I find it difficult in this java programming language that's why I need your help to defend this code in my class.
I got this code from internet with comments and explanation. but there are some codes and explanation that I don't understand.
/* Read me:
Note: As I have said this program has already comments.
If you see this "==" sign in a comment like this "//==",
it means that is my question.
*------------------------------------------------------------------------------/
/*
This module contains the recursive descent
parser that does not use variables.
*/
// Exception class for parser errors.
class ParserException extends Exception {
String errStr; // describes the error
public ParserException(String str) {
errStr = str;
}
public String toString() {
return errStr;
}
}
class Parser { //==what is the concept of parser?
// These are the token types.
final int NONE = 0; //== why NONE is initially zero
final int DELIMITER = 1; //== why DELIMITER is initially to 1
final int VARIABLE = 2; //== what is purpose of these token type
final int NUMBER = 3;
// These are the types of syntax errors.
final int SYNTAX = 0;
final int UNBALPARENS = 1;
final int NOEXP = 2;
final int DIVBYZERO = 3;
// This token indicates end-of-expression.
final String EOE = "\0"; //== purpose of this and why "\0"?
//== Can you explain what is end-of-expression.
//== What is exp, expIndx, token and toktype and its purpose
private String exp; // refers to expression string
private int expIdx; // current index into the expression
private String token; // holds current token
private int tokType; // holds token's type
// Parser entry point.
public double evaluate(String expstr) throws ParserException
{
double result;
exp = expstr;
expIdx = 0;
getToken();
if(token.equals(EOE))
handleErr(NOEXP); // no expression present
// Parse and evaluate the expression.
result = evalExp2(); //== What is parse? and why result is equals to evalExp2
if(!token.equals(EOE)) // last token must be EOE //== Meaning of this
handleErr(SYNTAX); //== Why we throw to SYNTAX
return result; //== Why we return to result
}
// Add or subtract two terms.
private double evalExp2() throws ParserException
{
char op;
double result; //== why we have so many result declaration
double partialResult; //== purpose of this, Why we have this?
result = evalExp3(); //== purpose of evalExp3
//== Meaning of chartAt(0)
while((op = token.charAt(0)) == '+' || op == '-') {
getToken();
partialResult = evalExp3(); //==why do we need to declare partialResult is equals to evalExp3?
switch(op) { //== meaning of switch adn why we switch op?
case '-':
result = result - partialResult;
break;
case '+':
result = result + partialResult;
break;
}
}
return result;
}
// Multiply or divide two factors.
private double evalExp3() throws ParserException
{
char op;
double result;
double partialResult;
result = evalExp4();
while((op = token.charAt(0)) == '*' ||
op == '/' || op == '%') {
getToken();
partialResult = evalExp4();
switch(op) {
case '*':
result = result * partialResult;
break;
case '/':
if(partialResult == 0.0)
handleErr(DIVBYZERO);
result = result / partialResult;
break;
case '%':
if(partialResult == 0.0)
handleErr(DIVBYZERO);
result = result % partialResult;
break;
}
}
return result;
}
// Process an exponent.
private double evalExp4() throws ParserException
{
double result;
double partialResult;
double ex;
int t;
result = evalExp5();
if(token.equals("^")) { //== Pls elaborate this exponent
getToken();
partialResult = evalExp4();
ex = result; //== What we have ex
if(partialResult == 0.0) {
result = 1.0;
} else
for(t=(int)partialResult-1; t > 0; t--) //== especially this one
//== why t-1 and t--
result = result * ex;
}
return result;
}
// Evaluate a unary + or -.
private double evalExp5() throws ParserException
{
double result;
String op;
op = "";
if((tokType == DELIMITER) &&
token.equals("+") || token.equals("-")) {
op = token;
getToken();
}
result = evalExp6();
if(op.equals("-")) result = -result;
return result;
}
// Process a parenthesized expression.
private double evalExp6() throws ParserException
{
double result;
if(token.equals("(")) {
getToken();
result = evalExp2();
if(!token.equals(")"))
handleErr(UNBALPARENS);
getToken();
}
else result = atom();
return result;
}
// Get the value of a number.
private double atom() throws ParserException
{
double result;
switch(tokType) {
case NUMBER:
try { // What is try command
result = Double.parseDouble(token);
} catch (NumberFormatException exc) { // What happen in this line
handleErr(SYNTAX);
}
getToken();
break;
default:
handleErr(SYNTAX);
break;
}
return result;
}
// Handle an error.
private void handleErr(int error) throws ParserException
{
String[] err = {"Syntax Error","Unbalanced Parenthesesa","No Expression Present","Division by Zero"
};
throw new ParserException(err[error]);
}
// Obtain the next token.
