comp1004: building better objects ii
DESCRIPTION
Comp1004: Building Better Objects II. Encapsulation and Constructors. Coming up. Recap Variables, Scope and Methods Accessor Methods Getters and Setters Encapsulation public and protected Constructors. Recap. Scope. public class Account{ int balance = 100; - PowerPoint PPT PresentationTRANSCRIPT
Comp1004: Building Better Objects IIEncapsulation and Constructors
Coming up• Recap– Variables, Scope and Methods
• Accessor Methods– Getters and Setters
• Encapsulation– public and protected
• Constructors
Recap
public class Account{
int balance = 100;
public void withdrawFiver(){balance = balance -5;
}
public void withdrawTenner(){int tenner = 10;balance = balance – tenner;
}
}
Scope
Member variables are visible in all the methods of a class.
They are created when an object is created, and destroyed when the object is garbage collected
public class Account{
int balance = 100;
public void withdrawFiver(){balance = balance -5;
}
public void withdrawTenner(){int tenner = 10;balance = balance – tenner;
}
}
Scope
Local variables are only visible in the method in which they are declared.
They are created when the method is invoked, and destroyed when it ends.
Member variables are visible in all the methods of a class.
They are created when an object is created, and destroyed when the object is garbage collected
Primitives Objects
Defined? defined in Java defined in Classes
Stored? stored directly in variables a reference is stored in the variable
Passed? Pass by copy Pass by reference
b
Elephant
a
int
10int a;a = 10;
Elephant b;b = new Elephant();
Primitives vs. References
Methods and Parameterspublic class Account{
int balance = 100;
public static void main(String[] args){Account myAccountObject = new Account();myAccountObject.withdraw(5);myAccountObject.withdraw(10);
}
public void withdraw(int amount){balance = balance - amount;
}}
Values received by a method are called parameters.
Within the method they can be used like any other local variable
Values passed into a method are called arguments
Methods and Parameterspublic class Account{
int balance = 100;int overdraft = 100;
public static void main(String[] args){Account myAccountObject = new Account();myAccountObject.withdraw(5);int funds = getAvailableFunds();System.out.println(funds);
}
public void withdraw(int amount){balance = balance - amount;
}
public int getAvailableFunds(){return balance + overdraft;
}}
You can use a return type to return a single value from a method
Overloadingpublic class Account{
int balance = 100;
public static void main(String[] args){Account myAccountObject = new Account();myAccountObject.withdraw(5);myAccountObject.withdraw(10);
}
public void withdraw(int amount){balance = balance - amount;
}
public void withdraw(int amount, String desc){balance = balance - amount;System.out.print(“Withdrew £”);System.out.print(amount);System.out.print(“ via ”);System.out.println(desc);
}}
Several Methods can have the same name as long as their signatures (types and order of parameters) are different.
Methods can take multiple parameters
Accessor Methods and Encapsulation
Getters and Setters
• Programming convention• Puts a method between users of an object and the
data inside the object
int a = student.age;int a = student.getAge();
student.age = 20;student.setAge(20);
aka
• These are also known as Getters and Setters– Or more formally as Accessors and Mutators.
• You’ll find them a lot
• The library classes that Java provides follow this convention often.
But Why Do It?
• What reason is there to write this extra code, when you can use the class variables directly?
public int getAge(){return age;
}
Encapsulation• This is the idea that a class
should be responsible for managing itself
• And should present a clean interface (set of methods) to the outside world
• An interface that does not reveal the inner workings of the class
Encapsulation• This is the idea that a class
should be responsible for managing itself
• And should present a clean interface (set of methods) to the outside world
• An interface that does not reveal the inner workings of the class
This means that you can change the way that the class works, without changing any other parts of the program
Results in cleaner, more portable code, that is more maintainable and reusable.
For exampleTake this example class where age is modelled as an int
public class Student {
int age = 20;
//code omitted
public static void main(String[] args){Student s1 = new Student();System.out.println(s1.getAge());
}
public int getAge(){return age;
}
}
For examplepublic class Student {
//int age = 20;Calendar dateOfBirth;
//code omitted
public static void main(String[] args){Student s1 = new Student();System.out.println(s1.getAge());
}
//public int getAge(){// return age;//}
public int getAge(){Calendar rightNow = Calendar.getInstance();int a = calculateAge(rightNow, dateofBirth);return a;
}}
Take this example class where age is modelled as an int
We might change the way that age is implemented – e.g. to make it based on the current date.
