In-fix Expression Evaluation - NoSuchElementFound Exception

Question

My assignment is to evaluate expressions read in from a file, such as (4-7)/2+6<3*(1-9).

< and > mean minimum and maximum, respectively, and have a higher precedence than / and *.

However, I am getting a NoSuchElementFound exception when attempting to evaluate an expression, pointing towards the stack I return at the end of the evaluate method. I'm assuming this means the stack is empty. I am having trouble seeing where the issue is. Here is my evaluate code. For what it matters, I have implemented my own stack classes (both strings) and am not using generics.

/**
 * Evaluates the expression and updates textArea appropriately
 */
private String evaluateLine()
{
    char token;
    String[] tokens = new String[expression[0].length()];
    for(int i = 0; i < expression[0].length(); i++)
    {
         token = expression[0].charAt(i);
         tokens[i] = String.valueOf(token);
    }

    OperatorStack operators = new OperatorStack();
    IntegerStack integers = new IntegerStack();

    for(int i = 0; i < tokens.length; i++)
    {
        if(isInteger(tokens[i]))
        {
            integers.push(tokens[i]);
        }
        else if(tokens[i] == "(")
        {
            operators.push(tokens[i]);
        }
        else if(tokens[i] == ")")
        {
            while(operators.top() != "(")
            {
                integers.push(applyOp(operators.pop(), integers.pop(), integers.pop()));
                operators.pop();
            }
        }
        else if(tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/" 
                || tokens[i] == "<" || tokens[i] == ">")
        {
            while(!operators.isEmpty() && hasPrecedence(tokens[i], operators.top(), operators.top()))
            {
                integers.push(applyOp(operators.pop(), integers.pop(), integers.pop()));
            }
            operators.push(tokens[i]);
        }
    }

    while(!operators.isEmpty())
    {
        integers.push(applyOp(operators.pop(), integers.pop(), integers.pop()));
    }

    return integers.pop();
}

public static boolean hasPrecedence(String op1, String op2, String op3)
{

    if(op2 == "(" || op2 == ")")
    {
        return false;
    }

    if((op1 == "<" || op1 == ">") && (op2 == "*" || op2 == "/") && (op3 == "+" || op3 == "-"))
    {
        return false;
    }
    else
    {
        return true;
    }
}

public static String applyOp(String op, String b, String a)
{
    switch (op)
    {
    case "+":
        return Integer.toString(Integer.parseInt(a) + Integer.parseInt(b));
    case "-":
        return Integer.toString(Integer.parseInt(a) - Integer.parseInt(b));
    case "*":
        return Integer.toString(Integer.parseInt(a) * Integer.parseInt(b));
    case "/":
        if(Integer.parseInt(b) == 0)
        {
            throw new
            UnsupportedOperationException("Cannot divide by 0");
        }
        return Integer.toString(Integer.parseInt(a) / Integer.parseInt(b));
    case "<":
        if(Integer.parseInt(a) < Integer.parseInt(b))
        {
            return b;
        }
        else if(Integer.parseInt(b) < Integer.parseInt(a))
        {
            return a;
        }
    case ">":
        if(Integer.parseInt(a) > Integer.parseInt(b))
        {
            return a;
        }
        else if(Integer.parseInt(b) > Integer.parseInt(a))
        {
            return b;
        }
    }

    return Integer.toString(0);

}

public boolean isInteger(String str)
{
    return str.matches("0123456789");
}

Answer 1

Hard to find the exact problem because you do not include enough code to get your example to compile but one thing I notice is that your isInteger method does not work. When I ran the input into your method it produced an empty stack for the integers. Use this instead:

public boolean isInteger(String str) {
    try {
        Integer.parseInt(str);
        return true;
    } catch (NumberFormatException ex) {
        return false;
    }
}

Alternatively I think you were attempting to use a Regex for determining if the string contains an integer. For that you would need to do this:

public boolean isInteger(String str) {
    return str.matches("[0-9]+");
}

With either of these implementations of isInteger I was able to get your code to run when I replace your stack with Stack<String> collections. Hope that helps.