import java.awt.FlowLayout;
import java.awt.event.ItemEvent;
import javax.swing.JCheckBox;
import javax.swing.JFrame;

public class MainWindow extends JFrame {

    JCheckBox check1 = new JCheckBox();
    
    public MainWindow() {
        this.check1.setText("alma");
        this.check1.addItemListener((e)->{
            check1_on_itemchanged(e);
        });

        this.add(check1);
        this.setLayout(new FlowLayout());
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(300, 200);
        this.setVisible(true);
    }

    private void check1_on_itemchanged(ItemEvent e) {
        System.out.println("változott");
    }
}