CS 211 Generics

1. CS 211Generics

2. Today’s lecture • Review of generics • Go over examples

3. Generics • What problem does using generics solve? • What do they look like? • What do they mean?

4. Problem: "lost" types • This issue would always arise when we remove things from the ArrayList. It's even more annoying with the for-each syntax:ArrayListpersonList = new ArrayList();// add many Person objects//NOT ALLOWED: for (Person p : personList) {p.whatever(); } • Instead, we must cast later, like this: // allowed, but annoying, harder to read, and error-prone. for (Object p : personList){ ((Person) p).whatever(); }

5. Declaring Generic Types We can add a generic type to a class definition: public class Foo <T> { // T can be any where: like field types.public T someField; public Foo(T t) {this.someField = t; } // can use T now for return types and param types. public TdoStuff(T t, int x) { … }}

6. Generics Example: ArrayList Here is an idealized definition of ArrayList. Fake versions are marked with †. public class ArrayList<E> { private int size; private E[] items; public ArrayList(){ items = new E[10]; //† size = 0; } public E get(inti) { return items[i]; } public void set(inti, E e) { items[i] = e; } public void add (E e) { if (size>=items.length) { E[] longer = new E[items.length*2]; //† for (inti=0; i<items.length; i++){ longer[i] = items[i]; } items = longer; } items[size++] = e; } }

7. Generics Example: ArrayList Let's look at how we actually get to use generics with ArrayList:→ we need to instantiate the class's type parameter: //instantiate the type parameter with <>'s: ArrayList<String>slist = new ArrayList<String>(); //now use all methods without having to specify again. slist.add("hello"); slist.add("goodbye"); String elt = slist.get(0); System.out.println("some element: " + elt); System.out.println("the list: " + slist);

8. Let’s go over the examples

9. Questions?