1. COMP204 Bernhard Pfahringer (with input from Robi Malik)
Java Generics
COMP204 Bernhard Pfahringer
(with input from Robi Malik)

2. Generic types for collections
Old Java (1.4 and older):
List strings = new ArrayList();
strings.add(“hello”);
String word = (String) strings.get(0);
New (since 1.5):
List<String> strings = new ArrayList<String>();
String word = strings.get(0);

3. Advantages Better readability
Better type-safety: no casts (runtime checks), compiler can already catch problems

4. Writing your own generic code
public class Stack<E> {
public void push(E element) {
contents.add(element); }
public E pop() {
int top = contents.size()-1;
E result = contents.get(top);
contents.remove(top);
return result;
private List<E> contents = new ArrayList<E>();

5. Formal type parameter public class Stack<E> { … }
convention: Short (single-char) uppercase
can be used wherever a Type is needed
will be replaced with actual Type

6. Problems with sub-types
class Student extends Person { .. }
List<Student> students = new ArrayList<Student>();
List<Person> people = students;
// should this be possible? -> no

7. No, because Person sam = new Person(); people.add(sam);
// if this was legal, we would have just sneaked a non-student onto the students list
on the other hand, how do you write generic code then accepting lists of sub-types of Persons ???

8. Wildcards void printCollection(Collection<?> c) {
for(Object o: c) {
System.out.println(o); }
// only read access (everything is an Object), but cannot add, because do not know correct type (except null, which is any type)

9. Bounded wildcards
public void drawAll(Collection<? extends Shape> c) {
for(Shape shape: c) {
shape.draw(); }
// again, only read access, allows collections of Shape or any sub type of Shape