Computer Science 209

1. Computer Science 209 Software Development Inheritance and Composition

2. Two Types of Class Relations • Inheritance: Class A inherits the attributes and operations of class B, and then adds some of its own; clients can run B’s methods as well as A’s methods • Composition: Class A contains an instance variable of class B; clients run only A’s methods, not B’s methods B A = extends = composes A B

3. Java Stack Class <<Interface>> Iterable <<Interface>> Collection Stack inherits List operations as well as Collection operations Gets Vector to manage its data and provide some of its methods, but at a price! Abstract Collection <<Interface>> List = extends AbstractList = implements Vector Stack

4. Use Composition <<Interface>> Iterable = extends <<Interface>> Collection = implements Abstract Collection <<Interface>> List = composes ArrayStack AbstractList ArrayStack inherits Collection operationsand uses List operations in its implementation Vector ArrayList Stack

5. Implement with Inheritance public class Stack<E> extends Vector<E>{ public E pop(){ return this.remove(this.size() – 1); } public void push(E newElement){ this.add(newElement); } public E peek(){ return this.get(this.size() – 1); } // The rest, including the constructor, // are inherited } Vector Stack

6. Implement with Composition public class ArrayStack<E> extends AbstractCollection<E>{ private List<E> list; public ArrayStack(){ list = new ArrayList<E>(); } public E pop(E newElement){ list.remove(list.size() - 1); } public void push(E newElement){ list.add(newElement); } public E peek(){ return list.get(list.size() – 1); } Use list instead of this AbstractCollection ArrayStack ArrayList

7. Implement with Composition public class ArrayStack<E> extends AbstractCollection<E>{ private List<E> list; public ArrayStack(){ list = new ArrayList<E>(); } public int size(){ return list.size(); } public boolean add(E newElement){ this.push(newElement) return true; } public Iterator<E> iterator(){ // We will get to this soon! } Last three methods required by AbstractCollection AbstractCollection ArrayStack ArrayList

8. Other Implementations of Stacks <<Interface>> Iterable <<Interface>> Collection Abstract Collection LinkedStack ArrayStack LinkedList ArrayList

9. Add a Single Stack Interface <<Interface>> Iterable <<Interface>> Collection = extends <<Interface>> TrueStack Abstract Collection = implements LinkedStack ArrayStack LinkedList ArrayList

10. Using the Stacks TrueStack<String> s1 = new ArrayStack<String>(); TrueStack<Integer> s2 = new LinkedStack<Integer>(); // Push a bunch of ints onto s2 for (int i : s2) s1.push(i + ""); TrueStack<String> s3 = new LinkedStack<String>(); s3.addAll(s1); The for loop and the method addAll come from AbstractCollection

11. Defining a Stack Interface public interface TrueStack<E> extends Collection<E>{ public E pop(); public void push(E newElement); public E peek(); } <<Interface>> Iterable <<Interface>> Collection <<Interface>> TrueStack

12. Implementing a Stack Interface public class ArrayStack<E> extends AbstractCollection<E> implements TrueStack<E>{ // As before } <<Interface>> Iterable <<Interface>> Collection <<Interface>> TrueStack Abstract Collection ArrayStack