How to get JButton label text by actionPerformed?

Question

 public Keypad() {
        setLayout(new GridLayout(4,3));
        for(int i=9; i>=0; i--){
            JButton tmp = new JButton(Integer.toString(i));
            add(tmp);
            tmp.addActionListener(this);
        }
        JButton btnPoint = new JButton(".");
        JButton btnEqual = new JButton("=");
        add(btnPoint);
        btnPoint.addActionListener(this);
        add(btnEqual);
        btnEqual.addActionListener(this);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        pack();
    }

    public void actionPerformed(ActionEvent event) {
        System.out.println(event.getSource());
    }

As a beginner of JFrame, I try to create some JButton with for loop. However, I have no idea how to handle the actionPerformed by corresponding button as they have the same variable name "tmp", so if(event.getSource() == tmp) may not suitable for this case.

In the actionPerformed, I try to print out the source by clicking different button, the result is: javax.swing.JButton[,0,0,75x29,alignmentX=0.0,alignmentY=0.5,border=com.apple.laf.AquaButtonBorder$Dynamic@4be40942,flags=288,maximumSize=,minimumSize=,preferredSize=,defaultIcon=,disabledIcon=,disabledSelectedIcon=,margin=javax.swing.plaf.InsetsUIResource[top=0,left=2,bottom=0,right=2],paintBorder=true,paintFocus=true,pressedIcon=,rolloverEnabled=false,rolloverIcon=,rolloverSelectedIcon=,selectedIcon=,text=9,defaultCapable=true]

It seem that it can get the button label text=9 correctly when I click button "9". so can I do something like if(event.getSource().getText() == "9") ?

Answer 1

how to handle the actionPerformed by corresponding button as they have the same variable name "tmp"

Actually the scope of tmp is the for loop in Keypad class constructor. Hence you can't refer to it in method actionPerformed .

You are assigning the same ActionListener to all your JButton s and you want a way to determine, in the actionPerformed method, which JButton was clicked. Since every JButton you create has a different text, your idea of getting the JButton text seems like one way to do this. So how can you get the text of the JButton that was clicked in your actionPerformed method?

A JButton has a ButtonModel which has an action command which is a string. By default, the text of the JButton is also its action command . The ActionEvent , ie the parameter of method actionPerformed , declares method getActionCommand which returns the action command of the JButton that was clicked. Hence the simplest way to get the text of the JButton , in your actionPerformed method, is to call method getActionCommand on the actionPerformed method parameter.

public void actionPerformed(java.awt.event.ActionEvent event) {
    String buttonText = event.getActionCommand();
    switch (buttonText) {
        case "1":
            // Handle button "1".
            break;
        // Remaining cases...
    }
}

This Web page may also be helpful:
How to Write an Action Listener

Alternatively, you can assign a separate ActionListener to each JButton . Since Java 8, this is easily done using method references because ActionListener is a functional interface.

public Keypad() {
    setLayout(new GridLayout(4,3));
    for(int i=9; i>=0; i--){
        JButton tmp = new JButton(Integer.toString(i));
        add(tmp);
        switch (i) {
            case 1:
                tmp.addActionListener(this::handleButton1);
                break;
            // Remaining cases...
        }
        tmp.addActionListener(this);
    }
}

private void handleButton1(java.awt.event.ActionEvent event) {
    // No need to check the "event" source since the source is always the same.
}

In order to be a valid method reference, method handleButton1 must return the same value that method actionPerformed returns – which is void and it must have the same parameters as method actionPerformed .