CODEWITHSUNDEEP
javaexceptionstack
dimlo
dimlo

Reputation: 25

Exception thrown when using stack.pop in Java

To begin with keep in mind that the EXPRESSION is "2 + 3 * 5" and the POSTFIX expression is 2 3 5 * +
Here's a method that I wrote in order to extract the operators and operand from the stack that has the postfix expression, and then evaluate the whole thing.

// This method evaluates the postFix expression
public void evaluateRPN() {

    int t1 = 0,
        t2 = 0,
        calculation = 0;
    String oper;

    double result;

    // Empty stack  
    Stack stack = new Stack();

    // Tokenize expression
    Scanner scan = new Scanner(this.postfix);
    char current;

    while ( scan.hasNext() ) {

        String token = scan.next();     

        if ( this.isNumber(token) ) { // if the token is an operand, push it onto the stack

            stack.push(token);


        } else {        // If the token is an operator          

            t1 = (char) stack.pop();
            t2 = (char) stack.pop();                

            calculation = (int) evalEx(t2, token, t1 );
            stack.push(calculation);    

        }

    }

    this.finalExpression = (String) stack.pop();

}

now when I run this code, it gives me an error on the line: t1 = (char) stack.pop(); where I start to pop the first element from the stack. Also the evalEx() method is already declare somewhere else and it works as fine. So my question is, what am I missing here? I know that I should use a try/catch in the (else) part but I don't think that's the problem. Thanks in advanced!

Upvotes: 0

Views: 235

Answers (1)

Siyuan Ren
Siyuan Ren

Reputation: 7844

Because you cast a String object into a char, and that is not allowed. Unless you are working on a very old java compiler, please use generics. Use Stack<String> instead of Stack and you will get the error message at compile time. Also, Stack is deprecated. Use Deque instead.

Upvotes: 0

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