Can't push values into stack from command line arguments in Java

Question

I have a weird problem in java. I am trying to make a math expression calculator which would evaluate an infix math expression and return the final result on the screen. I have used stacks to accomplish the task and so far, I was using a hardcoded string for input, but then I changed it to command line arguments for the same purpose. My problem is that, the stacks which I am using just won't push values into themselves from the commandline inputs although the datatype and the strings are exactly the same.

Here is the code to push data into the stack.

public class Expression 
{
    public static void main(String argv[]){

        Stack<String> operator = new Stack<String>();
        Stack<String> operand = new Stack<String>();

        /*
        String push[] = new String[argv.length];
        for (int i = 0; i<push.length; i++){
            push[i] = argv[i];
        }
        */

        for(int i = 0; i<argv.length; i++){
            if(argv[i] == "+" || argv[i] == "-" || argv[i] == "*" 
               || argv[i] == "/" || argv[i] == "^") {
                operator.push(argv[i]);
            } else if(argv[i] == "0" || argv[i] == "1" || argv[i] == "2" 
                      || argv[i] == "3" || argv[i] == "4" || argv[i] == "5" 
                      || argv[i] == "6" || argv[i] == "7" || argv[i] == "8" 
                      || argv[i] == "9") {
                operand.push(argv[i]);
            }
        }

        System.out.println(operand);
        System.out.println(operator);

        Stack<String> result = evaluateStack(operand, operator);
        System.out.println(result.toString());
    }
}

When I used the debugger using harcoded string, it was showing that the stack was filled normally, but with commandline arguments, it always shows the stacks with 0 elements even when the for loop is completed. What am I doing wrong?

`argv[i]` is a `String`, you need to use `equals` when comparing — , Sep 11 '12 at 05:41
So == will only work if I declare a String like String push[] = {"4", "+", "2"}; Because the stack was working when I was using a hardcoded String array like this... — pslayer89, Sep 11 '12 at 05:46
see also http://stackoverflow.com/questions/767372/java-string-equals-versus — , Sep 11 '12 at 05:50

score 1 · Answer 1 · answered Sep 11 '12 at 05:51

Ideally , you have not pushed anything into your stack, because of the equality operator you have used. You modify your argv[i] == "<any_str>" to argv[i].equals("<any_str>") . Then it should work. Also during string comparisons, always use equals() method because you will never know whether a string is interned or not

score 1 · Answer 2 · answered Sep 11 '12 at 05:54

You have two problems here. The first is that a String in Java should be compared using equals. Using == in Java actually compares the addresses of Strings, NOT their contents. The second problem is that when you call you program like java Expression 1*2+3, argv[0] in this case will be the total 1*2+3. To fix this, you'll need to iterate over each character in the String.

Edit: After posting this I realized the arguments were probably whitespace delimited, as in 1 * 2 + 3.

score 1 · Accepted Answer · answered Sep 11 '12 at 06:00

Be sure when to use "==" and when to use "equals"....

     "==" will be used to check the equality of the String objects

and

    "equals(...)" will be used to the check the content of the String objects.

your problem will be solved when you use the equals(...) method for checking the operands

operators

Can't push values into stack from command line arguments in Java

3 Answers3