Calculator GUI for java

import java.awt.*;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JPanel;
import javax.swing.JTextField;

/*Just add this to a JFrame for an instant Calculator GUI*/

public class GUIPanel extends JPanel {

    public GUIPanel(){

        addDisplay();

        addPanels();

    }

    private void addDisplay(){

        JTextField numberDisplay = new JTextField("0");

        numberDisplay.setHorizontalAlignment(JTextField.RIGHT);

        numberDisplay.setBorder(BorderFactory.createLoweredBevelBorder());

        numberDisplay.setPreferredSize(new Dimension(200,50));

        numberDisplay.setBackground(new Color(204,255,204));

        this.add(numberDisplay);

    }

    private void addPanels(){

        String [] inputButtons = {"MC","M+","M-", "MR","C","±","÷","×","1","2","3","-","4","5","6","+","7","8","9","0","="};

        JPanel topPanel = new JPanel();

        JPanel bottomPanel = new JPanel(new GridBagLayout());

        GridBagConstraints gbc = new GridBagConstraints();

        topPanel.setLayout(new GridLayout(4,4));

        for(String x:inputButtons){

            if(x.equals("0")){

                addWideButton(x,bottomPanel,gbc);

            }

            else if(x.equals("=")){

                addTallButton(x,bottomPanel,gbc);

            }

            else if(x.equals("7") || x.equals("8")|| x.equals("9")){

                gbc.gridwidth = 1;

                gbc.gridheight = 1;

                if(x.equals("7")){

                    gbc.gridx = 0;

                    gbc.gridy = 0;

                }

                else if(x.equals("8")){

                    gbc.gridx = 0;

                    gbc.gridy = 1;

                }

                else if(x.equals("9")){

                    gbc.gridx = 0;

                    gbc.gridy = 2;

                }

                addButton(x,bottomPanel,gbc);

            }

            else{

                addButton(x,topPanel,gbc);

            }

        }

        // this button would not go tot he right place for some reason!!

        gbc.gridx = 2;

        gbc.gridy = 1;

        JButton button = new JButton(".");

        button.setPreferredSize(new Dimension(55,30));

        bottomPanel.add(button, gbc);

        // end of the "." button fiasco!

        topPanel.setOpaque(false);

        bottomPanel.setOpaque(false);

        setPreferredSize(new Dimension(225,250));

        this.add(topPanel);

        this.add(bottomPanel);

    }

    private void addButton(String text, Container container, GridBagConstraints gbc){

        JButton button = new JButton(text);

        button.setPreferredSize(new Dimension(55,30));

        button.setOpaque(false);

        container.add(button);

    }

    private void addWideButton(String text, Container container, GridBagConstraints gbc){

        JButton button = new JButton(text);

        button.setOpaque(false);

        button.setPreferredSize(new Dimension(110,30));

        gbc.gridx = 0;

        gbc.gridy = 1;

        gbc.gridheight = 1;

        gbc.gridwidth = 2;

        container.add(button, gbc);

    }

    private void addTallButton(String text, Container container, GridBagConstraints gbc){

        JButton button = new JButton(text);

        button.setOpaque(false);

        button.setPreferredSize(new Dimension(55,60));

        gbc.gridx = 3;

        gbc.gridy = 0;

        gbc.gridheight = 2;

        gbc.gridwidth = 1;

        container.add(button, gbc);

    }

}