Changing contents of jLabel from another class

Question

I know this seems simple but I have read every other version of this question on stack and other sites and cannot figure it out. I even simplified the issue and made a brand new separate program with only the 6 lines of code yet it still isn't working, I'll explain...

I am in the middle of creating a game and it requires two jframes. When I click the button on FRAME1 I want the jlabel on FRAME2 to change. Originally I wanted the labels' contents to change to the text inputted into a textfield on FRAME1 but after I started to struggle I have simplified it even more to try and get to the bottom of the problem.

Please find attached my code for FRAME1 and FRAME2.

FRAME1:

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    FRAME2 set = new FRAME2();
    set.setval("f");
}

I have only included the code I have added, the rest was computer generated in the NetBeans IDE.

FRAME2:

public void setval(String v){
    val=v;
    System.out.println(val);
}

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt)   {                                         
    jLabel1.setText(val);         
}

UPDATE - full program

class 1

package crossjframe.image.change.test;


public class CrossjframeImageChangeTest {


public static void main(String[] args) throws InterruptedException {
   a a = new a ();
   a.setVisible(true);
     b b = new b ();
   b.setVisible(true);
}

}

Class 2

package crossjframe.image.change.test;


public class a extends javax.swing.JFrame {


public a() {
    initComponents();
}


// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

    jButton1 = new javax.swing.JButton();

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

    jButton1.setText("jButton1");
    jButton1.addActionListener(new java.awt.event.ActionListener() {
        public void actionPerformed(java.awt.event.ActionEvent evt) {
            jButton1ActionPerformed(evt);
        }
    });

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
            .addContainerGap(97, Short.MAX_VALUE)
            .addComponent(jButton1, javax.swing.GroupLayout.PREFERRED_SIZE, 171, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addGap(132, 132, 132))
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addGap(77, 77, 77)
            .addComponent(jButton1)
            .addContainerGap(198, Short.MAX_VALUE))
    );

    pack();
}// </editor-fold>                        

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
b set = new b();
set.setval("f");
}                                        


public static void main(String args[]) {

    //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
    /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
     * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
     */
    try {
        for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
            if ("Nimbus".equals(info.getName())) {
                javax.swing.UIManager.setLookAndFeel(info.getClassName());
                break;
            }
        }
    } catch (ClassNotFoundException ex) {
        java.util.logging.Logger.getLogger(a.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (InstantiationException ex) {
        java.util.logging.Logger.getLogger(a.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (IllegalAccessException ex) {
        java.util.logging.Logger.getLogger(a.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (javax.swing.UnsupportedLookAndFeelException ex) {
        java.util.logging.Logger.getLogger(a.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    }
    //</editor-fold>


    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new a().setVisible(true);
        }
    });
}

// Variables declaration - do not modify                     
private javax.swing.JButton jButton1;
// End of variables declaration                   
}

Class 3

package crossjframe.image.change.test;


public class b extends javax.swing.JFrame {
String val="k";

public b() {
    initComponents();
}

// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

    jLabel1 = new javax.swing.JLabel();

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

    jLabel1.setText("jLabel1");
    jLabel1.addMouseListener(new java.awt.event.MouseAdapter() {
        public void mouseClicked(java.awt.event.MouseEvent evt) {
            jLabel1MouseClicked(evt);
        }
    });

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addContainerGap()
            .addComponent(jLabel1, javax.swing.GroupLayout.DEFAULT_SIZE, 376, Short.MAX_VALUE)
            .addContainerGap())
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addContainerGap()
            .addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, 49, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addContainerGap(238, Short.MAX_VALUE))
    );

    pack();
}// </editor-fold>                        

private void jLabel1MouseClicked(java.awt.event.MouseEvent evt)   {                                     
    // TODO add your handling code here:
}                                    
public void setval(String v){
val=v;
System.out.println(val);
jLabel1.setText(val); 
}

public static void main(String args[]) {

    //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
    /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
     * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
     */
    try {
        for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
            if ("Nimbus".equals(info.getName())) {
                javax.swing.UIManager.setLookAndFeel(info.getClassName());
                break;
            }
        }
    } catch (ClassNotFoundException ex) {
        java.util.logging.Logger.getLogger(b.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (InstantiationException ex) {
        java.util.logging.Logger.getLogger(b.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (IllegalAccessException ex) {
        java.util.logging.Logger.getLogger(b.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (javax.swing.UnsupportedLookAndFeelException ex) {
        java.util.logging.Logger.getLogger(b.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    }
    //</editor-fold>


