Java / SWT GUI Project on Calculator

import org.eclipse.swt.SWT;
import org.eclipse.swt.custom.ScrolledComposite;
import org.eclipse.swt.events.MouseAdapter;
import org.eclipse.swt.events.MouseEvent;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.layout.GridData;
import org.eclipse.swt.layout.GridLayout;
import org.eclipse.swt.widgets.Button;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Event;
import org.eclipse.swt.widgets.Group;
import org.eclipse.swt.widgets.Label;
import org.eclipse.swt.widgets.Listener;
import org.eclipse.swt.widgets.Shell;
import org.eclipse.swt.widgets.Text;

public class Main {

public class calculator {
	private Display display;
	private Shell shell;
	private Text txt1;
	private Text txt2;
	private Text txtOutput;
	private Button btnClear;
	private Button btnSubmit;
	private Label lbl1;
	private Label lbl2;
	private Label lbl3;

	private Group grpStart;
	private ScrolledComposite scrolledComposite;
	private Composite shellComposite;

	public calculator() {
		display = new Display();
		shell = new Shell(display);
		shell.setSize(700, 400);
		shell.setText("Calculator");
		shell.setLayout(new FillLayout());
		scrolledComposite = new ScrolledComposite(shell, SWT.H_SCROLL | SWT.V_SCROLL);
		scrolledComposite.setExpandVertical(true);
		scrolledComposite.setExpandHorizontal(true);
		shellComposite = new Composite(scrolledComposite, SWT.NONE);
		shellComposite.setLayout(new GridLayout(2, false));
		scrolledComposite.setContent(shellComposite);
		scrolledComposite.addListener(SWT.Resize, new Listener() {
			public void handleEvent(Event event) {
				scrolledComposite.setMinSize(shellComposite.computeSize(SWT.DEFAULT, SWT.DEFAULT));
			}
		});

		grpStart = new Group(shellComposite, SWT.NONE);
		grpStart.setText("Input Number in Text Box");
		grpStart.setLayout(new GridLayout(2, false));
		GridData gd_grpStart = new GridData(SWT.FILL, SWT.FILL, false, false, 1, 1);
		gd_grpStart.heightHint = 213;
		gd_grpStart.widthHint = 374;
		grpStart.setLayoutData(gd_grpStart);

		lbl1 = new Label(grpStart, SWT.NONE);
		lbl1.setText("Enter a : ");
		txt1 = new Text(grpStart, SWT.BORDER);
		txt1.setTextLimit(30);

		lbl2 = new Label(grpStart, SWT.NONE);
		lbl2.setText("Enter b : ");
		txt2 = new Text(grpStart, SWT.BORDER);
		txt2.setTextLimit(30);

		lbl3 = new Label(grpStart, SWT.NONE);
		lbl3.setText("Output : ");
		txtOutput = new Text(grpStart, SWT.READ_ONLY | SWT.BORDER);
		txtOutput.setTextLimit(30);

		btnSubmit = new Button(grpStart, SWT.NONE);
		btnSubmit.setToolTipText("Schedule");
		btnSubmit.setText("Submit");// 229e");

		btnClear = new Button(grpStart, SWT.NONE);
		btnClear.setToolTipText("Schedule");
		btnClear.setText("Clear");// 229e");

		btnSubmit.addMouseListener(new MouseAdapter() {
			@Override
			public void mouseDown(MouseEvent e) {
				System.out.println("button clicked");
				System.out.println(txt1.getText());
				System.out.println(txt2.getText());
				int sum = Integer.parseInt(txt1.getText()) + Integer.parseInt(txt2.getText());
				txtOutput.setText(Integer.toString(sum));

			}
		});

		btnClear.addMouseListener(new MouseAdapter() {
			@Override
			public void mouseDown(MouseEvent e) {

			txtOutput.setText("");
			txt1.setText("");
			txt2.setText("");

			}
		});

		btnClear.addMouseListener(new MouseAdapter() {
			@Override
			public void mouseDown(MouseEvent e) {
				System.out.println("button clicked");

			}
		});

		shell.open();
		while (!shell.isDisposed()) {
			if (!display.readAndDispatch()) {
				display.sleep();
			}
		}
		display.dispose();
	}
}

public Main() {
	new calculator();
}

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

}

Project Setup & Output