I have this program and I have made the changes. Now I am trying to implement the stack within my code and im confused. I need to push the environment_types onto the stack but I dont know how. Here is what I have

int X=0, Y=0, Z=0 ;  
			/* Evironment classes are setup in order to implement the actions of the stack*/
			Environment_Type envMain = new Environment_Type();
			Environment_Type envA = new Environment_Type();
			Environment_Type envB = new Environment_Type();
			Environment_Type envC = new Environment_Type();
			
			Stack envStack = new Stack();
			 // create a fileScan Scanner for a file, Simple.txt
      
         // create a fileScan Scanner for a file, Simple.txt
         Scanner fileScan = new Scanner(new File("simple.txt"));
         String inputString = "";
         String commandString;
      
         inputString =  fileScan.next();
      
      //   System.out.println (inputString);
      
         if (inputString.equals ("begin"))
         {
            System.out.println ("Parsing begin");
         	
                // check whether the fileScan has the next item (i.e., next line)
            while (fileScan.hasNext())	// check if there is any data to read
            {
            
               inputString =  fileScan.next();
               System.out.println (inputString);
               		
               if (inputString.equals ("end;"))
               {
                  System.out.println ("Parsing End; Program");
                  System.exit(0);
               }
               
               else if (inputString.equals ("invoke")) //***************
               {
                  System.out.print ("Parsing invoke  ");
                  commandString = fileScan.next ();
                  System.out.println ("The Command is: " + commandString);
                  if (commandString.equals ("A;"))
                  {
                     envStack.push(envA); // Push environment A  
                  }
                  else if (commandString.equals ("B;"))
                  {	
                     envStack.push(envB);//push environment B
                  }
                  else if (commandString.equals ("C;"))
                  {	
                     envStack.push(envC);//push environment C
                  }
               }
               
               else if (inputString.equals ("increment"))
               {
                  System.out.print ("Parsing increment  ");
                  commandString = fileScan.next ();
                  System.out.println ("The Command is: " + commandString);
               
                  if (commandString.equals ("X;"))
                  {
                     X++;
                  }
                  else if (commandString.equals ("Y;"))
                  {	
                     Y++;
                  }
                  else if (commandString.equals ("Z;"))
                  {	
                     Z++;
                  }
               }
               
               else if (inputString.equals ("decrement"))
               {
                  System.out.print ("Parsing decrement  ");
                  commandString = fileScan.next ();
                  System.out.println ("The Command is: " + commandString);
               
                  if (commandString.equals ("X;"))
                  {
                     X--;
                  }
                  else if (commandString.equals ("Y;"))
                  {	
                     Y--;
                  }
                  else if (commandString.equals ("Z;"))
                  {	
                     Z--;
                  }
               }
               
               else if (inputString.equals ("output"))
               {
                  System.out.print ("Parsing output  ");
                  commandString = fileScan.next ();
                  System.out.println ("The Command is: " + commandString);
               
                  if (commandString.equals ("X;"))
                  {
                     System.out.println("Value of X:  "+ X);
                  }
                  else if (commandString.equals ("Y;"))
                  {	
                     System.out.println("Value of Y:  "+ Y);
                  }
                  else if (commandString.equals ("Z;"))
                  {	
                     System.out.println("Value of Z:  "+ Z);
                  }
               
               }
               
               else if (inputString.equals ("stop_sub"))
               {
                  System.out.print ("Parsing Stop_sub;  ");
               }
            
            
            }
                     
                      
         }
            
         else				//if information is not found in file then the program is false
            System.out.println ("Bad program");
      
      }
   }

Huh? That is exactly what you are doing here

envStack.push(envA);

Perhaps you need to restate the question?

The java.util.Stack class is a legacy class. You should use the java.util.Deque interface instead, which supports LIFO.

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