Method-Local Inner Class question

[Joe Lemmer]

Hi there,

In K&B 6, Ch.8 p.671 (Method-Local Inner Classes) it states that the Inner Class Object can't use the local variables of the method that it's in

Because the local variables aren't guaranteed to be alive as long as the method-local inner class object... ...Unless the local variables are marked final.

I don't understand why marking the local variable as final would make a difference. My understanding is that marking an object reference variable as final means that you can't assign another object to that reference variable, but it has no effect on the variable's scope.

I wrote this code to test:

interface MethodLocal {
    public void doStuff();
}

public class TestFinal {

	private MethodLocal inner;


    public static void main(String[] args) {
        TestFinal fin = new TestFinal();
		fin.go();
		fin.goAgain();
	}
	
	public void go() {
		final StringBuilder bob = new StringBuilder("Bob");
	    class MyInner implements MethodLocal {
			public void doStuff() {
			    bob.append(" the Builder");
				System.out.println(bob);
			}
		}
		MyInner in = new MyInner();
		in.doStuff();
		inner = in;
	}
	
	public void goAgain() {
	    inner.doStuff();
	}
}    

the output is:

Bob the Builder
Bob the Builder the Builder    

I can understand the first output as the StringBuilder object 'bob' is on the stack. But surely after the go() method has run, the stack frame is popped from the stack (if thats the right terminology) and no reference to 'bob' exists. So when the goAgain() method runs, I'm not sure how the doStuff method manages to access an object to which no reference exists. But it does, so there's something I'm not understanding here.

Anyone care to enlighten me?

Thanks in advance

Joe

[Ankit Garg]

This has been discussed many times before like here and here...

[Joe Lemmer]

Hi Ankit,

Thank you for those links. It makes sense now. Apologies for not doing a proper search.

[Ankit Garg]

Joe Lemmer wrote:Apologies for not doing a proper search.

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