Hello. There is an error in this program. Why???

Question

Hello. Trying to make a basic calculator GUI. Can anyone figure out the problem, as when i try to compile, it says:
compile-single:
run-single:
java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:621)
at java.awt.Container.add(Container.java:307)
at calculator1.calculator1.<init>(calculator1.java:31)
at calculator1.calculator1.main(calculator1.java:48)
Exception in thread "main"
Java Result: 1
BUILD SUCCESSFUL (total time: 7 seconds)
Here is the sourec code.
package calculator;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class calculator1 extends JFrame {
JLabel welcome;
JLabel value1;
JTextField value1text;
JLabel value2;
JTextField value2text;
JTextField result;
JButton addition;
JButton subtract;
JButton multiply;
JButton divide;
JLabel operation;
JLabel equals;
public calculator1() {
super("Calculator");
setSize(600, 400);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
FlowLayout flow = new FlowLayout(FlowLayout.CENTER);
getContentPane().setLayout(flow);
getContentPane().add(value1);
getContentPane().add(value1text);
getContentPane().add(operation);
getContentPane().add(value2);
getContentPane().add(value2text);
setVisible(true);
public static void main(String args[]) {
try {
UIManager.setLookAndFeel(
UIManager.getSystemLookAndFeelClassName());
} catch (Exception e) { }
new calculator1();
calculator1 window = new calculator1();
window.setTitle("Input Two Values Calculator");
window.pack();
window.show();
Please help.

Answer 1

compile-single:
run-single:
java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:621)
at java.awt.Container.add(Container.java:307)
at calculator1.calculator1.<init>(calculator1.java:31)
at calculator1.calculator1.main(calculator1.java:48)
Exception in thread "main"
Java Result: 1
BUILD SUCCESSFUL (total time: 7 seconds)
package calculator;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class calculator1 extends JFrame {
JLabel welcome;
JLabel value1;
JTextField value1text;
JLabel value2;
JTextField value2text;
JTextField result;
JButton addition;
JButton subtract;
JButton multiply;
JButton divide;
JLabel operation;
JLabel equals;
public calculator1() {
super("Calculator");
setSize(600, 400);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
FlowLayout flow = new FlowLayout(FlowLayout.CENTER);
getContentPane().setLayout(flow);
getContentPane().add(value1);
getContentPane().add(value1text);
getContentPane().add(operation);
getContentPane().add(value2);
getContentPane().add(value2text);
setVisible(true);
public static void main(String args[]) {
try {
UIManager.setLookAndFeel(
UIManager.getSystemLookAndFeelClassName());
} catch (Exception e) { }
new calculator1();
calculator1 window = new calculator1();
window.setTitle("Input Two Values Calculator");
window.pack();
window.show();
}