Changing label text using ActionListener

Question

I'm trying to make a program that allows me to change the text of a label when clicking different buttons, however regardless of what button I click the text of the label changes to the last line of actionPerformed.

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class Gui extends JFrame{
private JButton add;
private JButton subtract;
private JButton multiply;
private JButton divide;
private JTextField box1;
private JTextField box2;
private JLabel label;
private JButton ansbutton;
public String operator;


public Gui(){
    super("Luke's Calculator");
    setLayout(new FlowLayout());
    add = new JButton("Add");
    subtract = new JButton("Subtract");
    multiply = new JButton("Multiply");
    divide = new JButton("Divide");
    add(add);
    add(subtract);
    add(multiply);
    add(divide);
    box1 = new JTextField(10);
    label = new JLabel();
    box2 = new JTextField(10);
    ansbutton = new JButton("Answer");
    add(box1);
    add(label);
    add(box2);
    add(ansbutton);

    HandlerClass handler = new HandlerClass();
    add.addActionListener(handler);
    subtract.addActionListener(handler);
    multiply.addActionListener(handler);
    divide.addActionListener(handler);
    ansbutton.addActionListener(handler);
}
public class HandlerClass implements ActionListener{
    public double sum;
    public double fn;
    public double sn;

    public void actionPerformed(ActionEvent e) {
        if(add.isSelected())
            operator = "+";
        label.setText("+");

        if(subtract.isSelected())
            operator = "-";
        label.setText("-");

        if(multiply.isSelected())
            operator = "x";
        label.setText("x");

        if(divide.isSelected())
            operator = "/";
        label.setText("/");

        if(ansbutton.isSelected())
            JOptionPane.showMessageDialog(null, "The answer is " + sum, "Answer", JOptionPane.PLAIN_MESSAGE);
        }       
    }
}

Use brackets to wrap each if clause and use "else if" after the first if — , May 11 '15 at 19:44

score 0 · Answer 1 · answered May 11 '15 at 19:42

0

In your code

if(add.isSelected())
        operator = "+";
    label.setText("+");

You have no curly brackets after the if, so only setting the operator is conditional, the label is ALWAYS changed. So the last label always win.

if(add.isSelected()) {
        operator = "+";
    label.setText("+");
}

Or

if(add.isSelected())
        operator = "+";
...
label.setText(operator)

answered May 11 '15 at 19:42

JP Moresmau

7,388
17
31

I'm really confused, I've done what you said and now instead of changing to the last one it does not change at all! Any ideas? – Luke Hebblethwaite May 11 '15 at 19:51

camelsaucerer · Accepted Answer · 2015-05-11T20:05:48.453

0

Your problem is that you are not including the label.setText() method inside your if statements. Because of this it will execute every label.setText() call and keep overriding the text until it defaults to the final call in your method. It is better to use braces, even for one line if statements, to avoid this problem.

 if(e.getSource() == add) {
    operator = "+";
    label.setText("+");
}

Also, since only one of the conditions is going to be true, you can use an else if.

edited May 11 '15 at 20:05

answered May 11 '15 at 19:43

camelsaucerer

301
3
12

I'm really confused, I've done what you said and now instead of changing to the last one it does not change at all! Any ideas? – Luke Hebblethwaite May 11 '15 at 19:51
@LukeHebblethwaite Try changing the condition in your `if` statements to this: `if(e.getSource() == add)` etc... – camelsaucerer May 11 '15 at 19:56
Yes that worked, thank you! Do you have any idea why my way didn't work? I'm interested as to why it would not work – Luke Hebblethwaite May 11 '15 at 20:01
@LukeHebblethwaite `isSelected()` only really works for toggle buttons. Using `e.getSource()` checks which object specifically triggered the `ActionEvent` and is therefore what you want to use for normal buttons. – camelsaucerer May 11 '15 at 20:03
See http://stackoverflow.com/questions/12337659/jbutton-isselected-method-doesnt-work – JP Moresmau May 11 '15 at 20:03

Changing label text using ActionListener

2 Answers2