Beginner problem. Cannot access class instance methods outside main method?

NMard :

I'm working through the book "Head first Java" and started chapter 12. When I tried to create the method changeButtonText() I cannot access any of the class methods from button.

Why is this? What have I done wrong in this code?

import javax.swing.*;

public class Main {

    public static void main(String[] args) {

        JFrame frame = new JFrame();
        JButton button = new JButton("Click Me");

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.getContentPane().add(button);

        frame.setSize(300,300);
        frame.setVisible(true);

    }

    public void changeButtonText(){
        button.setText("I've been clicked");
    }
}

AntoineB :

The reason you cannot access the variable is because of its scope (tutorial about the scope of variables: https://www.baeldung.com/java-variable-scope).

You declare the variable JButton button in the main method, therefore it is not accessible anywhere outside of it, even in a method that main calls itself.

To make changeButtonText aware that the button variable exists, you have to pass it as parameters of this method:

import javax.swing.*;

public class Main {

    public static void main(String[] args) {
        JButton button = new JButton("Click Me");

        changeButtonText(button);
    }

    public static void changeButtonText(JButton button){
        button.setText("I've been clicked");
    }
}

I also added the static keyword in front of the changeButtonText method, because main is also a static method. Check this link for example to have more details about the difference: https://www.geeksforgeeks.org/static-methods-vs-instance-methods-java/

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