Question

I have two classes,

1. Main

2. Sub

In main class, i have a button and jtextarea

In Sub class, i hava a button

when i click on the button in main class, the Sub class runs and shows a button. When i press on the button in the Sub class, the jtextarea should show the value, “Sample text”, but jtextarea is not showing any text.

Sub class code,

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
    Main main = new Main();
    main.jTextArea1.setText("Sample text");
}

Answer 1

You shouldn’t new Main, then you get a different object (that is probably not set to be visible), and nothing will be shown. What you need to do is create a local variable for your Main-object (the one that is showing), as well as a constructor in Sub, like this:

private Main main;
public Sub(final Main main) {
    this.main = main();
}

then, when you instantiate Sub from Main:

final Sub sub = new Sub(this);

Then your action in Sub can just say:

main.jTextArea1.setText("Sample text")

or even better:

main.getjTextArea1().setText("Sample text");

You should always keep your variables private, and use methods to manipulate them, getters and setters, or something else. You could for instance do it like this:

main.displayText("Sample text");

This way, Sub does not need to know anything about Mains text area, which is a good thing.