jTabbedPane1StateChanged update the jLabel text from another class

[Jhovarie]

Hello im stock again in my project and i need help.

im trying to update the text of jLabel1 from NewJPanel1.java using jLabel1.setText using the jTabbedPane1StateChanged from NewJFrame.java see my code i hope its easy to understand the problem.

//Code from NewJFrame.java

 private void jTabbedPane1StateChanged(javax.swing.event.ChangeEvent evt) {                                          
int currentTab = jTabbedPane1.getSelectedIndex();
switch(currentTab){
    case 0:
        currtab = 0;
        new NewJPanel1("Tab1");
        break;
    case 1:
        currtab = 1;
        new NewJPanel1("Tab2");
        break;
    case 2:
        currtab = 2;
        new NewJPanel1("Tab3");
        break;
    case 3:
        currtab = 3;
        new NewJPanel1("Tab4");
        break;
    case 4:
        currtab = 4;
        new NewJPanel1("Tab5");
        break;
}
    }

//Code from NewJPanel1

public class NewJPanel1 extends javax.swing.JPanel {

    public NewJPanel1(String curTabText){ 
        System.out.println(curTabText); //<- this is working it prints the text
      jLabel1.setText(curTabText); //<- this one cause the errors
    }
       public NewJPanel1() {
        initComponents();
        
       }
    @SuppressWarnings("unchecked")

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at test.NewJPanel1.<init>(NewJPanel1.java:8)
at test.NewJFrame.jTabbedPane1StateChanged(NewJFrame. java:137)
at test.NewJFrame.access$100(NewJFrame.java:18)
at test.NewJFrame$2.stateChanged(NewJFrame.java:59)
at javax.swing.JTabbedPane.fireStateChanged(JTabbedPa ne.java:400)
at javax.swing.JTabbedPane$ModelListener.stateChanged (JTabbedPane.java:253)
at javax.swing.DefaultSingleSelectionModel.fireStateC hanged(DefaultSingleSelectionModel.java:116)
at javax.swing.DefaultSingleSelectionModel.setSelecte dIndex(DefaultSingleSelectionModel.java:50)
at javax.swing.JTabbedPane.setSelectedIndexImpl(JTabb edPane.java:599)
at javax.swing.JTabbedPane.setSelectedIndex(JTabbedPa ne.java:574)
at javax.swing.JTabbedPane.insertTab(JTabbedPane.java :710)
at javax.swing.JTabbedPane.addTab(JTabbedPane.java:77 7)
at test.NewJFrame.initComponents(NewJFrame.java:64)
at test.NewJFrame.<init>(NewJFrame.java:25)
at test.NewJFrame$3.run(NewJFrame.java:166)
at java.awt.event.InvocationEvent.dispatch(Invocation Event.java:209)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 597)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:122)

thanks..

[Fubarable]

//Code from NewJFrame.java

 private void jTabbedPane1StateChanged(javax.swing.event.ChangeEvent evt) {                                          
int currentTab = jTabbedPane1.getSelectedIndex();
switch(currentTab){
    case 0:
        currtab = 0;
        new NewJPanel1("Tab1");
        break;
    case 1:
        currtab = 1;
        new NewJPanel1("Tab2");
        break;
    case 2:
        currtab = 2;
        new NewJPanel1("Tab3");
        break;
    //.... etc...

Not sure what you're trying to do here as you don't seem to do anything with the NewJPanel1 that you create. Also, what's with all the switch/case statements? Wouldn't it be easier to simply write:

currtab = jTabbedPane1.getSelectedIndex();
new NewJPanel1("Tab" + (currtab + 1));

And regarding:

//Code from NewJPanel1

      jLabel1.setText(curTabText); //<- this one cause the errors

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at test.NewJPanel1.<init>(NewJPanel1.java:8)

This error is telling you that jLabel1 is obviously null at this location. So look in the rest of your code to figure out why.

[Jhovarie]

I add jLabel1 = new JLabel(); before the jLabel1.setText(curTabText); error message is gone but jLabel1 doest not update the text.

[camickr]

You can't just randomly create a new component.

