问题:

This question already has an answer here:

Difference between final and effectively final 13 answers

The documentation on Anonymous Classes states

An anonymous class cannot access local variables in its enclosing scope that are not declared as final or effectively final.

I don't understand what does a variable being "effective final" mean. Can someone provide an example to help me understand what that means?

回答1:

Effectively final means that it is never changed after getting the initial value.

A simple example:

public void myMethod() {
    int a = 1;
    System.out.println("My effectively final variable has value: " + a);
}

Here, a is not declared final, but it is considered effectively final since it is never changed.

Starting with Java 8, this can be used in the following way:

public void myMethod() {
    int a = 1;
    Runnable r = new Runnable() {
        @Override
        public void run() {
            System.out.println("My effectively final variable has value: " + a);
        }
    };
}

In Java 7 and earlier versions, a had to be declared final to be able to be used in an local class like this, but from Java 8 it is enough that it is effectively final.