Implementing the List ADT

1. Implementing the List ADT Computer Science 4 Mr. Gerb Reference: Objective: Understand how to implement an ADT.

2. Review • We created an ADT called a list • We defined a number of operations on list • We decided the inputs and outputs of those operations • At no time did we discuss how those operations would be implemented. • But we could use the operations perfectly well without knowing. • Encapsulation!!!

3. How could we implement a list? // Stores a list as an array class MyList implements List { Object[] storage = new Object[10]; int listSize=0; ... //Methods } • Some notes • “implements List” tells Java that List has been defined as an interface, a set of operations • Array initially given a size of 10. We will deal later with the case when this is not enough.

4. Implementing the clear method //Very simple, list size = 0 public void clear() { listSize=0; }

5. Implement the size operator public int size() { return listSize; }

6. Implementing the get operator // This won’t work: public Object get(int N) { return storage[N]; } • What happens if N >= listSize?

7. A better implementation of get public Object get(int N) } if (N>=listSize || N<0) throw new ArrayIndexOutOfBoundsException(); else return storage[N]; } • Can implement set in a similar way

8. Implementing remove, move all subsequent elements down public Object remove(int N) } if (N>=listSize || N<0) throw new ArrayIndexOutOfBoundsException(); else { for (int (m=N+1;m<listSize;++m) storage[m-1]=storage[m]; --listSize; } }

9. Implementing add, array might be smaller than listSize //Double the size of the array if no space public void add(Object obj) { if (storage.length<=listSize) { Object[] s=new Object[storage.length*2]; for (m=0; m<listSize; ++m) s[m]=storage[m]; storage=s; } storage[listSize] = obj; ++listSize; }

10. Two parts of the implementation • Algorithms • Process by which operations occur • Examples: Moving down items during a remove or doubling the array during add. • Data structures • How data in the ADT is stored • In this case, an array and an integer

11. Summary • Implement an ADT with a class • In Java, define the ADT as an interface • Use the implements keyword in the class definition. • Two parts of an implementation • Algorithms • Data structures • Implementation deals with many details that the ADT doesn’t need to deal with.