Inner class and local variables

Question

Why do I need to declare a local variable as final if my Inner class defined within the method needs to use it ?

Example :

class MyOuter2 {

private String x = "Outer2";

void doStuff() {
    final String y = "Hello World";

    final class MyInner {

        String z = y;

        public void seeOuter() {
            System.out.println("Outer x is "+x);
            System.out.println("Local variable is "+y);
            MyInner mi = new MyInner();
            mi.seeOuter();
        }
    }
}

}

Why the String y needs to be a final constant ? How does it impact ?

possible duplicate of Cannot refer to a non-final variable inside an inner class defined in a different method — user177800
– user177800, Commented Apr 26, 2012 at 17:53

mprabhat · Accepted Answer · 2012-04-26 17:55:15Z

17

Local variables always live on the stack, the moment method is over all local variables are gone.

But your inner class objects might be on heap even after the method is over (Say an instance variable holds on to the reference), so in that case it cannot access your local variables since they are gone, unless you mark them as final

answered Apr 26, 2012 at 17:55

mprabhat

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Comments

ControlAltDel · Accepted Answer · 2012-04-26 17:53:45Z

13

The answer is the two are in different scopes. So that variable could change before the inner class accesses it. Making it final prevents that.

answered Apr 26, 2012 at 17:53

ControlAltDel

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1 Comment

ohcibi Over a year ago

This has changed in java 8: docs.oracle.com/javase/tutorial/java/javaOO/…

charles_ma · Accepted Answer · 2013-07-10 20:48:00Z

9

It's because the inner class inside a function actually makes a copy of the local variable because local variable may be destroyed after the function call while the instance of the local class still exists. To make the two copies of the local variable always have the same value(to make them seem identical), you have to declare the variable as final.

answered Jul 10, 2013 at 20:48

charles_ma

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1 Comment

zubergu Over a year ago

There are tens if not hundreds of explanations why this rule was put into compiler. This is the only one that both makes sense and is easily understandable.

Martijn Courteaux · Accepted Answer · 2012-04-26 17:57:39Z

3

I think this is to prevent that the programmer would make mistakes. This way, the compiler reminds the programmer that the variable will not be accessible anymore when you leave that method. This is critical with looping. Imagine this code:

public void doStuff()
{
    InnerClass cls[] = new InnerClass[100];
    for (int i = 0; i < 100; ++i)
    {
        cls[i] = new InnerClass()
        {
            void doIt()
            {
                System.out.println(i);
            }
        };
    }
}

When the doIt() method is called, what will be printed? i changed while looping. To prevent that confusion, you have to copy the variable to a local variable or create the variable final, but then you will be unable to loop.

edited Apr 26, 2012 at 17:57

answered Apr 26, 2012 at 17:52

Martijn Courteaux

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Comments

abson · Accepted Answer · 2012-04-26 17:53:47Z

1

Your inner class is final and so you shouldn't be able to modify anything from inside your final entity.

answered Apr 26, 2012 at 17:53

abson

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Vishal · Accepted Answer · 2014-07-20 06:27:14Z

0

According to Java memory model use of final variable guarantees that the final variable are always initialized. We get error when we try to use local variable without initialization. using final guarantees the inner class that local variable which is been used is initialized.

answered Jul 20, 2014 at 6:27

Vishal

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