选择特定的 JComboBox 项目时如何向 JPanel 添加额外的 JButton

Question

I have been facing this small issue regarding making my JCombobox as dynamic as it can be.
For an example, when a selected item in combobox is selected, it will dynamically change the number of buttons accordance to the number of days within the month and be added in the panel

The problem i am facing is that it is not automatically changing the display of the panel , but when i tried to see if the code runs in my console log. it runs smoothly . ive tried my best to find a solution but to no avail.

The main problem is within the actionListener, for eg if February is selected , it will display 28 buttons, if January is selected, it will display 31 days etc etc but when i run the code, my system.out.println states it runs but my Gui shows no button .

private static JButton method_Btn(int i){
    JButton btn = new JButton(Integer.toString(i));
    return btn;
}

public static void day(){
    JFrame frame = new JFrame();
    JPanel topPanel = new JPanel();
    JPanel centerPanel = new JPanel();
    JButton days = new JButton();
    JLabel days_label = new JLabel();


    //-- Top Panel
    String month[] = {"--Select Month--" , "January", "February"};
    JComboBox month_combo = new JComboBox(month);
    topPanel.add(month_combo, BorderLayout.PAGE_START);

    //-- Center Panel
    centerPanel.setLayout(new FlowLayout());
    centerPanel.add(days_label);



    //------- Change when jcombo is selected
    month_combo.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent arg0) {
            if(month_combo.getSelectedItem().equals("January")){
                for(int i = 0;i < 31;i++){
                    centerPanel.add(method_Btn(i));
                }
            }

            if(month_combo.getSelectedItem().equals("February")){
                for(int i = 0;i < 28;i++){
                    centerPanel.add(method_Btn(i));
                }
            }
        }
    });

    frame.add(topPanel, BorderLayout.PAGE_START);
    frame.add(centerPanel , BorderLayout.CENTER);

    frame.setSize(400,400);
    frame.setVisible(true);
}

public static void main(String[] args){
    day();

}

Addtional Note, Ive come to realised an additional problem i faced is that it will stack up the number of button created after selecting the month 2nd time. How i solve it is i added centerPanel.removeAll(); and centerPanel.repaint();

month_combo.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent arg0) {
            int count = 0;
            //---- gettind days of month selected in comboBox


                if (month_combo.getSelectedItem().equals("February")) {
                    centerPanel.removeAll();
                    centerPanel.repaint();
                    for (int i = 1; i <= 28; i++) {
                        centerPanel.add(method_Btn(i));
                        System.out.println("days in feb " + i);
                    }
                    centerPanel.revalidate();
                }



            if (month_combo.getSelectedItem().equals("March")) {

                centerPanel.removeAll();
                centerPanel.repaint();
                for (int i = 1; i <= 31; i++) {
                    centerPanel.add(method_Btn(i));
                }
                centerPanel.revalidate();

            }
        }
    });

Hope this help anybody who is in need. :)

Answer 1

You need to revalidate() the component you have added like following:

centerPanel.revalidate();

You need to change the following code:

month_combo.addActionListener(new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent arg0) {
        if(month_combo.getSelectedItem().equals("January")){
            for(int i = 0;i < 31;i++){
                centerPanel.add(method_Btn(i));
            }
        }

        if(month_combo.getSelectedItem().equals("February")){
            for(int i = 0;i < 28;i++){
                centerPanel.add(method_Btn(i));
            }
        }

        centerPanel.revalidate(); // Need to add this for revalidation for the component
    }
});