• INode.java

public interface INode { 
    public void parse(Context context); 
} 

• ProgramNode.java

// <program> ::= PROGRAM <command list> 
public class ProgramNode implements INode { 
    private INode commandListNode; 
    public void parse(Context context) { 
        context.skipToken("PROGRAM"); 
        commandListNode = new CommandListNode(); 
        commandListNode.parse(context); 
    } 

    public String toString() { 
        return "[PROGRAM " + commandListNode + "]"; 
    } 
}  

• CommandListNode.java

import java.util.Vector; 

// <command list> ::= <command>* END 
public class CommandListNode implements INode { 
    private Vector list = new Vector();

    public void parse(Context context) { 
        while (true) { 
            if (context.currentToken() == null) { 
                System.err.println("Missing 'END'"); 
                 break; 
            } else if (
                    context.currentToken().equals("END")) { 
                context.skipToken("END"); 
                break; 
            } else { 
                INode commandNode = new CommandNode(); 
                commandNode.parse(context); 
                list.add(commandNode); 
            } 
        } 
    }

    public String toString() { 
        return "" + list; 
    } 
}  

• CommandNode.java

// <command> ::= <repeat command> | <primitive command> 
public class CommandNode implements INode { 
    private INode node;

    public void parse(Context context) { 
        if (context.currentToken().equals("REPEAT")) { 
            node = new RepeatCommandNode(); 
            node.parse(context); 
        } else { 
            node = new PrimitiveCommandNode(); 
            node.parse(context); 
        } 
    }

    public String toString() { 
        return node.toString(); 
    } 
}  

• RepeatCommandNode.java

public class RepeatCommandNode implements INode { 
    private int number; 
    private INode commandListNode; 

    public void parse(Context context) { 
        context.skipToken("REPEAT"); 
        number = context.currentNumber(); 
        context.nextToken(); 
        commandListNode = new CommandListNode(); 
        commandListNode.parse(context); 
    }

    public String toString() {
        return "[REPEAT " + number + " " 
                  + commandListNode + "]"; 
    } 
}

• PrimitiveCommandNode.java

// <primitive command> ::= PRINT <string> 
//                           | SPACE | BREAK | LINEBREAK 
public class PrimitiveCommandNode implements INode {
    private String name;
    private String text;

    public void parse(Context context) { 
        name = context.currentToken(); 
        context.skipToken(name); 
        if (!name.equals("PRINT") && !name.equals("BREAK") 
             && !name.equals("LINEBREAK") 
             && !name.equals("SPACE")) { 
            System.err.println("Undefined Command"); 
        }

        if (name.equals("PRINT")) { 
            text = context.currentToken(); 
            name += text; 
            context.nextToken(); 
        } 
    }

    public String toString() { 
        return name; 
    } 
}  

• Context.java

import java.util.*; 

public class Context { 
    private StringTokenizer tokenizer; 
    private String currentToken; 

    public Context(String text) { 
        tokenizer = new StringTokenizer(text); 
        nextToken(); 
    } 

    public String nextToken() { 
        if (tokenizer.hasMoreTokens()) { 
            currentToken = tokenizer.nextToken(); 
        } else { 
            currentToken = null; 
        } 
        return currentToken; 
    } 

    public String currentToken() { 
        return currentToken; 
    } 

    public void skipToken(String token) { 
        if (!token.equals(currentToken)) { 
            System.err.println("Warning: " + token + 
                          " is expected, but " + 
                          currentToken + " is found."); 
        } 
        nextToken(); 
    } 

    public int currentNumber() { 
        int number = 0; 
        try { 
            number = Integer.parseInt(currentToken); 
        } catch (NumberFormatException e) { 
            System.err.println("Warning: " + e); 
        } 
        return number; 
    } 
}  

• Main.java

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

public class Main { 
    public static void main(String[] args) { 
        try { 
            BufferedReader reader = new 
                  BufferedReader(new FileReader(args[0])); 
            String text; 
            while ((text = reader.readLine()) != null) { 
                System.out.println("text = \"" + 
                                       text + "\""); 
                INode node = new ProgramNode(); 
                node.parse(new Context(text)); 
                System.out.println("node = " + node); 
            } 
        } 
        catch (ArrayIndexOutOfBoundsException e) { 
            System.err.println(
                   "Usage: java Main yourprogram.txt"); 
        } 
        catch (Exception e) { 
            e.printStackTrace(); 
        } 
    } 
} 

• program.txt

PROGRAM PRINT xxx END
PROGRAM REPEAT 4 PRINT xxx END END 
PROGRAM REPEAT 4 PRINT xxx PRINT "yyy" END END

• INode.java

public interface INode {
    public void parse(Context context);
    public void execute();
} 

