Updated version of Infix-Postfix

updatedInfixPostfixAlgo.java

// could also have a string that contains operators and check if contains;
public static HashMap<Character, Integer> operators = new HashMap<>();
// only push to stack if order higher
public static Stack<Character> operatorStack = new Stack<Character>();
public static StringBuilder postFixNotation = new StringBuilder();

public InfixPostFixAlgo() {
// assign numerical order depending on order of operator in BODMAS
    operators.put('^', 5);
    operators.put('/', 4);
    operators.put('*', 3);
    operators.put('+', 2);
    operators.put('-', 1);
}

private static String infixToPostfix(final String equation){
    int pointer = 0;
    char[] equationChars = equation.toCharArray();
    while (pointer < equationChars.length) {
        char currentChar = equationChars[pointer];
        if (currentChar == ' ') {
            pointer++;
            continue;
        }
        if (currentChar == '(') {
            operatorStack.add(currentChar);
        } else if (isOperator(currentChar)) {
            while(!operatorStack.isEmpty() && checkPrecedence(checkStack()) >= checkPrecedence(currentChar)) {
                postFixNotation.append(operatorStack.pop());
            }
            operatorStack.add(currentChar);
        } else if (currentChar == ')'){
            while(!operatorStack.isEmpty() && checkStack() != '(') {
                postFixNotation.append(operatorStack.pop());
            }
            operatorStack.pop();
        } else {
            postFixNotation.append(currentChar);
        }
        pointer++;
    }

    for (int i=0; i<= operatorStack.size(); i++) {
        postFixNotation.append(operatorStack.pop());
    }
    return postFixNotation.toString();
}

private static int checkPrecedence(final char value) {
    return operators.getOrDefault(value, -1);
}

private static boolean isOperator(final char value) {
    return operators.containsKey(value);
}

private static char checkStack() {
    if (!operatorStack.isEmpty()) {
        return operatorStack.peek();
    }
    return ' ';
}