How do I Change or Assign a Value to a Private Variable JTextField from another class?

Question

Sorry if this is another stupid-idiotic questions for you, but I'm still a newbie in Java Programming Language.

I have 3 classes: InputClass , PreviewClass , and MainClass .

MainClass contains the main method to run the program. InputClass contains a private JTextField for input and a JButton for setting a text into the JTextField in the PreviewClass . PreviewClass contains a private JTextField to show the inputted text in the InputClass .

How exactly can I do that (assigning value to JTextField in PreviewClass ) without creating an instance of the InputClass and then using getter-method-like to obtain the value it has, or, without making JTextField in the InputClass a static variable so I can access it with some static method ?

Just to show you my point, here is the code:

1. InputClass

     import javax.swing.*; import java.awt.*; import java.awt.event.*; public class InputClass extends JPanel implements ActionListener{ private JTextField inputName; private JButton inputButton; public InputClass() { setLayout(new FlowLayout()); inputName=new JTextField(15); inputButton=new JButton("INPUT"); inputButton.addActionListener(this); add(inputName); add(inputButton); } @Override public void actionPerformed(ActionEvent event) { // How do I change/assign a text to the PreviewClass from here? } } 

2. PreviewClass

     import javax.swing.*; import java.awt.*; public class PreviewClass extends JPanel{ private JTextField namePreview; public PreviewClass() { setLayout(new FlowLayout()); namePreview=new JTextField(15); namePreview.setEditable(false); add(namePreview); } } 

3. MainClass

    import javax.swing.*; import java.awt.*; public class MainClass extends JFrame{ private static final int FRAME_WIDTH=250; private static final int FRAME_HEIGHT=150; private static final int FRAME_X_ORIGIN=400; private static final int FRAME_Y_ORIGIN=300; private InputClass inputPanel; private PreviewClass previewPanel; private JTabbedPane tabbedPane; private Container contentPane; public MainClass() { contentPane=getContentPane(); contentPane.setLayout(new BorderLayout()); setTitle("How to Assign Value from Another Class"); setSize(FRAME_WIDTH, FRAME_HEIGHT); setLocation(FRAME_X_ORIGIN, FRAME_Y_ORIGIN); inputPanel=new InputClass(); previewPanel=new PreviewClass(); tabbedPane=new JTabbedPane(); tabbedPane.add("Input Name", inputPanel); tabbedPane.add("Preview Name", previewPanel); contentPane.add(tabbedPane, BorderLayout.CENTER); setDefaultCloseOperation(EXIT_ON_CLOSE); } public static void main(String[] args) { MainClass frame=new MainClass(); frame.setVisible(true); } } 

Answer 1

You have any number of possible solutions, all with pros and cons.

If I was facing an issue like this, I would be tempted to create a interface which would describe what could be changed and how and maybe even provide event notification when the internal state of the interface changes.

I would then create an instance of this interface and pass it to each of the classes. Those classes that need values from it, would read the values they need from it and, if available, attach some kind of change/property listener, so that they can monitor changes to the interface .

Those classes that need to make changes to the interface would do so as they need to.

As it's changed, the interface would fire updates letting any one who is listening know that changes have been made.

In this way, you decouple the classes from each and reduce the unnecessary exposure of other classes.

This is the "Model" in the MVC paradigm and describes the observer pattern

Answer 2

You are right to avoid making it static - that would be a bad idea.

The solution here is to pass a reference to your preview class into your input class when you create the input class.

The input class stores that reference and then can do preview.inputRecieved(str) when the text field changes

Create the inputRecieved method which can then update the JLabel and/or do whatever other processing is needed.

Note that this also means that you can change how your preview window displays and organizes itself without having to change the input window. That encapsulation is an important Object Oriented design principle.

Answer 3

Pass the reference of previewClass to the constructor of Inputclass to set the desired value.

InputClass

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class InputClass extends JPanel implements ActionListener{

    private JTextField inputName;
    private JButton inputButton;

    public InputClass(final PreviewClass perviewClassObj) {
        setLayout(new FlowLayout());

        inputName=new JTextField(15);
        inputButton=new JButton("INPUT");
        inputButton.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent arg0) {
                perviewClassObj.setNamePreview(inputName.getText());
            }
        });

        add(inputName);
        add(inputButton);
    }

    @Override
    public void actionPerformed(ActionEvent event) {
        // How do I change/assign a text to the PreviewClass from here?
    }

}

PreviewClass

import javax.swing.*;
import java.awt.*;

public class PreviewClass extends JPanel{
    private JTextField namePreview;

    /**
     * @param namePreview the namePreview to set
     */
    public void setNamePreview(String textContent) {
        this.namePreview.setText(textContent);
    }

    public PreviewClass() {
         setLayout(new FlowLayout());

         namePreview=new JTextField(15);
         namePreview.setEditable(false);

         add(namePreview);
    }

}

MainClass

import javax.swing.*;
import java.awt.*;

public class MainClass extends JFrame{
    private static final int FRAME_WIDTH=250;
    private static final int FRAME_HEIGHT=150;
    private static final int FRAME_X_ORIGIN=400;
    private static final int FRAME_Y_ORIGIN=300;

    private InputClass inputPanel;
    private PreviewClass previewPanel;

    private JTabbedPane tabbedPane;
    private Container contentPane;

    public MainClass() {
        contentPane=getContentPane();
        contentPane.setLayout(new BorderLayout());

        setTitle("How to Assign Value from Another Class");
        setSize(FRAME_WIDTH, FRAME_HEIGHT);
        setLocation(FRAME_X_ORIGIN, FRAME_Y_ORIGIN);

        previewPanel=new PreviewClass();
        inputPanel=new InputClass(previewPanel);

        tabbedPane=new JTabbedPane();
        tabbedPane.add("Input Name", inputPanel);
        tabbedPane.add("Preview Name", previewPanel);

        contentPane.add(tabbedPane, BorderLayout.CENTER);

        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {
        MainClass frame=new MainClass();
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

Answer 4

In InputClass, access the parent JFrame and get it's preview object and set the string value:

@Override
public void actionPerformed(ActionEvent event) {
    // How do I change/assign a text to the PreviewClass from here?
    ((MainClass)SwingUtilities.getWindowAncestor(this)).getPreviewPanel().setValue(inputName.getText());
}

In PreviewClass (expose the setter method for changing the text):

public void setValue(String s){
    namePreview.setText(s);
}

In MainClass (expose the getter method for accessing the Preview panel object):

public PreviewClass getPreviewPanel(){
    return previewPanel;
}

Answer 5

You can set InputClass an abstract class, override the actionPerformed method at instantiation and create a setter in your PreviewClass :

PreviewClass

public class PreviewClass extends JPanel{

    ...

    public void setNamePreview(String name) {
        inputName.setText(name);
    }
}

InputClass

public abstract class InputClass extends JPanel implements ActionListener{

    private JTextField inputName;
    private JButton inputButton;

    public InputClass() {
        setLayout(new FlowLayout());

        inputName=new JTextField(15);
        inputButton=new JButton("INPUT");
        inputButton.addActionListener(this);

        add(inputName);
        add(inputButton);
    }

    public String getInputNameValue(){
        return inputName.getText();
    }
}

MainClass

    ...

    previewPanel=new PreviewClass();
    inputPanel=new InputClass() {
        @Override
        public void actionPerformed(ActionEvent event) {
             previewPanel.setNamePreview(inputPanel.getInputNameValue());
        }
    };

    ...