// Program Calculator.java
// 	A basic calculator program that allows a user to 
//      add, subtract, multiply, and divide integers ( dividing by 0
//      produces an error message ). Note: when wishing to start a new 
//      operation hit the clear button or you will end up doing
//      some operation on the contents of the current accumulator 
//      ( i.e. if you added 14 + 30 = 44, you have 44 in the textfield
//      ... if you now try to hit * and then 3 you get 264 instead of
//      132 because the accumulator adds 44 to 44 and then multiplies
//      by 3. You can only do one operation at a time. Do an operation
//      on two numbers, hit clear, and proceed again. )

import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;

public class Calculator extends Applet implements ActionListener {

	private String currentOperation, currentText;
	private int accumulator;
	private TextField display;
	private Button operator[], number[], clear;
	private GridBagLayout gbLayout;
	private GridBagConstraints gbConstraints;
	private String[] operatorNames = { "/", "*", "-", "=", "+" };
		
	public void init()
	{
		// Initialize accumulator as 0
		accumulator = 0;		

		// Use a combination of border layout
		// and grid layout to implement the 
		// calculator design

		// Start with a border layout 
		setLayout( new BorderLayout() );

		currentText = new String( "" );
		currentOperation = new String( "" );
		operator = new Button[5];
		number = new Button[10];
		display = new TextField();
		display.setEditable( false ); // user must use the mouse to use the calculator
		clear = new Button( "Clear" );
			
		// Make a panel that uses grid layout
		Panel calculatorPanel = new Panel();
		
		// Operator buttons on the panel /, *, -, +
		for ( int i = 0; i < 5; i++ ) {
			operator[i] =  new Button( operatorNames[i] );
			operator[i].addActionListener( this );
		}

		// Number buttons on the panel 0, 1, ..., 9
		for ( int k = 0; k <= 9; k++ ) {
			number[k] = new Button( (Integer.toString( k ) ) );
			number[k].addActionListener( this );
		}		

		// Panel with 4 rows and 4 columns
		calculatorPanel.setLayout( new GridLayout( 4, 4 ) );
		
		// Add the buttons to the panel
		// Buttons 7 through 9
		for ( int i = 7, x = 0; i <= 9; i++, x++ ) {
			calculatorPanel.add( number[i] );
		}

		// Divide operator
		calculatorPanel.add( operator[0] );

		// Buttons 4 through 6
 		for ( int i = 4, x = 0; i <= 6; i++, x++ ) {
			calculatorPanel.add( number[i] );
		}
 
		// Multiplication operator
		calculatorPanel.add( operator[1] );

		// Buttons 1 through 3
		for ( int i = 1, x = 0; i <= 3; i++, x++ ) {
			calculatorPanel.add( number[i] );
		}

		// Subtraction operator
			calculatorPanel.add( operator[2] );

		// Button 0
			calculatorPanel.add( number[0] );

		// Clear textfield and accumulator button
		clear.addActionListener( this );
		calculatorPanel.add( clear );
	
		// Equals operator
		calculatorPanel.add( operator[3] );
	
		// Addition operator
		calculatorPanel.add( operator[4] );
	
		// Complete the Layout
		add( "North", display ); // North position
		add( "Center", calculatorPanel ); // Center position
	}	
		
	

	public void actionPerformed( ActionEvent e )
	{
		// The '+' button is pressed
		if ( e.getSource() == operator[4] ) {
			accumulator += Integer.parseInt( display.getText() );
			currentOperation = "addition";
			currentText = "";
			display.setText( currentText );
		}
		
		// The '-' button is pressed
		if ( e.getSource() == operator[2] ) {
			accumulator += Integer.parseInt( display.getText() );
			currentOperation = "subtraction";
			currentText = "";
			display.setText( currentText );
		}		

		// The '*' button is pressed
		if ( e.getSource() == operator[1] ) {
			accumulator += Integer.parseInt( display.getText() );
			currentOperation = "multiplication";
			currentText = "";
			display.setText( currentText );
		}	

		// The '\' button is pressed
		if ( e.getSource() == operator[0] ) {
			accumulator += Integer.parseInt( display.getText() );
			currentOperation = "division";
			currentText = "";
			display.setText( currentText );
		}		

		// The '=' button is pressed
		if ( e.getSource() == operator[3] ) {
			if ( currentOperation == "addition" ) {
				accumulator += Integer.parseInt( display.getText() ); 
				currentText = Integer.toString( accumulator );
			}
			if ( currentOperation == "subtraction" ) {
				accumulator -= Integer.parseInt( display.getText() ); 
				currentText = Integer.toString( accumulator );
			}
			if ( currentOperation == "multiplication" ) {
				accumulator *= Integer.parseInt( display.getText() ); 
				currentText = Integer.toString( accumulator );
			}
			if ( currentOperation == "division" ) {
				if ( ( Integer.parseInt( display.getText() ) ) == 0 ) {
					currentText = "Can't divide by Zero. Hit Clear";
				}
				else {
					accumulator /= Integer.parseInt( display.getText() ); 
					currentText = Integer.toString( accumulator );
				}
			}
			display.setText( currentText );
		}
	
		// Handle the case where a 'number' is pressed
		for ( int i = 0; i <= 9; i++ ) {
			if ( e.getSource() == number[i] ) {
				currentText = currentText.concat( Integer.toString( i ) );
				display.setText( currentText );
			}
		}

		// The 'clear' button is pressed
		if ( e.getSource() == clear ) {
			currentText = "";
			accumulator = 0;
			display.setText( currentText );
		}
	}
}