1. Coming up Constructors Overloading With one parameter
With two parameters
Overloading
Constructors and methods

2. Lecture 5
Objects Ahoy OO

3. Coming up Constructors Overloading With one parameter
With two parameters
Overloading
Constructors and methods

4. Constructors OO Look a bit like methods: Arguments No return type
public class DVD{
public DVD(){ }
Arguments
No return type
Same name as class

5. What are they used for? OO
The constructor contains the code that is run when you create an object
The constructor runs before the object is assigned to a reference
It lets you set it up for use

6. Why not just use set methods?
Okay, so lets make a Table (one that you’d eat at) OO

7. public class Table{ int size; public void sitAt(int numOfPeople){ //Sit some people at the table } public void setSize(int theSize){ size = theSize; public class UseTheTable{ public static void main(String[] args){ Table t = new Table(); t.sitAt(8); t.setSize(3);
What happens if....

8. The problem OO The object creation is two step
Make the object
Set the size
If someone else uses your class
Will they remember?
Or will there be 8 people trying to eat off a coffee table?

9. It’s worse OO When you declare a data type
It defaults to its default value...
Numbers default to 0
Boolean defaults to false
Objects default to null
So the last example would have 8 people seated at a table of size 0!

10. These constructors seem pretty important....OO
These constructors seem pretty important....
So why haven’t I used/heard of them before?
You have.
Table t = new Table();
new calls the constructor

11. But I didn’t write any constructors when I called new!
Java helpfully puts in a blank constructor for you.

12. public class Table{ int size; public void sitAt(int numOfPeople){ //Sit some people at the table } public void setSize(int theSize){ size = theSize; public Table(){
Java’s helpful hidden constructor

13. So now we know its there... .....We can use it
Set the table size to a small table when it is created

14. public class Table{ int size; public void sitAt(int numOfPeople){ //Sit some people at the table } public void setSize(int theSize){ size = theSize; public Table(){ size = 1;

15. Or we can pass it an argument....

16. public class Table{ int size; public void sitAt(int numOfPeople){ //Sit some people at the table } public void setSize(int theSize){ size = theSize; public Table(int sizeToSet){ size = sizeToSet;

17. Or two arguments...

18. Uh oh... They have the same name...
public class Table{ int size; String wood; public void sitAt(int numOfPeople){ //Sit some people at the table } public void setSize(int theSize){ size = theSize; public Table(int sizeToSet){ size = sizeToSet; public Table(int sizeToSet, String woodType){ wood = woodType;
Uh oh...
They have the same name...

19. That can’t work, can it? OO
How would the program know which method I wanted to call? They both have the same name
Actually it does work
You can do the same thing with methods

20. public void sitAt(int numOfPeople){ numOfSitters = numOfPeople; }
public class Table{
int size;
String wood;
String partyName;
int numOfSitters;
public void sitAt(int numOfPeople){
numOfSitters = numOfPeople; }
public void sitAt(int num, String name){
partyName = name
public void setSize(int theSize){
size = theSize;
public Table(int sizeToSet){
size = sizeToSet;
public Table(int sizeToSet, String woodType){
wood = woodType;

21. Compiler error! Which method is called? OO
If you call someTable.sitAt(4);
If you call someTable.sitAt(4, “Martin”);
If you call someTable.sitAt();
public void sitAt(int numOfPeople){
numOfSitters = numOfPeople; }
public void sitAt(int num, String name){
partyName = name
Compiler error!

22. This is called OO Overloading
A method is recognised by its return type and its signature
ie the stuff in the brackets, like (int, String)

23. Coming up Constructors Overloading With one parameter
With two parameters
Overloading
Constructors and methods

24. Overloading Rules OO Return types can be different
As long as the arguments are different too
i.e. You can’t just change the return type
You can change the access levels to more or less restrictive
i.e. You can change public to private, and vice versa
Overloading is different to another principle, overiding, which we cover in inheritance.