Any way to set max size of a collection?

Question

Is there any way to set a maximum size of a Collection in Java?

Answer 1

You can do this:

List<X> list = Arrays.asList(new X[desiredSize]);
// where X is any Object type (including arrays and enums,
// but excluding primitives)

The resulting list is modifiable, but not resizable (i.e. add(e) and remove(e) don't work, but set(index, e) does).

Reference:

• Arrays.asList(T ...)

---

Or: using Guava, here's a static method that decorates an existing List with a maximum size

public static <T> List<T> setMaxSize(
    final List<T> input, final int maxSize){

    return new ForwardingList<T>(){

        @Override
        public boolean addAll(Collection<? extends T> collection){
            return standardAddAll(collection);
        }

        @Override
        public boolean addAll(int index, Collection<? extends T> elements){
            return standardAddAll(index, elements);
        }

        public boolean add(T e) {
            checkMaxSize();
            return delegate().add(e);
        }

        @Override
        public void add(final int index, final T e){
            checkMaxSize();
            delegate().add(index, e);
        }

        private void checkMaxSize(){
            if(size() >= maxSize){
                throw new UnsupportedOperationException("Maximum Size "
                    + maxSize + " reached");
            }
        }

        @Override
        protected List<T> delegate(){
            return input;
        }
    };
}

Since ForwardingXxx classes exist for all standard collection types, you can write yourself similar decorators for other collections as well.

Obviously this will only work if your client code uses the decorated collection. If you change the underlying collection you are screwed (just like the Collections.unmodifiableXXX methods)

Reference:

• ForwardingList