Question

In java, which LIFO data structure class allows me to specify a MAX item size which automatically discards old items whenever adding an item will cause it to exceed MAX size.

Answer 1

You can subclass Stack to get the desired behavior. You just need to override push() to check whether the size is greater than N, and discard old items:

@Override
public void push(E elt) {
    super.push(elt);
    while (this.size() > this.maxSize) {
        this.removeElementAt(this.size() - 1);
    }
}

will probably be close to what you want.