Supplier Classes

1
Supplier Classes

• Chapter 6
• The Object of Java

2
Clients and Suppliers

• A client class borrows the facilities (i.e.
  accesses state or methods) from a supplier class.
• Supplier classes can be used by the client class
  in any of the following ways
• A variable type
• A parameter type
• A return type

3
Composition

• When a client class uses a supplier class as one
  of its variables, this is known as composition.
• In UML notation, this is shown by

Supplier class
Client class
4
Client-Supplier Example (1)

• public class LibraryBook
• String title
• List waitingList
• ...
• String and List are supplier classes used by the
  client class LibraryBook

5
Client-Supplier Example (2)
LibraryBook

String
List


6
Creating Supplier Classes

• Use defensive programming protect supplier code
  from undesirable use by the client
• One way to do this is to use appropriate
  accessibility modifiers on variables and methods
• Many times, methods are the only public members
  of a supplier class. But in general, use the
  need to know concept to determine visibility.
• All members of a class are said to be
  encapsulated in that class.

7
Need To Know Example (1)

• public class LibraryBook
• private String title
• private List waitingList
• ...
• public Member nextMember()
• if (!waitingList.isEmpty())
• return waitingList.get(0)
• else
• return null

8
Need To Know Example (2)

• Clients of LibraryBook do not need to know that
  LibraryBook contains a List.
• They only really need to know that we can return
  the next library member on the waiting list
  through the nextMember() method call.
• This is why waitingList is declared with the
  private accessibility modifier.

9
Another Need To Know Example (1)

• You should declare local variables (i.e.
  variables declared inside the body of methods)
  whenever possible instead of declaring instance
  variables.
• If a variable only has relevance inside a certain
  method, dont share it with other methods.
• Even if you know what youre doing, other coders
  can subclass your class and incorrectly use
  instance variables that couldve been declared
  local.
• This is another example of defensive programming.

10
Another Need To Know Example (2)

• public class Circle
• protected float area
• protected float radius
• public void setRadius(float radius)
• this.radius radius
• public float area()
• area Math.PI radius radius
• return area

11
Another Need To Know Example (3)

• public class Circle
• // protected float area
• protected float radius
• public void setRadius(float radius)
• this.radius radius
• public float area()
• float area Math.PI radius radius
• return area

12
Information Hiding

• Information hiding is when you declare variables
  local or private.
• In this way, clients cannot access those
  variables.
• Information hiding leads to thin interfaces.
• In general, an interface to a class is composed
  of the variables and methods that are declared
  public, protected, and default.
• These members are accessible to clients. By
  declaring some variables local or private, we
  keep the interface thin by reducing the size of
  the contract with clients.

13
Accessor Methods

• Accessor methods are methods that provide read
  and write access to private variables.
• Accessor methods are sometimes referred to as
  getter (read) and setter (write) methods.

14
Read-Only Access Example (Getter)

• public class Person
• private int age
• public int getAge()
• return age

15
Write-Only Access Example (Setter)

• public class Person
• private int age
• public void setAge(int age)
• this.age age