Java: Immutable Stack

Posted on April 1, 2008 by imjorgeferreira

Here is the challenge: write an immutable stack in Java.

You might be thinking that a stack is by its very nature something that changes. A stack is an abstract data type with the interface:

IStack<T> push(T value);

IStack<T> pop();

T peek();

boolean isEmpty();

You push stuff onto it, you pop stuff off of it, it changes. How can it be immutable?

Every time you need to make a data structure immutable, you use basically the same trick: an operation which “changes” the data structure does so by constructing a new data structure. The original data structure stays the same.

How can that possibly be efficient? Surely we’ll be allocating memory all over the place! Well, actually, in this case, no. An immutable stack is every bit as efficient as a mutable stack. Even better: in some cases, it can be considerably more efficient, as we’ll see.

Let’s start by defining an interface for our immutable structure. While we’re at it, we’ll fix a problem with the stack ADT above, namely that you cannot interrogate the stack without changing it. And we’ll make the stack enumerable just for the heck of it:

interface IStack<T> extends Iterable<T> {
IStack<T> push(T value);

IStack<T> pop();

T peek();

boolean isEmpty();
}

 

Pushing and popping give you back an entirely new stack, and Peek lets you look at the top of the stack without popping it.

Now let’s think about constructing one of these things here. Clearly if we have an existing stack we can construct a new one by pushing or popping it. But we have to start somewhere. Since every empty stack is the same, it seems sensible to have a singleton empty stack.

class Stack<T> implements IStack<T> {
private T head;
private IStack<T> tail;
public Stack(final T head, final IStack<T> tail) {
this.head = head;
this.tail = tail;
}
public static <U> IStack<U> empty(final Class<U> type) { return new EmptyStack<U>(); }

public boolean isEmpty() { return false; }

public T peek() { return this.head; }

public IStack<T> pop() { return this.tail; }

public IStack<T> push(T value) { return new Stack<T>(value, this); }

public Iterator<T> iterator() { return new StackIterator<T>(this); }
private static class StackIterator<U> implements Iterator<U> {
private IStack<U> stack;
public StackIterator(final IStack<U> stack) { this.stack = stack; }

public boolean hasNext() { return !this.stack.isEmpty(); }

public U next() {
U result = this.stack.peek();
this.stack = this.stack.pop();
return result;
}

public void remove() { }
}
private static class EmptyStack<U> implements IStack<U> {

public boolean isEmpty() { return true; }

public U peek() { throw new RuntimeException(“empty stack”); }

public IStack<U> pop() { throw new RuntimeException(“empty stack”); }

public IStack<U> push(U value) { return new Stack<U>(value, this); }

public Iterator<U> iterator() { return new StackIterator<U>(this); }
}
}

 

And now we can easily create stacks and push stuff onto them. Notice how the fact that we have immutability means that stacks with the same tail can share state, saving on memory:

IStack<Integer> s1 = Stack.empty(Integer.class);
IStack<Integer> s2 = s1.push(10);
IStack<Integer> s3 = s2.push(20);
IStack<Integer> s4 = s2.push(30); // shares its tail with s3.
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3 Responses to Java: Immutable Stack

  1. Unknown says:
    March 29, 2010 at 10:59 pm

    Gracias por tu code. Es muy bueno!!(apologies for my bad spanish)

    Reply
  2. Unknown says:
    March 29, 2010 at 11:01 pm

    Pure awesome. Nicely done.

    Reply

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