public void actionPerformed(ActionEvent arg0) {
			String command = arg0.getActionCommand();
			System.out.println(command);
			
			String current = screen.getText();
			current = current.replaceAll("\\s+", " ");
			
			if (current.contains("ERROR")) {
				current = "";
				screen.setText("");
			}
			
			if (command.equals("Calculate")){
				current += " =";
				try {
					screen.setText(calculate(current));
				} catch (TooManyValuesException e) {
					screen.setText("ERROR: TOO MANY VALUES");
					System.out.println(e.toString());
				} catch (NumberFormatException e) {
					screen.setText("ERROR: SYMBOL FORMAT");
					System.out.println(e.toString());
				} catch (EmptyStackException e) {
					screen.setText("ERROR: TOO MANY OPERATORS");
					System.out.println(e.toString());
				} catch (Exception e) {
					screen.setText("UNHANDLED ERROR");
					System.out.println(e.toString());
				}
			} else if (command.equals("C")) {
				// Clear screen
				screen.setText("");
			} else if (command.equals("_")) {
				screen.setText(current + " ");
			} else {
				screen.setText(current + command);
			}
			
			
		}