Convert Integer to int in Java as a return type of stack

Question

In some code, I have to push some numbers in a stack and print them later. So I have tried this:

package bf;

import java.util.Stack;

public class BF {
    public static void main(String[] args) {
        Stack<Integer> stack = null ;
        stack.push(1);
        int a = Integer.parseInt("" + stack.pop());
        System.out.println(a);
    }
}

Each time I try to do this, I get this error message:

Exception in thread "main" java.lang.NullPointerException
    at bf.BF.main(BF.java:12)
C:\Users\user\AppData\Local\NetBeans\Cache\8.1\executor-snippets\run.xml:53: Java returned: 1

My questions:

1. Is there any way to convert Integer to int ? How?

2. I must use a stack. Later I need to pop and save it to an int variable, so what else I can do? Is there not any way (I have to create a stack myself)?

There were some questions with the same/nearly the same title in Stack Overflow, but I couldn't find an answer that worked in my case.

Answer 1

A couple of problems:

First, as Alex stated above, you need to initialize your stack before you use it. If it's null, then how could you expect to add anything to it?

Second, as I stated above, you can basically use Integer as interchangeable with int when boxing/unboxing due to autoboxing in Java.

Stack<Integer> stack = new Stack<>(); // initialize first
stack.push(1);
int a = stack.pop(); // just pop, no need to convert to string and parse
System.out.println(a);

Answer 2

Yes, there is a way, it is called auto-unboxing , which is the process that will automatically convert a wrapper class (here Integer ) to its corresponding primitive type (here int ) so actually you have nothing to do.

Here is the correct code:

public static void main(String[] args) throws Exception {
    Stack<Integer> stack = new Stack<>();
    stack.push(1);
    int a = stack.pop();
}

NB: The opposite also exists, it is called auto-boxing which is the process that will automatically convert a primitive type to its corresponding wrapper class