Need help with starting a simple calculator

Question

I have this simple calculator I just started on, and I'm having just a few problems. First of all, I don't know how to append text to a textfield. Maybe I would have to create a string buffere or something. Then, when the user clicks one of the buttons, the compiler goes crazy, and nothing is displayed.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Calculator extends JFrame implements ActionListener
     JButton[] btnNums;
     JButton btnBack;
     JButton btnClear;
     JButton btnCalculate;
     String[] strNames;
     JTextField txtDisplay;
     public Calculator()
          Container content = getContentPane();
          setSize(210,250);
          setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          /*  construct a JPanel and the textfield that
           *  will show the inputed values and outputed
           *  results
          JPanel jpDisplay = new JPanel();
          jpDisplay.setLayout(new BorderLayout());
          txtDisplay = new JTextField(15);
          txtDisplay.setHorizontalAlignment(JTextField.RIGHT);
          jpDisplay.add(txtDisplay);
          /*  contstruct a JPanel that will contain a back
           *  button and a clear button.
          JPanel jpRow2 = new JPanel();
          btnBack = new JButton("Backspace");
          btnClear = new JButton("Clear");
          btnClear.addActionListener(this);
          jpRow2.add(btnBack);
          jpRow2.add(btnClear);
          /*  construct a string array with all the names of the
           *  buttons, then in a for loop create the new buttons
           *  and add their name and an actionListener to them.
          String[] strNames = {"7","8", "9","/", "4", "5", "6","*", "1", "2", "3","+", "0", "+/-", ".", "-"};
          btnNums = new JButton[16];
          JPanel jpButtons = new JPanel();
          jpButtons.setLayout(new GridLayout(4,4));
            for (int i = 0; i < 16; i++)
                 btnNums[i] = new JButton(strNames);
               btnNums[i].addActionListener(this);
               jpButtons.add(btnNums[i]);
          /* construct the final JPanel and add a
          * calculate button to it.
          JPanel jpLastRow = new JPanel();
          btnCalculate = new JButton("Calculate");
          btnCalculate.addActionListener(this);
          jpLastRow.add(btnCalculate);
          /* set the contentPane and create the layout
          * add the panels to the container
          setContentPane(content);
          setLayout(new FlowLayout());
          setResizable(false);
          content.add(jpDisplay, BorderLayout.NORTH);
          content.add(jpRow2);
          content.add(jpButtons);
          content.add(jpLastRow);
          setTitle("Mini Calculator");
          setVisible(true);
     public void actionPerformed(ActionEvent ae)
          for (int i =0; i < 16; i++)
               if (ae.getSource() == btnNums[i])
                    txtDisplay.setText(strNames[i]);
     public static void main(String[] args)
          Calculator calc = new Calculator();

Answer 1

First of all, I don't
know how to append text to a textfield.
textField.setText( textField.getText() + TEXT_YOU_WANT_TO_APPEND );
Then,
when the user clicks one of the buttons, the compiler
goes crazy, and nothing is displayed.No, the compiler doesn't go crazy, the compiler has done it's job by then. However, you do get a NullPointerException, and the stacktrace tells you that the problem is on the following line:
txtDisplay.setText(strNames);
It turns out that strNames is null. I leave it to you to find out why. ;-)