Question

I am learning stacks right now, I my code compiles find. When I run it, code will not print my debug println’s and the error

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Arrays.java:2760)
at java.util.Arrays.copyOf(Arrays.java:2734)
at java.util.Vector.ensureCapacityHelper(Vector.java:226)
at java.util.Vector.addElement(Vector.java:573)
at java.util.Stack.push(Stack.java:50)
at stacks.main(stacks.java:56)

is displayed.

My code looks like:

import ch03.stacks.*;

import java.util.*;

public class stacks {

public static void main (String []args){

    System.out.printf("Enter a math equation in reverse polish notation:\n");

    Stack<Double> pemdas = new Stack<Double>();

    Scanner input = new Scanner(System.in);
    String in = input.next();

    double temp1, temp2, resultant = 0;


    while(input.hasNext()){
        if(in == "+"){
        temp1 = pemdas.peek();  
        pemdas.pop();
        temp2 = pemdas.peek();
        pemdas.pop();
        resultant = temp1 + temp2;
        pemdas.push(resultant);
        System.out.println(resultant);
        }

        if(in == "-"){
        temp1 = pemdas.peek();  
        pemdas.pop();
        temp2 = pemdas.peek();
        pemdas.pop();
        resultant = temp1 - temp2;
        pemdas.push(resultant);
        System.out.println(resultant);
        }

        if(in == "*"){
        temp1 = pemdas.peek();  
        pemdas.pop();
        temp2 = pemdas.peek();
        pemdas.pop();
        resultant = temp1 * temp2;
        pemdas.push(resultant);
        System.out.println(resultant);
        }

        if(in == "/"){
        temp1 = pemdas.peek();  
        pemdas.pop();
        temp2 = pemdas.peek();
        pemdas.pop();
        resultant = temp1 / temp2;  
        pemdas.push(resultant);
        System.out.println(resultant);
        }

        else
        pemdas.push(Double.parseDouble(in));
        System.out.println(resultant);

        }



    System.out.println("Answer:"+ resultant);
    }
}

So i first read in the the string of integers in Reverse Polish Notation and then if it is not an operatand, then i pop it on to my stack. At least that’s what I think it is doing. Any help is greatly appreciated.

Answer 1

You are using Scanner hasNext/next incorrectly. You should always precede each next() with a hasNext().

In your code, you are calling next() in front of the while loop. Then you are using the return value of hasNext() to terminate the the while loop. But … you are never calling ‘next()’ within the while loop. Therefore hasNext() always returns true and you are in an infinite loop. This – combined with the peek issue described earlier – may be growing your stack until you run out of memory.

The fix is simple. Move next() right after hasNext() within the while loop

Scanner input = new Scanner(System.in);
double temp1, temp2, resultant = 0;

while(input.hasNext()) {
    String in = input.next();

Unfortunately the program still does not work because you are not doing string compares correctly. Replace lines like:

if (in == "+")

with

if (in.equals("+"))

Hopefully this was just a typo. If you don’t understand why using == for string compares is a problem, you will need to review Java equality.

Finally, there is a problem with your if logic. Hint: to handle mutually exclusive cases, use code like:

if () 
{
}
else if ()
{
}
else if ()
{
}
else
{
}