RosettaCodeData/Task/Arithmetic-evaluation/Java/arithmetic-evaluation.java

import java.util.Stack;

public class ArithmeticEvaluation
{
  public static enum Parentheses { LEFT, RIGHT }

  public static enum BinaryOperator
  {
    ADD('+', 1) {
      public BigRational eval(BigRational leftValue, BigRational rightValue) {  return leftValue.add(rightValue);  }
    },
    SUB('-', 1) {
      public BigRational eval(BigRational leftValue, BigRational rightValue) {  return leftValue.subtract(rightValue);  }
    },
    MUL('*', 2) {
      public BigRational eval(BigRational leftValue, BigRational rightValue) {  return leftValue.multiply(rightValue);  }
    },
    DIV('/', 2) {
      public BigRational eval(BigRational leftValue, BigRational rightValue) {  return leftValue.divide(rightValue);  }
    };

    public final char symbol;
    public final int precedence;

    BinaryOperator(char symbol, int precedence)
    {
      this.symbol = symbol;
      this.precedence = precedence;
    }

    public abstract BigRational eval(BigRational leftValue, BigRational rightValue);
  }

  public static class BinaryExpression
  {
    public Object leftOperand = null;
    public BinaryOperator operator = null;
    public Object rightOperand = null;

    public BinaryExpression(Object leftOperand, BinaryOperator operator, Object rightOperand)
    {
      this.leftOperand = leftOperand;
      this.operator = operator;
      this.rightOperand = rightOperand;
    }

    public BigRational eval()
    {
      BigRational leftValue = (leftOperand instanceof BinaryExpression) ? ((BinaryExpression)leftOperand).eval() : (BigRational)leftOperand;
      BigRational rightValue = (rightOperand instanceof BinaryExpression) ? ((BinaryExpression)rightOperand).eval() : (BigRational)rightOperand;
      return operator.eval(leftValue, rightValue);
    }

    public String toString()
    {  return "(" + leftOperand + " " + operator.symbol + " " + rightOperand + ")";  }
  }

  public static void createNewOperand(BinaryOperator operator, Stack<Object> operands)
  {
    Object rightOperand = operands.pop();
    operands.push(new BinaryExpression(operands.pop(), operator, rightOperand));
    return;
  }

  public static Object createExpression(String inputString)
  {
    int curIndex = 0;
    boolean afterOperand = false;
    Stack<Object> operands = new Stack<Object>();
    Stack<Object> operators = new Stack<Object>();
inputStringLoop:
    while (curIndex < inputString.length())
    {
      int startIndex = curIndex;
      char c = inputString.charAt(curIndex++);
      if (Character.isWhitespace(c))
        continue;
      if (afterOperand)
      {
        if (c == ')')
        {
          Object operator = null;
          while (!operators.isEmpty() && ((operator = operators.pop()) != Parentheses.LEFT))
            createNewOperand((BinaryOperator)operator, operands);
          continue;
        }
        afterOperand = false;
        for (BinaryOperator operator : BinaryOperator.values())
        {
          if (c == operator.symbol)
          {
            while (!operators.isEmpty() && (operators.peek() != Parentheses.LEFT) && (((BinaryOperator)operators.peek()).precedence >= operator.precedence))
              createNewOperand((BinaryOperator)operators.pop(), operands);
            operators.push(operator);
            continue inputStringLoop;
          }
        }
        throw new IllegalArgumentException();
      }
      if (c == '(')
      {
        operators.push(Parentheses.LEFT);
        continue;
      }
      afterOperand = true;
      while (curIndex < inputString.length())
      {
        c = inputString.charAt(curIndex);
        if (((c < '0') || (c > '9')) && (c != '.'))
          break;
        curIndex++;
      }
      operands.push(BigRational.valueOf(inputString.substring(startIndex, curIndex)));
    }

    while (!operators.isEmpty())
    {
      Object operator = operators.pop();
      if (operator == Parentheses.LEFT)
        throw new IllegalArgumentException();
      createNewOperand((BinaryOperator)operator, operands);
    }
    Object expression = operands.pop();
    if (!operands.isEmpty())
      throw new IllegalArgumentException();
    return expression;
  }

  public static void main(String[] args)
  {
    String[] testExpressions = { "2+3", "2+3/4", "2*3-4", "2*(3+4)+5/6", "2 * (3 + (4 * 5 + (6 * 7) * 8) - 9) * 10", "2*-3--4+-.25" };
    for (String testExpression : testExpressions)
    {
      Object expression = createExpression(testExpression);
      System.out.println("Input: \"" + testExpression + "\", AST: \"" + expression + "\", eval=" + (expression instanceof BinaryExpression ? ((BinaryExpression)expression).eval() : expression));
    }
  }
}