Main.java

import java.util.*;
import java.io.File;
import java.io.IOException;

public class Main{
	
	private Stack<Map<String, String>> stackTrace; // we are going to use this to store our maps
	private Scanner sc; // scanner that we will use to read through the file
	private StringBuilder output;
	
	Main(String filename){
		stackTrace  = new Stack<>();
		output = new StringBuilder();
		sc = null;
		try{
			sc = new Scanner(new File(filename));
		}catch(IOException ex){
			System.out.println(ex.toString());
		}
	}
	
	/**
	*	Method will read the next instruction and return us the instruction from the file
	**/
	public String getNextInstruction(){
		if(sc == null){
			System.out.println("Scanner is null");
			return null;
		}
		if(sc.hasNext())
			return sc.nextLine();
		else{
			return null;
		}
	}
	
	/**
	* Method to parse the instruction read and perform the action
	*/
	public void execute(String instruction){
		String [] tokens = instruction.split(" ");
		String instructionType = tokens[0]; // this gives us which instruction we are performing
		output.append(instruction);
		switch(instructionType){
			case "define":{
				String var_name = tokens[1];
				String value = tokens[2];
				insertToStackTrace(var_name, value);
				output.append("\n");
				break;
			}case "use":{
				String var_name = tokens[1];
				String value = latestStackTraceHasName(var_name);
				if(value == null)
					value = "undefined";
				output.append(" = ").append(value).append("\n");
				break;
			}case "beginscope":{
				createScope();
				output.append("\n");
				break;
			}case "endscope":{
				endScope();
				output.append("\n");
				break;
			}
		}
	}
	
	public String latestStackTraceHasName(String var_name){
		if(stackTrace.isEmpty())
			return null;
		return stackTrace.peek().getOrDefault(var_name, null);
	}
	public void insertToStackTrace(String var_name, String value){
		if(stackTrace.isEmpty()){
			//we need to create a new global Symbol table
			stackTrace.push(new HashMap<>()); // we just put an empty map
		}else{
			
		}
		Map<String, String> map = stackTrace.peek(); // get the lates symbol table and use that 
		map.put(var_name, value);
	}
	public void createScope(){
		Map<String, String> map;
		if(!stackTrace.isEmpty())
			map = new HashMap<>(stackTrace.peek());
		else
			map = new HashMap<>();
		stackTrace.push(map);
	}
	
	public void endScope(){
		stackTrace.pop();
	}
	
	public static void main (String [] args){
		Main main;
		if(args.length > 0){
			main = new Main(args[0]);
			String instruction = main.getNextInstruction();
			while(instruction != null){
				main.execute(instruction);
				instruction = main.getNextInstruction();
			}
			System.out.println(main.output.toString());
		}
	}
}