Recursive Descent Parser for LL(1) Grammar. A mini project for my course, CSC569 Principles of Compiler. This codes are taken from the text book, but we modified the Grammar Rules according to the questions given.

RDP.java

import java.io.* ;
import java.util.* ;

class RDP // Recursive Decent Parser 
{
	char inp;
	public static void main(String[] args) throws IOException 
	{
		System.out.print("Please enter an input string : ");
		InputStreamReader stdin = new InputStreamReader(System. in );
		RDP rdp = new RDP();
		rdp.parse();
	}

	void parse() 
	{
		inp = getInp();
		S(); // Call starting non-terminal, which is S
		if (inp == '\r') // end marker, computer read the end marker as "\r"
		{
			accept(); // if the string reached the end marker without reject, accept the string
		}
		else 
		{
			reject(); // if not found end marker, reject the string
		}
	}

	// Rule 1 & Rule 2
	void S() 
	{
		if (inp == 'b') // if found terminal b
		{
			inp = getInp(); // get next input
			A(); // call non-terminal
		}
		else if (inp != 'a' | inp != 'b') 
		{
			A();
			if (inp == 'b') 
			{
				inp = getInp();
				B();
			}
			else 
			{
				reject();
			}
		}
		else 
		{
			reject(); // if not match, reject the string
		}
	}

	// Rule 3
	void A() 
	{
		if (inp == 'a') 
		{
			inp = getInp();

			if (inp == 'b') 
			{
				inp = getInp();
			}
			else 
			{
				reject();
			}
		}
		else 
		{
			reject();
		}
	}

	// Rule 4
	void B() 
	{
		if (inp == 'b') 
		{
			inp = getInp();
			A();
		}
		else 
		{
			reject();
		}
	}

	// method for print Accept message
	void accept() // Accept the input
	{
		System.out.println("This input string is accepted by the grammar.");
	}

	// method for print Reject message
	void reject() // Reject the input
	{
		System.out.println("This input string is rejected by the grammar.");
		System.exit(0); // terminate parser 
	}

	// method for reading the input string character
	char getInp() 
	{
		try 
		{
			return (char) System.in.read();
		}
		catch(IOException errormsg) 
		{
			System.out.println("IO error " + errormsg);
		}
		return '#'; // must return a char 
	}
}