Java で数式を評価する

package evaluateexpression;

import java.util.Scanner;
import java.util.Stack;

public class EvaluateExpression {
  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);

    // Creating stacks for operators and operands
    Stack<Integer> operator = new Stack();
    Stack<Double> value = new Stack();

    // Let's create some temparory stacks for operands and operators
    Stack<Integer> tmpOp = new Stack();
    Stack<Double> tmpVal = new Stack();

    // Enter an arthematic expression
    System.out.println("Enter expression");
    String input = scan.next();
    System.out.println(
        "The type of the expression is " + ((Object) input).getClass().getSimpleName());
    input = "0" + input;
    input = input.replaceAll("-", "+-");

    // In the respective stacks store the operators and operands
    String temp = "";
    for (int i = 0; i < input.length(); i++) {
      char ch = input.charAt(i);
      if (ch == '-')
        temp = "-" + temp;
      else if (ch != '+' && ch != '*' && ch != '/')
        temp = temp + ch;
      else {
        value.push(Double.parseDouble(temp));
        operator.push((int) ch);
        temp = "";
      }
    }
    value.push(Double.parseDouble(temp));

    // Create a character array for the operator precedence

    char operators[] = {'/', '*', '+'};

    /* Evaluation of expression */
    for (int i = 0; i < 3; i++) {
      boolean it = false;
      while (!operator.isEmpty()) {
        int optr = operator.pop();
        double v1 = value.pop();
        double v2 = value.pop();

        if (optr == operators[i]) {
          // if operator matches evaluate and store it in the temporary stack
          if (i == 0) {
            tmpVal.push(v2 / v1);
            it = true;
            break;
          } else if (i == 1) {
            tmpVal.push(v2 * v1);
            it = true;
            break;
          } else if (i == 2) {
            tmpVal.push(v2 + v1);
            it = true;
            break;
          }
        } else {
          tmpVal.push(v1);
          value.push(v2);
          tmpOp.push(optr);
        }
      }
      // pop all the elements from temporary stacks to main stacks
      while (!tmpVal.isEmpty()) value.push(tmpVal.pop());
      while (!tmpOp.isEmpty()) operator.push(tmpOp.pop());
      // Iterate again for the same operator
      if (it)
        i--;
    }
    System.out.println("\nResult = " + value.pop());
  }
}

Enter expression
2+7*5-3/2
The type of the expression is String

Result = 35.5