1. Java and OOP
Part 5 – More

2. More OOP More OOP concepts beyond the introduction: constants this
clone and equals
inheritance
exceptions
reflection
interfaces

3. Constants final is used for classes which cannot be sub-classed
Data members can also be declared final
Which means they are constants – for example
public final double π = ;
Also note you can use Unicode in source code

4. field1 of the object carrying out someMethod
this
Keyword this is a reference to the object executing the current method
Usually this can be omitted
public void someMethod() {
field1=99;
field2=32; }
field1 of the object carrying out someMethod
We could have said
this.field1=99;

5. Example of this public void putInQueue() { q.add(this); }
This might be a method to place an object in a queue.
The queue has a method add, and we need to pass to it a reference to the object to be added to the queue

6. Exercise Set up code to do the last example. Follow these steps -
Define a Queue class. This holds objects in an array. Have 3 data members - the current size (how many objects currently in the array), a max size, and the array itself. Have a constructor which takes as argument the max size required. Have an add method which adds an object to the array, and a display method
Define a QueueableObject class. It has just one data member - which Queue object it will be stored in. It has just one method - putInQueue
In main(), make 1 Queue. Then make some Queueable objects, add them to the queue, and display the queue.

7. object = object? Two distinct questions -
are 2 objects the same? (Do 2 references refer to th esame object)
do 2 objects contain equal data?

8. Example .equals Integer i1 = new Integer(4);
if (i1.equals(i2)) YES
System.out.println("Contain same data");
if (i1 == i2) NO
System.out.println("Same object");

9. Example = Integer i1 = new Integer(4); Integer i2=i1;
if (i1.equals(i2)) YES
System.out.println("Contain same data");
if (i1 == i2) YES
System.out.println("Same object");

10. Over-riding clone public class CloneableClass {
public CloneableClass(int i) {
field1=i; }
public CloneableClass clone() {
CloneableClass newOne = new CloneableClass(field1);
return newOne;
private int field1;
CloneableClass c1 = new CloneableClass(9);
CloneableClass c2 = c1.clone();

11. Danger - why is this wrong?
public class CloneableClass {
public CloneableClass clone() {
CloneableClass newOne = new CloneableClass();
newOne.field1=this.field1;
return newOne; }
public int[] field1 = {1,2,3};

12. Because.. CloneableClass c1 = new CloneableClass();
CloneableClass c2 = c1.clone();
c1.field1[1]=99;
System.out.println(c2.field1[1]); // outputs 99
c1 object
c2 object
same array - we didn't make a new one
the field1 array

13. Exercise
Copy the CloneableClass and fix its clone method

14. Exceptions exception = unusual situation
here program cannot execute 'normally'
with a more-or-less external cause
not a program bug
eg file not found
connection with server lost
out of memory
broken Java machine

15. Exceptions Some methods are declared to 'throw' an exception
Compiler will insist code 'catches' checked exception
Code form -
try {
.. call of method that may throw ExceptionType }
catch ( ExceptionType e) {
.. code to deal with exception

16. Example try catch FileReader fr = null; try {
fr= new FileReader("test.dat"); }
catch (FileNotFoundException fe) {
System.out.println("Cant find file");
FileReader is in package java.io
Note declare fr outside try and initialise
Else compiler complains fr may not be initialised

17. Exercise A FileReader inherits a method called read
read takes a char[] as argument
Look at the documentation
Extend the code on the last slide to display the file

18. More on inheritance
Suppose we have a base class called BaseClass, and that SubClass extends this.
What do we get if we say:
BaseClass s = new SubClass();

19. More on inheritance
Suppose we have a base class called BaseClass, and that SubClass extends this.
What do we get if we say:
BaseClass s = new SubClass();
Then s is an object which is 'really' a SubClass
But its been declared to be a BaseClass
What happens?
We get the subclass methods, but
The base class fields
For example

20. Example public class BaseClass { public void method() {
System.out.println("Base class method"); }
public int field=1;}
public class SubClass extends BaseClass {
public void method()
System.out.println("Subclass method"); }
public int field = 2;
BaseClass s = new SubClass();
s.method();
System.out.println(s.field);
outputs:
Subclass method 1

21. Reflection
Sometimes your code must deal with objects when you do not know what type they are
Reflection enables you to ask an object what class it is
A Class object carries info about a class
The Object class has a method called getClass
So all classes have this - eg
BaseClass s = new SubClass();
Class theClass= s.getClass();
System.out.println(theClass); .. outputs
class test.SubClass (SubClass was defined in package test)

22. This example shows how we can trace up a class hierarchy
See the doc of Class for more info
public static void main(String[] args) {
BaseClass s = new SubClass();
Class theClass= s.getClass();
upTree(theClass); }
public static void upTree(Class someClass) {
if (someClass==null) return;
System.out.println(someClass);
upTree(someClass.getSuperclass());
output:
class test.SubClass
class test.BaseClass
class java.lang.Object
Using Class objects

23. Reflection exercise Look at the doc of Class
Use Class.forName and .getMethods to output the methods of a given class
Note .forName needs package eg
theClass= Class.forName("java.lang.Class");
and need to catch exception

24. No multiple inheritance
In some languages (C++) a class can inherit from more than one super class
Called multiple inheritance
Cannot do it in Java
Idea of interface kind of substitute
Enables a class to 'be like' more than one class interface
Interfaces heavily used in Swing - simpler than what follows..

25. What is an interface Like a ghostly class
An interface has a set of methods (names, arguments, return types)
The interface declares but does not define the methods
A class can be declared to implement the interface
It must then define the methods in the interface
Or the compiler will complain
Classes can implement several interfaces

26. Example interface Comparable just one method -
int compareTo(Object obj)
should return negative, 0 or positive depending on whether obj comes before, is =, or comes after

27. Example implementation
public class Order implements Comparable {
public Order(double amount, int id, int day, int month, int year)
value=amount;
key = id;
date=new GregorianCalendar();
date.set(year, month, day); }
public int compareTo(Object obj)
Order otherOne = (Order) obj;
return date.compareTo(otherOne.date);
private GregorianCalendar date;
private double value;
private int key;
Calendar implements Comparable

28. Using a Comparable class
Order order1 = new Order(34.50, 1, 2, 1, 2004);
Order order2 = new Order(44.50, 2, 1, 1, 2004);
System.out.println(order1.compareTo(order2));
get 1
Exercise
Copy the Order class
Change it so it compares on order value not order date
Test it

29. Variables of interface type
You can declare variables of a type which is interface not a class eg
Comparable someObj;
But you can't refer to any method other than that in the interface
Enables you to write routines which work on any object which implements the interface
So you could write a Sorter class which would sort arrays of any Comparable type (like a template in C++):

30. Example : the Sorter class
public class Sorter {
public static void sort(Comparable[] things) { ..
.. the sort method.. }
in use..
String[] array = {"ppp", "a", "aaa", "ccc"};
Sorter.sort(array);
for (int i = 0; i<array.length; i++)
System.out.println(array[i]);
output - a
aaa
ccc
ppp

31. Exercise Copy the Sorter class
Fill in the sort method eg a bubble sort
Check it works
Check it works with arrays of other Comparable types eg Integer