=1134

[Tim Cooke]

Get yourself a pencil and paper and go through it step by step keeping a note of the field values as you go. What is the default value for an Object field? For example: What will Holder link be initialised as when the object is constructed?

[Roel De Nijs]

Ioanna Katsanou wrote:I really cannot understand at all this question.
Could anyone help??

This topic contains an excellent explanation with lots of code snippets about exactly the same question. Definitely worth reading!

[Chris J Allen]

I'll admit that this one took me a bit to figure out as well. I did look at the other thread mentioned but the statement in the last post 'The method "setInt" does not even get executed' is incorrect. Here is the way I determined the answer:

On line 2, a new instance of Holder is created and assigned to the reference variable 'b'. Note that at this point, the value of b.link is null since nothing has been assigned to it yet
On line 4, the setIt method is invoked. Inside this method, the following statement is executed:

x.link = y.link;

This is where the value of a.link (referred to in this method as x.link) is set to null. It is because b.link (or y.link in this method) is null at this point. Although b refers to a valid object, the b.link property has not been assigned anything at this point
The return value of the setIt() method is the a reference to the holder object 'x' (which is pointing the same object as the reference variable 'a') which is then assigned to b.link (so now b.link has a value but a.link still does not)

On line 5, the statement tries to invoke a method on a null object (b.link) and throws a NullPointerException