SUNY Geneseo Department of Computer Science

Alternative Design for Class Stack

Previous Lecture

Misc

Second hour exam is Friday (April 1)

• Lists, their algorithms and analyses, related structures
• Rules and format similar to first exam
• Sample Exam on Web

Questions?

Returning yellow tile count?

Walk through treasure hunt

    if robot is on white tile
        move forward
        int i = hunt()      // returns # yellow
        move forward
        return i

Is Stack Really a Subclass of List?

Class = set of (potential) objects

Subclass = subset

Are stacks conceptually a subset of lists?

• Methods rename list methods, i.e., stack does nothing more than list
• Should stacks do less than lists?
  • Restrict access to pop, push
  • vs lists: getRest, delete, find
• So stack really shouldn't be a subset (subclass) of list

Subclass as subset vs subclass for reusing code

Implementing Stacks without Subclassing List

    import geneseo.cs.sc.List;
    class Stack {
        private List items;      // holds items on stack
        public Stack() {
            items = new List();
        }
        public void push( Object x ) {
            items.addItems( x );
        }
        public Object pop() {
            return items.getAndRemove();
        }
        public boolean isEmpty() {
            return items.isEmpty();
        }
    }

Adaptor Pattern

Pre-Exam Questions?

Is every element of a list the same (Java)

    // In ExtendList subclass of List
    public boolean allSame() {
        if ( this.isEmpty() ) {
            return true;
        }
        else if ( this.getRest().isEmpty() ) {
            return true;
        }
        else {
            return this.getFirst().equals(this.getRest().getFirst()) && ((ExtendList)this.getRest()).allSame();
        }
    }

Next

(After exam)

Trees

Read Sections 13.1 and 13.2

Next Lecture