Notes for Java(Part d)

八、Enhancing classes

Overloading Constructors

•         Recall method overloading?

•         Like methods, constructors can be overloaded

•         An overloaded constructor provides multiple ways to set up a new object

Die die1 = new Die();         //creates a 6 sided die

Die die2 = new Die(20);       //creates a 20 sided die

Object-Oriented Analysis

•         One of the simplest ways to do object-oriented analysis involves finding nouns and verbs in the program’s specification.

•         Many of the nouns will represent classes, and many of the verbs will represent methods.

•         This simple technique doesn’t work perfectly, but it can be useful for generating ideas.

•         So an initial analysis of a banking system might reveal:

•         Nouns ie suggested classes in the specification for a banking system:

•         Customer

•         Account

•         Verbs ie suggested methods in the specification for a banking system:

•         Open (an account)

•         Deposit (money into an account)

•         Withdraw (money from an account)

•         Close (an account)

Categories of Methods

•         Most instance methods fall into one of several categories:

–        Manager methods

–        Implementor methods

–        Access methods

Manager Methods

•         Initialise objects

–        Constructors

–        Having additional constructors makes a class more useful.

Implementor Methods

•         Implementor methods represent useful operations that can be performed on instances of the class.

•         Implementor methods usually change the state of an object

•         The Account class  implementor methods:

–       open,

–       deposit,

–       withdraw,

–        close.

Access Methods

•         For access to instance variables, the choice is:

–        provide instance methods or

–        make the variables public (but this violates good object orientation)

•         Accessors (or getters)

–        methods that return the value of a private variable.

–        by convention, names of getters start with the word get

•         Mutators (or setters)

–        methods that change the value of a private variable.

–        by convention, names of setters start with the word set

The this Reference

•         Because an instance method is called by an object, the correct instance data is accessed

            double balance  = acct1.getBalance();

•         The keyword this is used to refer to the object that invoked the method

•         In the above, the this reference refers to acct1

•         The following two methods are actually identical. The first is using the this reference implicitly, the second explicitly

public double getBalance ()

{

   return balance;

}

public double getBalance ()

{

   return this.balance;

}

Variable Type Differences

Local variables and parameter variables

                        int result = 0;

                        public Account (String owner, int account, double initial)

•         Are declared within a method

•         Only exist for the duration of the method’s execution

•         Need to be initialised

–        A method is not allowed to access the value stored in a local variable until the variable has been initialised

Instance variables

•         belong to an object

–        are created when an object is constructed

–        exist as long as the object exists

•         each object has its own copy

–        that’s what allows objects to be different

•         are automatically initialised (if not explicitly set in a constructor),

–        zero for numeric and char variables

–        false for boolean variables

–        null for reference type variables

•         every instance variable should be initialised in a constructor

Hidden Variables

•         We cant declare a variable with the same name as one already in scope

–        always use different variable names except in headers of for loops

•         Beware of variable names with instantiable classes!!!

•         An instance variable and a local variable could have the same name –

–        the local variable with the inner scope will take precedence over (or hide) the variable in the outer scope (instance variable)

this used in Constructors

•         The this reference can be used to refer to the instance variable of the object

                        private long acctNumber;

                        private double balance;

                        private String name;

                        public Account(String owner, long account, double balance)

                        {         

                             this.balance = balance;    // and so on for the other variables

                        }

•         This approach is eliminates the need for different yet equivalent names

•         It is often found in constructors

The static Modifier

•         We have used methods that do not require objects

–         eg methods of Math class

•         These static or class methods are invoked through the class name - not through an object name

•         To make a method static, we apply the static modifier to the method definition

                        static double sqrt (double num)

•         And call the method by the class name

            System.out.print (“sq rt of 9: “ + Math.sqrt(9));

Static (or Class) Variables

•         The static modifier can be applied to variables as well

•         It associates a variable with the class rather than an object

•         They are declared with the word static

•         Normally, each object has its own data space

•         If a variable is declared as static, only one copy of the variable exists

•         Eg in the Account class a useful class variable may be

                        private static double totalDeposits;

•         All objects created from the class share access to the static variable

•         Changing the value of a static variable in one object changes it for all others

•         Eg to allocate a unique asset number every time a new asset object is created

–        A static variable could hold the last used asset number

–        Any new asset could increment this number by 1

Arrays of objects

•         Because each object has a unique identifier we cant use them in loops as we would do with variables

                        Account acct1 = new Account((“J Bond", 72354, 102.56);

                        acct1.deposit(100.00);

•         By storing objects in an array we can use the power of loops to process multiple objects

•         Create an array of objects        

                        Account[ ] acctArray = new Account[3];

•         No actual objects have been created at this time

–        This is merely an array waiting to receive Account objects

•         We need to create each object in the usual way

–        Call the constructor of the Account class

–        Pass it any parameters it requires

–        Assign it to a position in the array

            acctArray[index] = new acctArray(name, number, balance);

•         After you have instantiated the array with actual objects

–        You can use getters and setters or other methods from the Account class to return or change instance variables

            balance = acctArray[index].getBalance();or

            System.out.println(acctArray[index].getBalance());

Using an array as a database

•         Use a for loop to search every position in an array

                        int index;

                        for (index = 0; index < acctArray.length; index ++)

                        {

                               if (acctArray[index].getAcctNumber == 72354)

                            break;

                        }

•         When the loop has terminated

–        Test whether index is less than acctArray.length

–        if so index indicates the position of the record in the array

–        Use index to modify that object

            acctArray[index].deposit(amount);

Comparing Strings for equality

•         Remember to use the equals() method not ==

                        if (acctArray[index].getName() .equals (“J Bond”)