Yes the NPE is gone but you didn't add the label to the GUI so you are setting the text of an undisplayed component.

Your ChangeListener code needs access to the label that you added to the GUI. Presumably you have code somewhere the create a label and a tabbed pane and added both of those components to the GUI. That is the label you need to reference.

[Jhovarie]

can any one help me from this code? i create new short sample but still not working see my codes.

import java.awt.BorderLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import java.awt.event.*;
import javax.swing.JLabel;
import javax.swing.JTabbedPane;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;
public class Frame1 extends JFrame{
    private JFrame m2;
    private JButton btn;
    private JTabbedPane tab;
    public Frame1(){
        setTitle("Frame1");
        setLayout(new java.awt.BorderLayout());
  
        tab = new JTabbedPane();
        tab.addTab("Tab1",new JLabel("Tab1"));
        tab.addTab("Tab2",new JLabel("Tab2"));
        tab.addTab("Tab3",new JLabel("Tab3"));
        add(tab,BorderLayout.CENTER);
        tab.addChangeListener(new ChangeListener(){
                public void stateChanged(ChangeEvent evt){
                    int currtab = tab.getSelectedIndex();
                    System.out.println("Frame1: "+currtab); //<- it works
                   new myPanel(currtab); 
                    //new myPanel(new JLabel(),currtab);
                }
        });
        
        m2 = new Frame2(); 
        m2.setBounds(300,20,400,300);
        
     btn = new JButton("Show Frame2");
     add(btn,BorderLayout.SOUTH);
      btn.addActionListener(new ActionListener(){
      public void actionPerformed(ActionEvent e){
       m2.setVisible(true);
      }
      }
     ); 
       
      setVisible(true);
      setBounds(250,10,500,400);
      setDefaultCloseOperation(EXIT_ON_CLOSE);
    }
    public static void main(String[]args){
        new Frame1();
    }
}

import java.awt.BorderLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Frame2 extends JFrame{
    private JPanel pan;
    public Frame2(){
        setAlwaysOnTop(true);
        setLayout(new java.awt.BorderLayout());
        pan = new myPanel();
        add(pan,BorderLayout.CENTER); 
    }
}

import java.awt.BorderLayout;
import java.awt.Color;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class myPanel extends JPanel{
    private JLabel label = new JLabel("Test jLabel changed");
  //private JLabel label;  //<- not working
    /*
     * this constructor still not working
    public myPanel(JLabel l,int i){
        label = l;
        System.out.println("myPanel: "+i);
        //label = new JLabel(""+i); //<- not working
        label.setText(String.valueOf(i)); //<- not working
    }
     */
   public myPanel(int i){
        System.out.println("myPanel: "+i);
        //label = new JLabel(""+i); //<- not working
        label.setText(String.valueOf(i)); //<- not working
    }
        
    public myPanel(){
       setBackground(Color.ORANGE);
       setLayout(new java.awt.BorderLayout());
       add(label,BorderLayout.NORTH);
    }
}

im trying to update the text of label using label.setText from myPanel.java using stateChanged of tabPane in Frame1.java

[Fubarable]

What is your goal with this code? Your problem is likely one of misplaced reference -- you can only change the state of an object if you have a reference to that object. You cannot change the state of a JLabel unless you have a reference to it, and your code above appears to lack appropriate references where needed.

I suggest that you put down GUI coding for a bit and go through a basic intro text on Java cover to cover to get a better handle on this and other core concepts. You won't regret doing this, trust me.

[Jhovarie]

What is your goal with this code?

my problem is to update the text of the label from myPane.java but now its ok was solve here with i done.

//code from my Frame.java

final myPanel pn = new myPanel();
        tab.addChangeListener(new ChangeListener(){
                public void stateChanged(ChangeEvent evt){
                    int currtab = tab.getSelectedIndex();
                    System.out.println("Frame1: "+currtab); //<- it works 
                    pn.label.setText(" Tab: "+currtab); //<- it is now working.
                }
        });

//code from myPanel.java

public static JLabel label = new JLabel("Test jLabel changed");