Interpreter_Java_version.java

// Java version

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;
import java.util.Stack;

public class Interpreter_Java_version {
    public static void main(String[] args) throws FileNotFoundException {
        File file = new File("brainf.txt");
        Scanner reader = new Scanner(file);
        StringBuilder code = new StringBuilder();
        while (reader.hasNextLine())
            code.append(reader.nextLine());
        ArrayList<Character> legalCharacters = new ArrayList<>(Arrays.asList('+', '-', '<', '>', '.', ',', '[', ']', '='));
        ArrayList<String> tokens = new ArrayList<>();
        int looper = 0;
        for (int i = 0; i < code.length(); i++) {
            if (legalCharacters.contains(code.charAt(i)))
                tokens.add("" + code.charAt(i));
            if (code.charAt(i) == '[')
                looper++;
            if (code.charAt(i) == ']')
                looper--;
        }
        if (looper != 0)
            throw new IllegalStateException("The number of '[' is not equal to the number of ']'");
        interpret(tokens);
    }

    private static void interpret(ArrayList<String> brainfTokens) {

        int[] memory=new int[30_000];
        int pointer=0;
        boolean isLooping=false;
        Stack<Integer> loopStack=new Stack<>();
        int innerLoops=0;

        Scanner scanner=new Scanner(System.in);

        for (int i = 0; i < brainfTokens.size(); i++) {

            String brainfToken = brainfTokens.get(i);

            if (isLooping) {
                if (brainfToken.equals("["))
                    innerLoops++;
                if (brainfToken.equals("]")) {
                    if (innerLoops == 0)
                        isLooping = false;
                    else innerLoops--;
                }
                continue;
            }

            switch (brainfToken) {
                case "+":
                    memory[pointer]++;
                    break;
                case "-":
                    memory[pointer]--;
                    break;
                case ">":
                    pointer++;
                    break;
                case "<":
                    pointer--;
                    if (pointer < 0)
                        throw new IllegalStateException("Pointer value less than 0 is not allowed");
                    break;
                case ",":
                    memory[pointer] =scanner.next().charAt(0);
                    break;
                case ".":
                    System.out.print((char)(memory[pointer]));
                    break;
                case "[":
                    if (memory[pointer] == 0)
                        isLooping = true;
                    else
                        loopStack.push(i);
                    break;
                case "]":
                        if (memory[pointer]!=0)
                            i=loopStack.peek();
                        else
                            loopStack.pop();
                    break;
            }
        }
    }
}