JButton是否自动更新？

Question

Say I have a JButton called b and I:

b.setText(""+someIntVariable)

And I add() it to the appropriate JFrame. If later my program changes the value of someIntVariable will the JButton's text automatically be updated in my GUI ? Or do I have to do something to update it ?

Answer 1

Once a button is added to the JFrame, it will show the original text that you gave it as a parameter. If you want to change the text, you will need to call b.setText(""+someIntVariable) again. However, you will not have to add it to the JFrame.

This is because you're referring to the value stored in someIntVariable , not to the variable itself. So if the value changes, it will not automatically update.

Answer 2

JButton's text will not automatically update. It gets a string representation that you created with the ""+someIntVariable . Even if you passed just the int variable itself (which isn't possible, but let's suppose it was), it would be a copy of the integer, not the original value. There's now way for it to get a pointer to the integer to see that the original has changed, and even if there was a way, the integer would have no way of notifying the JButton that it had changed.

There may be ways to create buttons like this. I don't think using a JButton is one of those ways, but there may be button classes in other frameworks that can handle something like this. But you'd need to use a more complicated data type as the variable that you passed in.

Answer 3

You can change the label of a button like this :

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;

public class demoframe extends JFrame implements ActionListener {

    String label=new String("Init Label");
    JButton b1=new JButton(label);
    JButton b2=new JButton("Action");
    demoframe()
    {
        this.add(b1);
        this.add(b2);
        b2.addActionListener(this);
    }
    public static void main(String arg[])
    {
        demoframe d=new demoframe();
        d.setSize(200, 200);
        d.setVisible(true);
        d.setLayout(new FlowLayout());
    }
    public void actionPerformed(ActionEvent e) 
    {
        label="New Label";
        b1.setText(label);
    }
}