Basic Emulator In Java

 public class BasicEmulator {

static void GOTO( int linenumber ) {
switch( linenumber ) {
case 10: System.out.println( "Hello, World" );
case 20: GOTO( 10 );
}
}

public static void main( String[] args ) {
GOTO( 10 );
}

 }

Please note that the fall-through feature of the switch-statement is actually necessary here.

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The above code will result in a stack overflow. Try this:

 import java.util.*;

 public abstract class BasicEmulator {

   public interface Command { void run(); }

   private int currentLine = 0;
   private SortedMap<Integer, Command> commands = new TreeMap<Integer, Command>();

   public Command PRINT(final String value) {
     return new Command() {
       public void run() { System.out.println(value); }
     };
   }
   public Command GOTO(final int value) {
     return new Command() {
       public void run() { currentLine = value - 1; }
     };
   }

   public void line(int line, Command command) {
     commands.put(line, command);
   }    

   public void execute() {
     //lines();
     int maxCommand = 0;
     try {
       maxCommand = commands.lastKey();
     catch (Exception e) {}

     for (currentLine=0; currentLine <= maxCommand; currentLine++) {
       Command command = commands.get(currentLine);
       if (command != null) command.run();
     }
   }

   public static void main(String[] args) {
     new BasicEmulator() {{
         line(10, PRINT("Hello, World!"));
         line(20, GOTO(10));
     }}.execute();
   }
 }

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CategoryJava CategoryLanguageImplementation