How can I only refer to an object ONLY if it actually exists?

Question

I am implementing a stack using linked list in java. The problem is that I get a nullPointerException when there is no element below, e.g. the StackNode.link does not exist. Therefore if I try assigning the StackNode.link I get the Exception.

Using an if statement to only run the code if it exists, I just get the Exception in the if statement. How do I go about this?

int pop() {

    StackNode temp = top;

    // update top
    top = belowTop;
    belowTop = top.link; // this is where I get the nullPointExcpetion


    return temp.data;

}

I would expect that when top.link does not exist (e.g. is null) then belowTop would just be null. This would be fine, but as described I get the exception.

EDIT: This is what I tried with the if-statement

if (top.link != null) {
        belowTop = top.link;
    }
else {
        belowTop = null;
    }

Answer 1

You need to check if the variable top has been initialized or not:

...
if (top != null) {
   belowTop = top.link;
} else {
   // Handle the not initialized top variable
}
...

Probably a good solution is to throw a runtime exception if belowTop if not initialized, like

...
if (top == null) {
   throw new IllegalStateException("Can't pop from an empty stack");
} 
belowTop = top.link;
...

In this case you have to prepare also a method that gives the ability to check if the stack is not empty or not initialized. Here a complete proposal:

public boolean isEmpty() {
   // Your logic here 
}

// Better have a public access because it seems an utility library and 
// it should be accessed from anywhere
public int pop() {

    StackNode temp = top;

    // update top
    top = belowTop;
    if (top == null) {
        throw new IllegalStateException("Can't pop from an empty stack");
    } 
    belowTop = top.link; // Now it works

    return temp.data;

}

And you can use it as follow:

if (!myStack.isEmpty()) {
   int value = myStack.pop();
   // Do something
}