Because we used an Accessor we do not need to alter main
Enforcing Good Behaviour
• So we have used methods to hide the way that our class is implemented
• But what happens if we forgot and accidently access the variable directly?
• Or if someone else uses our class in their program and ignores the accessor methods?
Protectedpublic class Student {
//int age = 20;protected Calendar dateOfBirth;
//code omitted
public static void main(String[] args){Student s1 = new Student();System.out.println(s1.getAge());
}
//public int getAge(){// return age;//}
public int getAge(){Calendar rightNow = Calendar.getInstance();int a = calculateAge(rightNow, dateofBirth);return a;
}}
The protected keyword tells Java that only methods in this class* can access this variable.
*and its sub-classes, but we’ll come to that later in the course…
Protectedpublic class Student {
//int age = 20;protected Calendar dateOfBirth;
//code omitted
public static void main(String[] args){Student s1 = new Student();System.out.println(s1.getAge());
}
//public int getAge(){// return age;//}
public int getAge(){Calendar rightNow = Calendar.getInstance();int a = calculateAge(rightNow, dateofBirth);return a;
}}
The protected keyword tells Java that only methods in this class* can access this variable.
*and its sub-classes, but we’ll come to that later in the course…
And yes, public means the opposite – that all other methods can access it!
Encapsulation• The idea that classes look after themselves and hide their
internal workings
• Is a key concept in Object Oriented Programming
• And is enforced using the public or protected keywords*
• Which can be applied to both member variables and methods
*There is also a private keyword (we come to that later too)
A Hint
• Good OO programmers are paranoid
• If you want to make good encapsulated classes treat everything as protected
• Only open up methods or member variables as public if you really want them to be accessed outside of the class.
Constructors
public class Student {
protected int age = 20;
//code omitted
public static void main(String[] args){Student s1 = new Student();System.out.println(s1.getAge());
}
public int getAge(){return age;
}}
Back at our simple Student Example
We have a problem – all our students will be age 20
public class Student {
protected int age = 20;
//code omitted
public static void main(String[] args){Student s1 = new Student();s1.setAge(19);System.out.println(s1.getAge());
}
public int getAge(){return age;
}
public void setAge(int a){age = a;
}}
Back at our simple Student Example
We have a problem – all our students will be age 20
One solution is to use a Setter method. But Is this good encapsulation?
public class Student {
protected int age = 20;
//code omitted
public static void main(String[] args){Student s1 = new Student();s1.setAge(19);System.out.println(s1.getAge());
}
public int getAge(){return age;
}
public void setAge(int a){age = a;
}}
Back at our simple Student Example
We have a problem – all our students will be age 20
One solution is to use a Setter method. But Is this good encapsulation?
No – object creation is in two steps. Someone could forget to set it before using the object.
We need a way to force a program to set the age at the point of object creation
Constructors
• Look a bit like methods
• Contain code that is called during the creation of objects
• Is used to initialise objects and set critical member variables
• This maintains Encapsulation
Constructors
public class Student {
protected age;
public Student() {age = 20;
}
//code omitted}
Constructor Rules:
• Must have the same name as the class
• Do not need a return type
Constructors
public class Student {
protected age;
public Student() {age = 20;
}
public Student(int a) {age = a;
}
//code omitted}
Constructor Rules:
• Must have the same name as the class
• Does not need a return type
• Can take parameters
• Can be overloaded
Constructors
public class Student {
protected age;
public Student() {age = 20;
}
public Student(int a) {age = a;
}
public static void main(String[] args){Student s1 = new Student(19);System.out.println(s1.getAge());
}
//code omitted}
Constructor Rules:
• Must have the same name as the class
• Does not need a return type
• Can take parameters
• Can be overloaded
• Are invoked at the point of creation using the new keyword
The Default Constructor• What happens if you do not declare a constructor?
• In these cases Java creates an invisible default constructor for you
• The default constructor takes no parameters and contains no behavior. But allows you to create Object instances
• We’ve been using them from day one– Dog d = new Dog();
Summary• Recap– Variables, Scope and Methods
• Accessor Methods– Getters and Setters
• Encapsulation– public and protected
• Constructors