    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new b().setVisible(true);
        }
    });
    }

// Variables declaration - do not modify                     
private javax.swing.JLabel jLabel1;
// End of variables declaration                   
}

You need to invoke your `Frame2.setval()` *on the instance of `Frame2` that is being displayed*. You're instead creating a new, separate instance and invoking `setval()` on that, which will have no visible effect. It follows, then, that your `Frame1` instance must have a reference to the relevant `Frame2` instance in order to be able to invoke methods on it. — John Bollinger, Jan 17 '18 at 22:23
If you need more help then post a proper [mcve] that demonstrates the problem. — camickr, Jan 17 '18 at 22:23
You're suffering from "magical" thinking. Just because a String is used to set a JLabel's text, doesn't mean that changing that String's reference will magically update the JLabel. Your code must do this. This question gets to the core of the difference between a variable and an object. Learn this. — Hovercraft Full Of Eels, Jan 17 '18 at 22:24
See [The Use of Multiple JFrames, Good/Bad Practice?](http://stackoverflow.com/q/9554636/418556) — Andrew Thompson, Jan 17 '18 at 22:27

score 1 · Answer 1 · answered Jan 17 '18 at 22:23

So, in order to change the value of any object, you need a reference to said object.

While some would suggest passing a reference of the JLabel from the first window to the second, I'd avoid this for a number of reasons...

It couples the code. You now have a direct dependency between the two windows, making it impossible to modify the code without affecting one or the other classes
It exposes the label to uncontrolled modification on the part of the second window (as well as provides a means through which the second window can gain access to other information about the first window it shouldn't know about and perform operations it should be allowed it).

It might be suggested that you create a interface which provides the functionality that the second windows needs (set label value), to which the first window conforms to, then pass a reference of the first window to the second window.

This is not a bad idea, but we can still do better.

One of the primary principles most UI developers drive for is the concept of "model-view-controller", which is part of the "separation of responsibility" concept.

The idea been, that it's not actually the UI's responsibility to carry the state (ie the score), that responsibility is actually the "models". This allows you to pass a reference of the model to as many view's as you want and they will use it's information to display what ever state they need.

In your case, this means that the first window would update the state of the model (via a controller), the model would then generate a suitable event notification, which the second window would be registered to, and when triggered, would update the state of the UI accordingly.

This is commonly know as a "observer pattern".

Based on the available snippet of the code, you're creating a new instance of FRAME2 every time that the button is clicked, which is a bad idea, and passing the value to it, but not actually applying the value to the label, which would probably be mute anyway as the frame might not be visible - or you know have three windows on the screen all displaying something different

camickr · Answer 2 · 2018-01-18T02:34:28.703

When I click the button on FRAME1

All your ActionListener code does is change the text of the "val" variable.

Nowhere do you actual update the text of the label.

If you want to update the text of the label then you need to update the label itself:

public void setval(String v) 
{
    //val=v;
    //System.out.println(val);
    jlabel1.setText( v );
}

Edit:

First of all learn an use proper Java naming conventions. Class names should start with an upper case character. Neither of your classes "a" or "b" follow these conventions making your code incredibly hard to read. If you expect people to take the time to look at your code, then make your code readable!!!

In your main() method you create an instance of class "a" and you create an instance of class "b" and make the frame visible. But you never keep a reference to class "b", so you can never invoke a method on class "b" again.

In the ActionListener of class "a" you create a second instance of the "b" class and invoke the setVal(...) method but you never make the frame visible, so you never see the change.

So the main problem you have is the design of your classes. If you want class "a" to invoke a method of class "b" then you class "a" needs to have a reference to class "b". The easiest way to do this is to create class "b" in the constructor of class "a" and then keep an instance variable for class "b".

You can't keep creating a new instance of a class to invoke a method on the class. You can't create an instance of

Then in the ActionListener code you access this instance variable for class "b" instead of creating a new instance of class "b".

@Kyle, `i actually added the code to update the label into the action listener` that is not what I suggested. The solution is to update the label in the setVal(...) method. — camickr, Jan 17 '18 at 22:28
i have created the project in netbeans so there is a lot of code as it has generated a tonne — Kyle, Jan 17 '18 at 22:34

Changing contents of jLabel from another class

2 Answers2