How do I make my calculator perform calculation

Question

I am new in programming, can someone help in making this calculator work? This code inputs something on the screen if you click the button, but my challenge is making add, subtract, divide and multiply buttons doing their functions. Am still new in programming, please help.

        /*
         * Simple Calculator Program
         * Author : RM
         * Date : 10/7/2016
         * Version : 1.0 
         */
        import java.awt.*;
        import java.awt.event.*;

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

        public class SimpleCalculator2 extends JFrame implements ActionListener{

            private JPanel contentPanel;
            private JPanel buttonPanel;
            private JTextField display;
            private JButton button;
            private String buttonOrder;
            private ActionListener buttonEvent;
            private Font font = new Font ("Ariel", Font.BOLD,52);
        public SimpleCalculator2 (){
                buttonEvent =new EventHandler();
                buttonPanel = new JPanel();
            String buttonOrder ="789+456-123*C0=/";

                    display = new  JTextField();
                    display.setFont(font);
                buttonPanel.setLayout(new GridLayout(4,4));
                for(int i=0; i< buttonOrder.length(); i++)

                {
                    String caption =  buttonOrder.substring (i,i+1);
                            button= new JButton(caption);
                            buttonPanel.add(button);
                            button.setFont(font);
                    button.addActionListener(buttonEvent);
                }

                contentPanel = new JPanel();
                contentPanel.setLayout(new BorderLayout());
                contentPanel.add(display, BorderLayout.NORTH);
                contentPanel.add(buttonPanel, BorderLayout.SOUTH);
                this.setContentPane(contentPanel);
                this.pack();
                this.setTitle("Simple Calculator2");
                 this.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
                this.setVisible(true);

        }



        public static void main (String args[]){
                new SimpleCalculator2();
            }

            @Override
            public void actionPerformed(ActionEvent e) {
                throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
            }
            public class EventHandler implements ActionListener{
        public void actionPerformed(ActionEvent e){
            display.setText(display.getText() +e.getActionCommand());


        }
            }
            }