Stack class in Java 8

Question

I have a exercise where I have to write a Stack class with the push and pop methods. The code compiles and works as it should. Are there better practices to use in my code? Is there something I should avoid doing?

public class Stack {

    private final int INCREMENTSIZE = 1024;
    Integer position = 0;

    int[] list = null;

    public Stack(){
        list = new int[INCREMENTSIZE];
    }

    public void push(Integer i) {
        if(position == list.length) {
            list = Arrays.copyOf(list, list.length + INCREMENTSIZE);
        }
        list[position++] = i;
    }

    public Integer pop(){
        return list[--position];
    }
}

Answer 1

Other answers correctly point out that using primitive types and not mixing them with Objects (=int instead of Integer), reducing visibility wherever possible (=adding private modifier to position and list) and preventing popping from empty stack are good practices. There are a few more subtleties which can be improved:

1. Rename INCREMENTSIZE to INCREMENT_SIZE. It's customary, when naming constants using full caps, to separate words with underscores.
2. Consider growing a stack by multiplying current size and start small, e.g. instead having new size be current+increment, make it current*factor, where factor can be 1.5 or 2, or even decreased as the stack grows. If you're implementing a general purpose stack, you don't know how small or large the user will want it to be—incrementing by a constant might be an overkill or too small, while starting small and growing it in multiples will conserve memory if user needs a small stack, and will grow it in large enough increments later on if user needs to store many elements. The two approaches can be mixed and fine-tuned for best performance.
3. Consider generifying the stack class so it can be a stack of anything, not just Integers. It's a small cost, but can be of large benefit.
4. If you do store arbitrarily typed objects, beware of memory leaks! The pop function as it stands won't free memory if a user decides to empty the stack. It's a merely unconservative approach when dealing with primitives or small, usually pooled Integers. It can be a real problem when storing something heavyweight—your stack will keep a reference to something which the user has popped and prevent the garbage collector from collecting it (also see Effective Java 3rd ed., Item 7). To prevent this, when popping an element, set the value of the popped element in array to null; additionally when a considerable proportion of the array is empty, deallocate a portion of it (e.g. using the Arrays.copyOf with a smaller second argument).
5. Guard against overflows. At some point, list.length + INCREMENTSIZE will overflow, and you'll get NegativeArraySizeException from Arrays#copyOf. Unfortunately, you can't have arrays which are indexed by longs, so best you can do is use Integer.MAX_VALUE as the new size, and throw an exception (e.g. IllegalStateException with a helpful message) if the caller wants to add more items to stack after it's grown to MAX_VALUE. You can see how ArrayDeque#grow(int) and ArrayDeque#newCapacity(int, int) are implemented by OpenJDK here.

Answer 2

This looks good, However I suggest you properly indent the code.

Further suggestions:

I would change following

private final int INCREMENTSIZE = 1024;

to

private static final int INCREMENTSIZE = 1024;

Since you are not changing this in the constructor (to a new value) we might as well make it unique for the whole class.

I would change following

Integer position = 0;

to

private int position = 0;

There is no reason to have an Integer when int will do. We can also make it private.

For push and pop functions you can also use int's instead of Integer.

Answer 3

Your class is small and does what you said it should, that's good.
However, there are a few things you could do better.

1. Use primitive types unless the boxed ones are specifically needed:

You are using the Integer type for counting the position and storing/returning data. If you actually used the fact, that it could be null, this would be acceptable use. However, any instance of null would break your code here. You are storing your integers in an int[], a primitive integer array. This leads to unboxing and a null reference will break your code.
A position of null (not zero 0) also isn't making any sense in the context of your class.

Use int instead of Integer unless you actually need null references.

2. Reduce the visibility of member fields

Other classes inside the same package as your class typically don't need access to your internal array. Make that array private.

3. Think about what you want to store in your stack.

Currently your stack only allows storing primitive ints. Maybe think about storing any type of data using generics. Extending your current class wouldn't be difficult.

Other than that: Your code is concise, not too complicated and serves its purpose. Think about formatting the code using your IDE's formatter and add comments and documentation to your code.

Answer 4

Your class is good (for me I would rename the variable list as arr), but you should consider case where you call pop on an empty stack. In this case your method could throw an exception like the code below:

public Integer pop(){
   if (position == 0) throw new RuntimeException("Empty Stack");
   return list[--position];
}

You can check from Java documentation that Stack pop() throws EmptyStackException, subclass of RuntimeException.

Answer 5

In addition to what other's have said:

Your constant-increment growth scheme causes push operations to be amortized O(n). You should grow by a constant factor, as ArrayList does (1.5x, if I recall correctly). Even better, just don't use a primitive array at all, and use ArrayList instead.