Java miniräknare med GUI

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

public class Calculator extends JFrame implements ActionListener
{
	
	private static final long serialVersionUID = 1L;
	//Instansvariabler
	
	private Container cont;
	private JTextField display;
	
	//main()-metod
	
	public static void main(String[] args)
	{
		Calculator calc = new Calculator();
		calc.setVisible(true);
	}
	
	//Konstruktor
	
	public Calculator()
	{
		initiateInstanceVariables();
		buildFrame();
	}
	
	//Metoder
	
	private void initiateInstanceVariables()
	{
		cont = getContentPane();
		display = new JTextField(20);
	}
	
	private void buildFrame()
	{
		setTitle("Mini Miniräknare");
		setSize(325, 100);
		setDefaultCloseOperation(EXIT_ON_CLOSE);
		cont.setLayout(new BorderLayout());
		buildOpButtonPanel();
		buildDispPanel();
	}
	
	private void buildOpButtonPanel()
	{
		JPanel opButtonPanel = new JPanel();
		JButton button;
		button = new JButton("C");
		button.addActionListener(this);
		opButtonPanel.add(button);
		button = new JButton("+");
		button.addActionListener(this);
		opButtonPanel.add(button);
		button = new JButton("/");
		button.addActionListener(this);
		opButtonPanel.add(button);
		button = new JButton("*");
		button.addActionListener(this);
		opButtonPanel.add(button);
		button = new JButton("-");
		button.addActionListener(this);
		opButtonPanel.add(button);
		button = new JButton("=");
		button.addActionListener(this);
		opButtonPanel.add(button);
		cont.add(opButtonPanel, BorderLayout.SOUTH);
	}
	
	private void buildDispPanel()
	{
		JPanel dispPanel = new JPanel(new GridLayout(4,3));
		dispPanel.add(display);
		cont.add(dispPanel, BorderLayout.NORTH);
	}
	
	public void actionPerformed(ActionEvent e)
	{
		String buttonText = e.getActionCommand();
		if(buttonText.equals("="))
		{
			calculate();
		}
		else
		{
			if(buttonText.equals("C"))
			{
				clearDisplay();
			}
			else
			{
				show(buttonText);
			}
		}
		
	}
	
	private void calculate()
	{
		String strExp = display.getText();
		int operatorPos = strExp.indexOf("+");
		if(operatorPos>-1)//addition
		{
			String firstTerm = strExp.substring(0,operatorPos);
			String secondTerm = strExp.substring(operatorPos+1, strExp.length());
			int nr1 = Integer.parseInt(firstTerm);
			int nr2 = Integer.parseInt(secondTerm);
			int sum = nr1 + nr2;
			display.setText(Integer.toString(sum));
		}
		
		else
		{	
			operatorPos = strExp.indexOf("-");
			String firstTerm = strExp.substring(0,operatorPos);
			String secondTerm = strExp.substring(operatorPos+1, strExp.length());
			int nr1 = Integer.parseInt(firstTerm);
			int nr2 = Integer.parseInt(secondTerm);
			int sum = nr1 - nr2;
			display.setText(Integer.toString(sum));
		}
		
		
	}
	
	private void clearDisplay()
	{
		display.setText("");
	}
	
	private void show(String bText)
	{
		display.setText(display.getText() + bText);
	}
}

if (operatorPos>-1) {}
else if(xxxx){}
else if(xxxx){}
else if(xxxx){}