1. Chapters 17-18 (ArrayList / Generic)
Big Java
Chapters 17-18
(ArrayList / Generic)

2. Generic Programming
The design and implementation of data structures and algorithms that work for multiple types.

3. Generic Class Uses a type variable to achieve genericity
public class ArrayList<E> {
public ArrayList() { . . .}
public void add(E element) { } }
may have one or more
Can instantiate type variable with class or
interface type, not primitive (use wrapper)

4. Example
ArrayList<BankAccount> accounts1 = new ArrayList<BankAccount>();
LinkedList accounts2 = new LinkedList();
// intended for BankAccounts
accounts1.add(“my savings”);
// compiler error – WHY ?
accounts2.add(“my savings”);
// not detected

5. Names for Type Variables
Often use short uppercase names, such as:
Type Name
Meaning E
Element in collection K
Key type in map V
Value type in map T
General type
S,U
Additional general types

6. Example – useful return type
public class Pair<T, S> {
public Pair(T firstElement, S, second Element)
first = firstElement;
second = second Element }
public T getFirst() { return first; }
public S getSecond() { return second; }
private T first;
private S second;

7. Generic Methods Method with a type variable
Useful for methods that differ only by one or more types
Can start with method that works on a specific types, then make generic

8. Generic Method Example
public static void print(String[] a) {
for (String e : a)
System.out.print(e + “ “);
System.out.println(); }
public static <E> void print(E[] a)
for (E e : a)
Rectangle[] rectangles = . . .;
ArrayUtil.print(rectangles);
Methods are not required to be static
Method call based on parameters,
doesn’t specify E explicitly
Assume in ArrayUtil class

9. Constraining Type Variables
actually means extends or implements
public static <E extends Comparable> E min(E[] a) {
E smallest = a[0];
for (int i=0; i<a.length; i++)
if (a[i].compareTo(smallest) < 0)
smallest = a[i];
return smallest; }
could also do:
<E extends Comparable & Cloneable>
if have more than one constraint

10. Genericity and Inheritance
If SavingsAccount is a subclass of BankAccount, is ArrayList<SavingsAccount> a subclass of ArrayList<BankAccount>?
NO. Necessary for type checking.
ArrayList<SavingsAccount> savingsAccounts =
new ArrayList<SavingsAccount>();
ArrayList<BankAccount> bankAccounts = savingsAccounts; // Not legal, but if it were…
BankAccount myChecking = new CheckingAccount();
BankAccounts.add(myChecking);
// just added checking account to savings account list

11. Wildcard Types – Advanced Topic
A wildcard type is a type that can remain unknown.
public void addAll(LinkedList<? extends E> other) {
ListIterator<E> iter = other.listIterator();
while (iter.hasNext())
add(iter.next()); }
Allows you to add any type that is a subtype of E.
Read Advanced Topic 17.1 for more details

12. Raw Types
The Java Virtual Machine (JVM) uses raw types in which type variables are replaced with ordinary types
The compiler converts generic classes and type variables into regular classes/raw types for the JVM
Type variables are replaced with Object, e.g.,
public Pair(Object firstElement, Object secondElement) { … }
Type erasure enables generic classes to interoperate with legacy code (e.g., original ArrayList).

13. Limitations
You cannot use type variables to define static fields, static methods or static inner classes
You cannot create a new object of a type variable type (remember it is replaced with Object)

14. Graphical User Interfaces…

15. BorderLayout JPanel panel = new JPanel();
panel.setLayout(new BorderLayout());
panel.add(component, BorderLayout.NORTH);
NORTH
WEST
CENTER
EAST
SOUTH

16. GridLayout JPanel buttonPanel = new JPanel();
buttonPanel.setLayout(new GridLayout(4, 3));
buttonPanel.add(button7);
buttonPanel.add(button8); … CE

17. Nesting Panels JPanel keypadPanel = new JPanel();
keypadPanel.setLayout(new BorderLayout());
JPanel buttonPanel = new JPanel();
buttonPanel.setLayout(new GridLayout(4, 3));
buttonPanel.add(button7);
buttonPanel.add(button8); …
keypadPanel.add(buttonPanel, BorderLayout.CENTER);
JTextField display = new JTextField();
keypadPanel.add(display, BorderLayout.NORTH); CE

18. Making Choices – Radios, Combos & Checks

19. Combo Box
editable – can type in
your own selection

20. Check Box & Border
Many other border types
available – see API

21. Radio Buttons Need ButtonGroup for grouping, only one
selected at a time

22. Control Panel for All Controls
It’s easy once the
other panels are
created!
controls are going in
the bottom

23. Controlling the Font access combo box for font name
access check boxes to
determine bold/italic settings
access radio buttons to
determine size settings – sometimes
separate field is used for this
repaint causes changes to be
visible

24. Putting it together
Same listener for
all components

25. Reading Assignment
How To 18.1 Laying Out a User Interface, pages – summarizes how to achieve an interface

26. Menus

27. File Menus
Creates menu item
Creates menu item
with listener

28. Menu and Item

29. Setting Font Note use of instance variables for size,
facename and style

30. Putting it all Together
Create a JMenuBar
and add menus to it