prefix - Evaluating Polish Notation in Java with 2 stacks

Question

I am writing an evaluation code for polish notation. When I overcome ClassCastException I get EmptyStack and when I solve Emptystack I get ClassCast I'm in a loop. Here is how I wanted to evaluate the Polish notation:

First I get a string from the user then put it into a stack. For evaluating I use a second stack. Why? Because: An example string: "* + * + 1 2 + 3 4 5 6" First operation here is to add up 3 and 4 but what I'm gonna do with 5 and 6? I put them in another stack. Let's say stack2. I start popping until I found an operator, in this condition I pushed 6 5 4 3 into stack2 and found a plus operator. Then I pop 2 numbers which I pushed to stack2 (the top 2 numbers are 3 and 4), add them up, and push them to stack again. I should evaluate but seems like I am missing a point or this wasn't a good idea at first. Here is my code:

public class Main {

    public static void stack(String srt){  // this function puts the string in srt stack
        String arr[] = srt.split(" "); // I am recognizing if there is a 2 or more digit number
        int size= arr.length;                // with receiving prefix with " " btw every number and operator
        Stack stack= new Stack();
        for (String s : arr) {
           // System.out.println(s);
            stack.push(s);
        }
       // for (int i = 0; i < size; i++) {
       //     System.out.println(stack.pop()); // I checked to see if any 
                                               // problems first now I dont start this
      //  }

          evaluate(stack);             // then calls the evaluate function
    }

    public static void evaluate(Stack stack){
        Stack stack2= new Stack();
        stack2.push(stack.pop());// cuz of there cant be an operator there I pop first 2 number
        stack2.push(stack.pop());
        for (int i = 0; i < (stack.capacity()*2); i++) { // I had a hard time calculating how many times
                                                        // I should cycle through this for and leave it 2 times as stack capacity
            if(stack.peek().toString().equals("*") || stack.peek().toString().equals("-") || stack.peek().toString().equals("/") || stack.peek().toString().equals("+")){
                System.out.println("Checkpoint1");
//                System.out.println(stack2.peek());
                int s2= Integer.parseInt((String) stack2.pop());
                int s1= Integer.parseInt((String) stack2.pop());
                double s3;
                String c = (String) stack.pop();
                switch (c) {
                        case "+":
                            s3=s1+s2;
                            stack2.push(s3);
                        case "-":
                            s3=s1-s2;
                            stack2.push(s3);
                        case "*":
                            s3=s1*s2;
                            stack2.push(s3);
                        case "/":
                            s3=s1/s2;
                            stack2.push(s3);
                    }

            }else{
                System.out.println("Checkpoint 2");
                stack2.push(Integer.parseInt((String) stack.pop()));
//                System.out.println(stack.peek());
            }

            }

//        System.out.println(stack.peek());
//        System.out.println(stack2.peek());
        }



    public static void main(String[] args) {
        String examplestr ="* + * + 1 2 + 3 4 5 6";
        stack(examplestr);
    }
}

Thank you for your help!!!

Answer 1

So turns out the switch-case doesn't operate correctly because I forgot a fundamental part "break". For EmptyStackException, I made a try-catch and put them inside so when it returns empty stack I know evaluation is done and print the result. BUT I still don't know how to deal with these little problems. It feels like I did not solve them properly. Gotta work more. Thanks.

Edit: I made some more adjustments and this is my final working code. I couldn't detect an error in the results yet.

Here is the working code;

import java.util.EmptyStackException;
import java.util.Stack;


public class Main2 {

    public static void stack(String srt) {     
        String arr[] = srt.split(" "); 
        int size = arr.length;                
        Stack stack = new Stack();
        for (int i = 0; i < size; i++) {
            if (arr[i].toString().equals("*") || arr[i].toString().equals("-") || arr[i].toString().equals("/") || arr[i].toString().equals("+"))
                stack.push(arr[i]);

            else
                stack.push(Double.parseDouble(arr[i]));

        }
//        for (int i = 0; i < size; i++) {
//            System.out.println(stack.pop()); 
//        }

        evaluate(stack);             
    }

    public static void evaluate(Stack stack) { 
        Stack stack1 = new Stack();
        stack1.push(stack.pop()); 
        stack1.push(stack.pop()); 

        for (int i = 0; i < stack1.capacity(); i++) {  
            try {
                if (stack.peek().toString().equals("*") || stack.peek().toString().equals("-") || stack.peek().toString().equals("/") || stack.peek().toString().equals("+")) {
//                    System.out.println("Checkpoint1");
                    double s1 = (double) stack1.pop();
                    double s2 = (double) stack1.pop();
                    double s3;
                    String operator = String.valueOf(stack.pop());
                    switch (operator) {
                        case "+":
                            s3 = s1 + s2;       
                            stack1.push(s3); 
                            break;            
                        case "-":               
                            s3 = s1 - s2;      
                            stack1.push(s3);   
                            break;     
                        case "*":              
                            s3 = s1 * s2;    
                            stack1.push(s3);    
                            break;
                        case "/":
                            s3 = s1 / s2;
                            stack1.push(s3);
                            break;
                    }

                } else {
//                    System.out.println("Checkpoint 2");
                    stack1.push(Double.parseDouble(String.valueOf(stack.pop())));
}

            } catch (EmptyStackException e) {
                System.out.println("Notasyonun sonucu: " + stack1.peek());

            }
        }
    }

    public static void main(String[] args) {
        String notasyon = scanner.nextLine();
        stack(notasyon);
    }
}