Thread: need help with my java program

I'm writing a program to calculate the final yield of a person investing a initial amount of money over a set amount of time with a set interest rate. I get the following error when compiling the code:

Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at calculator.<init>(calculator.java:62)
at calculator.main(calculator.java:92)

Here is the code for my java program:

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
 
public class calculator extends JFrame{
	private JButton calcButton, clearButton;
	private JLabel originalDeposit, interestRate, timeInvested;
	private JLabel resultDisplay;
	private JTextField originalField, interestField, timeField;
	private JPanel panel1, panel2, panel3,panel4,panel5;
	double deposit,interest,time,newBalance;
 
	public calculator(){
		super("App to calculate the yield of an invesment");
		setSize(400,400);
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setLayout(new GridLayout(6,1));
		originalDeposit = new JLabel("Enter amount of original deposit");
		originalField = new JTextField(7);
		interestRate = new JLabel("Enter the annual interest rate");
		interestField = new JTextField(5);
		timeInvested = new JLabel("Enter the years of invesment as a whole number");
		calcButton = new JButton("Calculate");
		clearButton = new JButton("Clear");
		resultDisplay = new JLabel("The new balance in the investment account will be" + newBalance);
 
		calcButton.addActionListener(new ActionListener(){
			public void actionPerformed(ActionEvent e){
 
			String caption = e.getActionCommand();
			if(e.getSource() instanceof JButton){
				Object String;
				if("Calculate".equals(caption))
					getNewBalance();
				resultDisplay.setText("The new balance in the investment account will be" + newBalance);
				}
			}
		});
 
		clearButton.addActionListener(new ActionListener(){
        	public void actionPerformed(ActionEvent e){
 
			String caption = e.getActionCommand();
			if(e.getSource() instanceof JButton){
				if("Clear".equals(caption))
					clearFields();
				}
        	}
        });
 
 
 
		panel1 = new JPanel();
		panel1.add(originalDeposit);
		panel1.add(originalField);
		panel2 = new JPanel();
		panel2.add(interestRate);
		panel2.add(interestField);
		panel3 = new JPanel();
		panel3.add(timeInvested);
		panel3.add(timeField);
		panel4 = new JPanel();
		panel4.add(calcButton);
		panel4.add(clearButton);
		panel5 = new JPanel();
		panel5.add(resultDisplay);
		add(panel1);
		add(panel2);
		add(panel3);
		add(panel4);
		add(panel5);
	}
 
	public double getNewBalance(){
		deposit = Double.parseDouble(originalField.getText().trim());
		interest = Double.parseDouble(interestField.getText().trim());
		time = Double.parseDouble(timeField.getText().trim());
 
		newBalance = deposit*(1+(interest/1));
 
		return newBalance;
	}
 
	public void clearFields(){
		originalField.setText(" ");
		interestField.setText(" ");
		timeField.setText(" ");
	}
 
	public static void main(String args[]){
		calculator cal = new calculator();
	}
}