import java.util.Stack;

public class Calculator {

public enum Operator { ADD, SUBTRACT, MULTIPLY, DIVIDE, BLANK }

public static void main(String[] args) { String expression = "4*2/4"; Calculator calc = new Calculator(); System.out.println(calc.compute(expression)); }

public double compute(String sequence) { Stack numberStack = new Stack<>(); Stack operatorStack = new Stack<>(); for (int i = 0; i < sequence.length(); i++) { char c = sequence.charAt(i); if (Character.isDigit(c)) { int number = parseNumber(sequence, i); numberStack.push((double) number); i += Integer.toString(number).length() - 1; } else if (c == '(') { operatorStack.push(Operator.BLANK); } else if (c == ')') { collapseTop(numberStack, operatorStack, Operator.BLANK); } else { Operator op = parseOperator(sequence, i); collapseTop(numberStack, operatorStack, op); operatorStack.push(op); } } collapseTop(numberStack, operatorStack, Operator.BLANK); return numberStack.pop(); }

private void collapseTop(Stack numberStack, Stack operatorStack, Operator futureTop) { while (numberStack.size() >= 2 && operatorStack.size() >= 1) { if (priorityOfOperator(futureTop) <= priorityOfOperator(operatorStack.peek())) { double second = numberStack.pop(); double first = numberStack.pop(); Operator op = operatorStack.pop(); double result = applyOp(first, op, second); numberStack.push(result); } else { break } } }

private double applyOp(double left, Operator op, double right) { switch (op) { case ADD return left + right; case SUBTRACT: return left - right; case MULTIPLY: return left * right; case DIVIDE: return left / right; default: return right; }

}

private int priorityOfOperator(Operator op) { switch (op) { case ADD return 1; case SUBTRACT: return 1; case MULTIPLY: return 2; case DIVIDE: return 2; case BLANK: return 0; } return 0; }

private int parseNumber(String sequence, int offset) { StringBuilder sb = new StringBuilder(); while (offset < sequence.length() && Character.isDigit(sequence.charAt(offset))) { sb.append(sequence.charAt(offset)); offset+++; } return Integer.parseInt(sb.toString()); }

private Operator parseOperator(String sequence, int offset) { if (offset < sequence.length()) { char op = sequence.charAt(offset); switch (op) { case '+': return Operator.ADD; case '-': return Operator.SUBTRACT; case '*': return Operator.MULTIPLY; case '/': return Operator.DIVIDE; } } return Operator.BLANK; }

} Analyze the code in detail