 

九、Inheritance

•         Genetic inheritance - facts

–        Eg Your blood type, hair colour, height, body shape

–        Many facts about you (ie, your instance data) are inherited

•         Genetic inheritance – behaviour

–        eg Sports and leisure interests, loudly or quietly spoken

–        Much of your behaviour (ie your methods) are inherited

•         Many objects have similarities to other objects

–        Eg cars and trucks are similar and examples of vehicles

•         A class (car) can have a ‘parent’ (vehicle) from which it inherits some of its data and behaviour

•         Inheritance allows a software developer to derive a new class from an existing one   

•         The existing class is called the parent class, base class or superclass

•         The derived class is called the child class or subclass

• As the name implies, the child inherits characteristics (the methods and data) of the parent

• Inheritance relationships are often shown graphically in a class diagram, with the arrow pointing to the parent class

• Inheritance should create an is-a relationship, meaning the child is a more specific version of the parent

•         Creating employee objects from the Employee class

                        Employee accountant = new Employee();

                        Employee programmer = new Employee();

•         Other employees may require additional data to number and salary eg company car registration number

•         We could change the class to have

–        3 instance variables (empNumber, empSalary, empCarRego)

–        6 methods (getters and setters for the 3 variables)

•         It is better programming to keep it as it is and reuse it

•         Create a new class EmployeeWithCar that inherits all the instance variables and methods of the Employee class

•         Then just add the extra instance variable and methods

Deriving Subclasses

•         In Java, we use the reserved word extends to establish an inheritance relationship

            public class EmployeeWithCar extends Employee

            {

                           // class contents

            }

Advantages of inheritance

•         Saves time

–        reusing data and methods that already exist

–        makes programs easier to write

•         Reduces errors

–        methods have already been used and tested

•         A simple Account class may have this instance variable:

            private double balance;

•         And these methods:

            public double deposit (double amount)

            public double withdraw (double amount, double fee)

            public double getBalance ()     

       A SavingsAccount class may want to use these features and also have an interest rate

•         A SavingsAccount is an Account, but with more features eg it has an interest rate

Writing Subclass Constructors

•         A subclass doesn’t inherit constructors from its superclass

•         The constructor for the SavingsAccount class will need to initialize both the balance and interestRate variables

–        But the superclass instance variables are likely to be private so they cannot be referenced directly from the subclass

•         The hard part of writing a constructor for a subclass is initializing the variables that belong to the superclass

•         Writing constructors for a subclass is the same, except for initialisation of superclass variables (if private)

                        public SavingsAccount (double initialBalance,  double initialRate)

                        {

                                    balance = initialBalance;      //ERROR

                                    interestRate = initialRate;

                        }

•         As balance was declared as private in the Account class this will not compile – the SavingsAccount class has no access to it

The super Reference

•         A child’s constructor is responsible for calling the parent’s constructor

•         The first line of a child’s constructor should use the super reference to call the parent’s constructor

•         To access a superclass private instance variable we need to use a superclass method…or in this case constructor

•         We want to use the parent's constructor to set up the "parent's part" of the object

•         The super reference can be used to refer to the parent class, and is often used to invoke the parent's constructor

•         The first line of a child’s constructor uses the super reference to call the parent’s constructor

Invoking the superclass constructor

•         The SavingsAccount constructor invokes the Account constructor using the word super

                        public SavingsAccount (double initialBalance,  double initialRate)

                        {

                                    super (initialBalance);    //invoke Account constructor

                                    interestRate = initialRate;

                        }

•         The account constructor will initialise the balance variable to the value stored in initialBalance

Superclass constructor parameters

•         The subclass constructor must provide all the parameters required by the superclass constructor in order eg.

•         If a subclass constructor fails to include super, the compiler will automatically insert super(); at the beginning of the constructor.

•         If a subclass has no constructors at all, the compiler will create a no-arg constructor that contains super(); but no other statements.

Controlling Inheritance

•         Visibility modifiers determine which class members get inherited and which do not

•         Variables and methods declared with public visibility are inherited

•         Those with private visibility are not actually inherited in that we need to use the superclass constructor and methods to access them

•         But public variables violate our goal of encapsulation!

The protected Modifier

•         There is a third visibility modifier that helps in inheritance situations: protected

•          protected  behaves the same as private within a class but allows a member of a base class to be inherited into the child class

private  can only be accessed in the same class

protected          can only be accessed in the same class or in a subclass

public   can be accessed in any class

•         Declaring instance variables protected exposes them to all subclasses

•         This is a potentially unlimited number and may weaken an object-oriented goal of encapsulation

•         If a subclass needs access to these variables it should be able to call a getter or setter method of the superclass

 

•         We will not use protected variables

•         Always declare instance variables private

Methods available to a subclass

•         Inherit methods from the superclass

–        superclass methods can be applied to subclass objects

                                    account1.deposit(200.00);

•         Define new methods

                        public void setInterestRate(double  rate);

•         Override methods from the superclass

–        ie specify a method with the same signature (that is the same name, return type and parameter types) in the subclass

–        this method defined in the subclass will take precedence

–        Use super to call an overridden method

•         Java looks first in the class of the calling object, then in the class’s superclass, then its superclass etc

•         Consider the following statement:

                         account1.deposit(500.00);      

•         Java first looks for the deposit method in the SavingsAccount class, then in the Account class.

Overloading vs. Overriding

•         Don't confuse the concepts of overloading and overriding

•         Overloading deals with multiple methods in the same class with the same name but different signatures

•         Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature

•         Overloading lets you define a similar operation in different ways for different data

•         Overriding lets you define a similar operation in different ways for different object types

 

Never finish on Java, even not start.