private void getToken()
{
tokType = NONE;
token = "";
// Check for end of expression.
if(expIdx == exp.length()) {
token = EOE;
return;
}
// Skip over white space.
while(expIdx < exp.length() &&
Character.isWhitespace(exp.charAt(expIdx))) ++expIdx;
// Trailing whitespace ends expression.
if(expIdx == exp.length()) {
token = EOE;
return;
}
if(isDelim(exp.charAt(expIdx))) { // is operator
token += exp.charAt(expIdx);
expIdx++;
tokType = DELIMITER;
}
else if(Character.isLetter(exp.charAt(expIdx))) { // is variable
while(!isDelim(exp.charAt(expIdx))) {
token += exp.charAt(expIdx);
expIdx++;
if(expIdx >= exp.length()) break;
}
tokType = VARIABLE;
}
else if(Character.isDigit(exp.charAt(expIdx))) { // is number
while(!isDelim(exp.charAt(expIdx))) {
token += exp.charAt(expIdx);
expIdx++;
if(expIdx >= exp.length()) break; //== why we use break
}
tokType = NUMBER;
}
else { // unknown character terminates expression
token = EOE;
return;
}
}
// Return true if c is a delimiter.
private boolean isDelim(char c) //== why we have char c
{
if((" +-/*%^=()".indexOf(c) != -1)) //== Meaning of this line, why != -1?
return true; //== Why return to true
return false;
}
}
/*== If you think I missed something in my questions that so important please let me know what it is and explain what you mean.
Thanks for reading....
*/Here's the link of explanation that I don't fully understand.
http://www.usaupload.net/d/yfxpm4fwp3x
---------------------------------------------------------------------------------------------------------
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
public class cal {
JLabel expression1 = new JLabel("Expression:");
JLabel result1 = new JLabel("Result: ");
JTextField expression_textf1 = new JTextField(22);
JTextField result_textf1 = new JTextField(10);
JButton clear1 = new JButton(" Clear ");
JButton calculate1 = new JButton(" Calculate");
JButton cancel1 = new JButton(" Cancel");
Parser p = new Parser();
public JPanel gui_2() {
JPanel northPanel1 = new JPanel();
JPanel northPanel2 = new JPanel();
northPanel1.setLayout(new GridLayout(2, 0));
northPanel1.add(expression1);
northPanel1.add(expression_textf1);
northPanel1.add(result1);
northPanel1.add(result_textf1);
northPanel2.setLayout(new FlowLayout(FlowLayout.CENTER));
northPanel2.add(northPanel1);
return northPanel2;
}
public JPanel gui_3() {
JPanel southPanel1 = new JPanel();
JPanel southPanel2 = new JPanel();
southPanel1.setLayout(new GridLayout(2, 2));
southPanel2.add(calculate1);
southPanel2.add(clear1);
southPanel2.add(cancel1);
result_textf1.setEditable(false); // Result field should not be editable
calculate1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
String expr = expression_textf1.getText().trim();
try {
result_textf1.setText(String.valueOf(p.evaluate(expr)));
}
catch(ParserException exc) {
result_textf1.setText(String.valueOf(exc));
}
}
});
clear1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
expression_textf1.setText("");
result_textf1.setText("");
expression_textf1.requestFocus();
}
});
cancel1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
System.exit(0);
}
});
southPanel2.setLayout(new FlowLayout(FlowLayout.CENTER));
southPanel2.add(southPanel1);
return southPanel2;
}
public JFrame buildNewFrame() {
try{
UIManager.setLookAndFeel(
UIManager.getSystemLookAndFeelClassName());
}catch(Exception e){}
cal re = new cal();
JPanel northPanel = new JPanel();
JPanel southPanel = new JPanel();
JPanel finalPanel = new JPanel();
northPanel = re.gui_2 ();
southPanel = re.gui_3 ();
finalPanel.setLayout( new BorderLayout());
finalPanel.add(northPanel, BorderLayout.NORTH);
finalPanel.add(southPanel, BorderLayout.SOUTH);
JFrame frame = new JFrame("Calculator");
frame.setSize(900, 900);
frame.getContentPane().add(finalPanel);
frame.addWindowListener( new WindowAdapter()
{
public void windowClosing( WindowEvent e) {
System.exit(0);
}
});
return frame;
}
}And lastly the runner.
import javax.swing.JFrame;
public class runner {
public static void main(String args[]){
JFrame lor = new JFrame();
cal rece = new cal();
lor = rece.buildNewFrame();
lor.pack();
lor.setVisible(true);
}
}
