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

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

PrimitivesObjects Defined?defined in Javadefined in Classes Stored?stored directly in variablesa reference is stored in the variable Passed?Pass by copyPass by reference b Elephant a int int a; a = 10; Elephant b; b = new Elephant(); Primitives vs. References

Methods and Parameters public 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 Parameters public 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

Overloading public 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 example Take 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 example public 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?

Protected public 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…

Protected public 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