Question about local final variable in Java

2018-06-29 16:33:33

I have the following code:

public class BookLib {
    void f() {
        final int x = 5; // Line 1
        class MyCLass {
            void print() {
                System.out.println(x);
            }
        }
    }
}

I don't understand why should use final variable in this case (Line 1)?

You've created an inner class here. Since the life-time of objects of this class can potentially be much greater than the runtime of the method invocation (ie the object can still exist long after the method has returned), it needs to "preserve" the state of local variables that it can access.

This preserving is done by creating an (invisible, synthetic) copy inside the inner class and automatically replacing all references to the local variable with references to that copy. This could lead to strange effects when the local variable were modified after the inner class object was created.

To avoid this, there is a requirement that all local variables that you access this way are final : this ensures that there is only ever one possible value for the local variable and no inconsistencies are observed.

This specific rule can be found in §8.1.3 Inner Classes and Enclosing Instances of the JLS:

Any local variable, formal method parameter or exception handler parameter used but not declared in an inner class must be declared final. Any local variable, used but not declared in an inner class must be definitely assigned (§16) before the body of the inner class.

Here you create an inner class. In order to access the context of execution, the references to the variables should stay intact, otherwise an error behaviour is observed. In order to guarantee this, you should declare your variable final: thus one can't change it.

See detailed explanation here.

From kathy sierra scjp Book

The local variables of the method live on the stack, and exist only for the lifetime of the method. We already know that the scope of a local variable is limited to the method the variable is declared in. When the method ends, the stack frame is blown away and the variable is history. But even after the method completes, the inner class object created within it might still be alive on the heap if, for example, a reference to it was passed into some other code and then stored in an instance variable. Because the local variables aren't guaranteed to be alive as long as the method-local inner class object, the inner class object can't use them. Unless the local variables are marked final!

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