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java referencing question

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  java referencing question John <xsli2@yahoo.com> - 2015-01-20 06:17 -0800
    Re: java referencing question Jeff Higgins <jeff@invalid.invalid> - 2015-01-20 10:03 -0500
    Re: java referencing question Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-01-20 10:23 -0500

#30756 — java referencing question

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.

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#30757

On 01/20/2015 09:17 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.
>

4.3 Reference Types and Values
There are three kinds of reference types: class types (§8), interface 
types (§9), and array types (§10). Reference types may be parameterized 
(§4.5) with type arguments (§4.4).

4.3.1 Objects
An object is a class instance or an array.

The reference values (often just references) are pointers to these 
objects, and a special null reference, which refers to no object.

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#30758

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

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