• ProgramNode.java

// <program> ::= PROGRAM <command list>
public class ProgramNode implements INode {
    private INode commandListNode;

    public void parse(Context context) {
        context.skipToken("PROGRAM");
        commandListNode = new CommandListNode();
        commandListNode.parse(context);
    }

    public void execute() {
        commandListNode.execute();
    }

    public String toString() {
        return "[PROGRAM " + commandListNode + "]";
    }
} 

• CommandListNode.java

import java.util.*;    

// <command list> ::= <command>* END
public class CommandListNode implements INode {
    private Vector list = new Vector();
    private INode commandNode;

    public void parse(Context context) {
        while (true) {
            if (context.currentToken() == null) {
                System.err.println("Missing 'END'");
                break;
            } else if(context.currentToken().equals("END")) {
                context.skipToken("END");
                break;
            } else {
                commandNode = new CommandNode();
                commandNode.parse(context);
                list.add(commandNode);
            }
        }
    }

    public void execute() {
        Iterator it = list.iterator();
        while (it.hasNext()) {
            ((CommandNode)it.next()).execute();
        }
    }

    public String toString() {
        return "" + list;
   }
} 

• CommandNode.java

// <command> ::= <repeat command> | <primitive command>
public class CommandNode implements INode {
    private INode node;

    public void parse(Context context) {
        if (context.currentToken().equals("REPEAT")) {
            node = new RepeatCommandNode();
            node.parse(context);
        } else {
            node = new PrimitiveCommandNode();
            node.parse(context);
        }
    }

    public void execute() {
        node.execute();
    }

    public String toString() {
        return node.toString();
    }
} 

• PrimitiveCommandNode.java

// <primitive command> ::= PRINT <string> 
//                           | SPACE | BREAK | LINEBREAK
public class PrimitiveCommandNode implements INode {
    private String name;
    private String text;

    public void parse(Context context) {
        name = context.currentToken();
        context.skipToken(name);
        if (!name.equals("PRINT") && !name.equals("BREAK") 
                           && !name.equals("LINEBREAK") 
                           && !name.equals("SPACE")) {
            System.err.println("Undefined Command");
        }

        if (name.equals("PRINT")) {
            text = context.currentToken();
            context.nextToken();
        }
    } 

    public void execute() {
        if(name.equals("PRINT"))
            System.out.print(text);
        else if(name.equals("SPACE"))
            System.out.print(" ");
        else if(name.equals("BREAK"))
            System.out.println();
        else if(name.equals("LINEBREAK"))
            System.out.println(
                  "\n------------------------------");
    }

    public String toString() {
        return name;
    }
} 

• RepeatCommandNode.java

public class RepeatCommandNode implements INode {
    private int number;
    private INode commandListNode;

    public void parse(Context context) {
        context.skipToken("REPEAT");
        number = context.currentNumber();
        context.nextToken();
        commandListNode = new CommandListNode();
        commandListNode.parse(context);
    }

    public void execute() {
        for(int i = 0; i < number; i++)
            commandListNode.execute();
    }

    public String toString() {
        return "[REPEAT " + number + " " + 
                             commandListNode + "]";
    }
} 

• Context.java

import java.util.*;

public class Context {
    private StringTokenizer tokenizer;
    private String currentToken;

    public Context(String text) {
        tokenizer = new StringTokenizer(text);
        nextToken();
    }

    public String nextToken() {
        if (tokenizer.hasMoreTokens()) {
            currentToken = tokenizer.nextToken();
        } else {
            currentToken = null;
        }
        return currentToken;
    }

    public String currentToken() {
        return currentToken;
    }

    public void skipToken(String token) {
        if (!token.equals(currentToken)) {
            System.err.println("Warning: " + token + 
                           " is expected, but " + 
                           currentToken + " is found.");
        }
        nextToken();
    }

    public int currentNumber() {
        int number = 0;
        try {
            number = Integer.parseInt(currentToken);
        } catch (NumberFormatException e) {
            System.err.println("Warning: " + e);
        }
        return number;
    }
} 

• Main.java

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

public class Main {
    public static void main(String[] args) {
        try {
            BufferedReader reader = new BufferedReader(
                               new FileReader(args[0]));
            String text;
            while ((text = reader.readLine()) != null) {
                System.out.println("text = \"" + text 
                                      + "\"");
                INode node = new ProgramNode();
                node.parse(new Context(text));
                node.execute();
            }
        }
        catch (ArrayIndexOutOfBoundsException e) {
            System.err.println(
                 "Useage: java Main yourprogram.txt");
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
}

• program.txt

PROGRAM REPEAT 4 LINEBREAK PRINT justin SPACE PRINT momor LINEBREAK END END