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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: java referencing question
Date: Tue, 20 Jan 2015 10:23:01 -0500
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On 1/20/2015 9:17 AM, John wrote:
>
> I ran into a "strange" problem. If I understood the problem correctly, my summary is here:
>
> public class Team {
>      private Person teamLeader;
>
>      public Team(Person theLeader) {
>         teamLeader = theLeader;
>      }
>
>      public void doIt() {
>         teamLeader.speak();
>         ...
>      }
>
> }
>
>
> public class Demo {
>       public static void main(String[] args) {
>            Person leader = null;
>            Team dreamTeam = new Team(leader); //teamLeader inside dreamTeam is referencing an object which is also referenced by 'leader'. Right now, the object is null
>
>            leader = new Person(); //the object being referenced by leader is a good object now. So I thought teamLeader inside dreamTeam is referencing this good object too, since the two are referencing the same object
>            dreamTeam.doIt(); //crashes at teamLeader.speak() due to teamLeader is null -- I don't understand it. Please help.
>       }
> }
>
> The design is not good. I will change the design. Here I am puzzled by what I saw. Thank you.

     The thing you're missing is that `leader' is not a Person object,
but a reference that can point to Person objects.  At the time of
`new Team(leader)', the `leader' variable is null: It has a value
(null) that refers to no object.  The constructor copies this null
value to `teamLeader', so `teamLeader' also refers to no object.

     Later, you create a new Person and make `leader' refer to it.
But the fact that `leader' now points to a Person does not cause
`teamLeader' also to point to that Person: Changing the value of
the variable `leader' does not magically change the value of every
other variable that was initialized from `leader's former value.

     Let's try a simplified version:

	public class Value {
	    private int number;
	    public Value(int theNumber) {
	        number = theNumber;
	    }
	    public void doIt() {
	        System.out.println("The number is " + number);
	    }
	}

	public class TryIt {
	    public static void main(String[] unused) {
	        int howMuch = 10;
	        Value value = new Value(howMuch);
	        value.doIt();  // Prints "The number is 10"
	        howMuch = 42;
	        value.doIt();  // What does this print? Why?
	    }
	}

Giving a new value to `howMuch' after its old value has already
been copied to `value.number' does not also change `value.number'.
It's exactly the same with your `leader' and `dreamTeam.theLeader'.

-- 
esosman@comcast-dot-net.invalid
"Don't be afraid of work. Make work afraid of you." -- TLM