T pop() {
            int size = this.size;
            if (size == 0) {
                return null;
            }
            size--;
            T ret = elements[size];
            elements[size] = null;
            map.remove(ret);
            this.size = size;
            return ret;
        }
Light-weight object pool based on a thread-local stack.

@param <T> the type of the pooled object
lts-core/src/main/java/com/github/ltsopensource/core/commons/utils/Recycler.java