How to Pass Parameters to an Anonymous Class in Java

Is it possible to access external variables in an anonymous class in Java?

int myVariable = 1;

myButton.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        // Accessing myVariable in the anonymous class
        System.out.println(myVariable);
    }
});

In Java, you can access final or effectively final variables from the enclosing scope in an anonymous class. This allows anonymous classes to utilize external parameters, provided those variables meet certain criteria. Here’s a detailed explanation of how this works.

final int myVariable = 1;

myButton.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        // Accessing myVariable here
        System.out.println(myVariable);
    }
});

Causes

• Anonymous classes can reference variables from their enclosing scope if those variables are final or effectively final, meaning they are not modified after being initialized.
• This allows the anonymous class to use the state of external variables without explicitly passing them as parameters.

Solutions

• Declare the variable as final or ensure it is effectively final (i.e., it is not changed after initialization). This allows it to be accessed safely within the anonymous class.
• Use an instance variable of the enclosing class instead, if the variable needs to be mutable and accessed by the anonymous class.

Mistake: Trying to access a non-final variable within the anonymous class.

Solution: Make the variable final or ensure it is effectively final by not reassigning it.

Mistake: Assuming the anonymous class can modify the external variable.

Solution: Recognize that anonymous classes can only read the variables, not modify them if they are final.