Variable should be final inside listener [duplicate]

Answer 1

This is because the inner class executes code only when an event is triggered. If the variable is not declared final, then the MyClass object referenced in variable can change, and the inner class will not know which Object it is supposed to be referencing if it needs the MyClass object.

Thus, it should be declared final so this reference will never change.

Imagine this without the final keyword:

variable is referencing MyClass Object with hashCode(): 12345 inner class is created, and variable in inner class is referencing MyClass Object with hashCode(): 12345

variable is changed to be now referencing MyClass Object with hashCode(): abcde variable in inner class is still referencing MyClass Object with hashCode(): 12345

How does Java run code from here when event is triggered? Use which MyClass object?

Now with the final keyword:

variable is referencing MyClass Object with hashCode(): 12345 inner class is created, and variable in inner class is referencing MyClass Object with hashCode(): 12345

Due to the final keyword, the reference cannot be changed.

Java always know which MyClass object to call when event is triggered. No problems.

Answer 2

It is because you are using anonymous inner class. What happens is that compiler creates class for you. It calls it as your outer class and adds $ and number, e.g. $, $2 etc.

The class has reference to outer class initialized automatically, so its instance can use methods and fields of outer class.

But your class is anonymous. It kind of defined inside method and can use its internal variables defined before this anonymous class. The question is "how can it do it?" Really, you cannot refer to "instance of running method" to access its variables. The answer is that all method variables referenced from anonymous inner class are copied to the anonymous inner class . Therefore the variables are required to be final: otherwise somebody can change their values from the outer class and the changes will not be visible into the inner class.