Question

I have a label and a button in a class called FrameTest, when i press the button, a method named buttonpressed get’s executed from the class Test. In this buttonpressed method i will set a text to the label found in the FrameTest class.

The problem i have is that, the text for the label is not getting set. The reason is that i am creating a separate object to call the buttonpressed method;

public void actionPerformed(ActionEvent arg0) {
                    Test t = new Test();
                    t.buttonpress();
                }

and i am creating a separate object in the main method of the Test class to create the UI.

public static void main(String[] args) {

         FrameTest f = new FrameTest();
         f.mainScreen();

    }

The full code as follows;

public class FrameTest extends JFrame {

private JPanel contentPane;
private JLabel lblLabel;
private FrameTest ft = this;

//private FrameTest frame;
/**
 * Launch the application.
 */
public  void mainScreen() {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                //FrameTest frame = new FrameTest();
                //setVisible(true);

                FrameTest frame = ft;
                frame.setVisible(true);

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}


public void writeLabel(String k){
    this.lblLabel.setText(k);

}

public FrameTest() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setExtendedState(JFrame.MAXIMIZED_BOTH); 

    //setBounds(100, 100, 450, 300);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    contentPane.setLayout(new BorderLayout(0, 0));
    setContentPane(contentPane);

    lblLabel = new JLabel("LABEL");
    contentPane.add(lblLabel, BorderLayout.CENTER);

    JButton btnNewButton = new JButton("Press");
    btnNewButton.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            Test t = new Test();
            t.buttonpress();
        }
    });
    contentPane.add(btnNewButton, BorderLayout.WEST);
    //pack();
    setLocationByPlatform(true);
}

}

Test Class

public class Test {


public static void main(String[] args) {

     FrameTest f = new FrameTest();
     f.mainScreen();

}

public void buttonpress(){
     FrameTest f = new FrameTest();

     f.writeLabel("Button was pressed");

}

Answer 1

1) Dont extend JFrame class unnecessarily.

2) dont use setContentPane() unless thats what you want. Rather just simply JFrame#add(..).

3) Steer away from EventQueue and use SwingUtilities block rather

4) Dont forget to call JFrame#pack(); before setting JFrame visible.

5) Java naming convention is CamelCase so buttonPress() is correct not buttonpress()

Here is an example I made (basically your code fixed):

Test.java: (This is the main class which will create an instance of your FrameTest and has the method to change JLabel text)

import javax.swing.SwingUtilities;

public class Test {

    private static FrameTest f;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                f = new FrameTest();
                f.mainScreen();
            }
        });
    }

    void buttonPress() {
        f.writeLabel("Hello");
    }
}

FrameTest.java: (This class will show the JFrame and create a new instance of class Test to call buttonPress()):

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;

public class FrameTest {

    private JPanel panel;
    private JLabel lblLabel;
    private JFrame frame;

    private void initComponents() {
        frame = new JFrame("Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH);

        panel = new JPanel();
        panel.setBorder(new EmptyBorder(5, 5, 5, 5));
        panel.setLayout(new BorderLayout(0, 0));

        lblLabel = new JLabel("LABEL");
        panel.add(lblLabel, BorderLayout.CENTER);

        JButton btnNewButton = new JButton("Press");
        btnNewButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                Test t = new Test();
                t.buttonPress();
            }
        });

        panel.add(btnNewButton, BorderLayout.WEST);

        frame.add(panel);

        frame.setLocationByPlatform(true);
        frame.pack();
        frame.setVisible(true);
    }

    public void writeLabel(String k) {
        this.lblLabel.setText(k);
    }

    void mainScreen() {
        initComponents();
    }
}