Calculator GUI in java

calculator.java

	import javax.swing.*;
	import java.awt.event.*;
	import java.awt.GridLayout;
	import java.util.*;
	class CalculatorFrame extends JFrame implements ActionListener{
		JTextField screen;
		JPanel buttonPanel;
		String operation;
		Integer result;
		List<String> operations = Arrays.asList("+","-","*","/","%","C","=");
		CalculatorFrame(){
			screen = new JTextField();
			screen.setBounds(10, 10, 200, 30);
			add(screen);
			
			buttonPanel = new JPanel(new GridLayout(5,4));
			buttonPanel.setBounds(10,50,200,125);
			
			for (int i=0; i<=9; i++)     addButton(""+i);
			for (String op : operations) addButton(op);
			add(buttonPanel);
		}
		void addButton(String txt){
			JButton btn = new JButton(txt);
			btn.addActionListener(this);
			buttonPanel.add(btn);
		}
		void doAction(String op){
			if(op=="C"){
				screen.setText("");
				operation=null;
				result = 0;
				return;
			}
			String screenText = screen.getText();
			if(screenText.length()==0) return;
			Integer operand = Integer.parseInt(screenText);
			if(operation!=null){
				switch(operation){
					case "+": result += operand; break;
					case "-": result -= operand; break;
					case "*": result *= operand; break;
					case "/": 
						try{
							result/=operand;
						}catch(ArithmeticException e){
							screen.setText(e.getMessage());
							return;
						}
						break;
					case "%": 
						try{
							result%=operand;
						}catch(ArithmeticException e){
							screen.setText(e.getMessage());
							return;
						}
						break;
				}
			}else
				result=operand;	
			if(op=="="){
				screen.setText(result.toString());
				operation=null;
			}else{
				screen.setText("");
				operation=op;
			}
		}
		public void actionPerformed(ActionEvent e){
			String sourceText = ((JButton)e.getSource()).getText();
			if(operations.contains(sourceText))
				doAction(sourceText);
			else
				screen.setText(screen.getText()+sourceText);
		}
	}

	class calculator{
		public static void main(String[] args) {
			CalculatorFrame frame = new CalculatorFrame();
			frame.setTitle("Calculator");
			frame.setLayout(null);
			frame.setSize(235, 225);
			frame.setVisible(true);
			frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		}
	}