Java tutorial 5

13. Inheritance

Inheritance can be defined as the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.

When we talk about inheritance, the most commonly used keyword would be extends andimplements. These words would determine whether one object IS-A type of another. By using these keywords we can make one object acquire
the properties of another object.

IS-A Relationship:

IS-A is a way of saying : This object is a type of that object. Let us see how the extendskeyword is used to achieve inheritance.

public class Animal{
}

public class Mammal extends Animal{
}

public class Reptile extends Animal{
}

public class Dog extends Mammal{
}

Now, based on the above example, In Object Oriented terms, the following are true:

Animal is the superclass of Mammal class.

Animal is the superclass of Reptile class.

Mammal and Reptile are subclasses of Animal class.

Dog is the subclass of both Mammal and Animal classes.

Now, if we consider the IS-A relationship, we can say:

Mammal IS-A Animal

Reptile IS-A Animal

Dog IS-A Mammal

Hence : Dog IS-A Animal as well

With use of the extends keyword the subclasses will be able to inherit all the properties of the superclass except for the private properties of the superclass.

We can assure that Mammal is actually an Animal with the use of the instance operator.

Example:

public class Dog extends Mammal{

public static void main(String args[]){

Animal a = new Animal();
Mammal m = new Mammal();
Dog d = new Dog();

System.out.println(m instanceof Animal);
System.out.println(d instanceof Mammal);
1ea19

System.out.println(d instanceof Animal);
}
}

This would produce the following result:

true
true
true

HAS-A relationship:

These relationships are mainly based on the usage. This determines whether a certain classHAS-A certain thing. This relationship helps to reduce duplication of code as well as bugs.

Lets us look into an example:

public class Vehicle{}
public class Speed{}
public class Van extends Vehicle{
private Speed sp;
}

This shows that class Van HAS-A Speed. By having a separate class for Speed, we do not have to put the entire code that belongs to speed inside the Van class., which makes it possible to reuse the Speed class in multiple applications.

In Object-Oriented feature, the users do not need to bother about which object is doing the real work. To achieve this, the Van class hides the implementation details from the users of the Van class. So basically what happens is the users would ask the Van
class to do a certain action and the Van class will either do the work by itself or ask another class to perform the action.

A very important fact to remember is that Java only supports only single inheritance. This means that a class cannot extend more than one class. Therefore following is illegal:

public class extends Animal, Mammal{}

However, a class can implement one or more interfaces. This has made Java get rid of the impossibility of multiple inheritance.
14. Overriding

In the previous chapter, we talked about super classes and sub classes. If a class inherits a method from its super class, then there is a chance to override the method provided that it is not marked final.

The benefit of overriding is: ability to define a behavior that's specific to the subclass type which means a subclass can implement a parent class method based on its requirement.

In object-oriented terms, overriding means to override the functionality of an existing method.

Example:

Let us look at an example.

class Animal{

public void move(){
System.out.println("Animals can move");
}
}

class Dog extends Animal{

public void move(){
System.out.println("Dogs can walk and run");
}
}

public class TestDog{

public static void main(String args[]){
Animal a = new Animal(); // Animal reference and object
Animal b = new Dog(); // Animal reference but Dog object

a.move();// runs the method in Animal class

b.move();//Runs the method in Dog class
}
}

This would produce the following result:

Animals can move
Dogs can walk and run

In the above example, you can see that the even though b is a type of Animal it runs the move method in the Dog class. The reason for this is: In compile time, the check is made on the reference type. However, in the runtime, JVM figures
out the object type and would run the method that belongs to that particular object.

Therefore, in the above example, the program will compile properly since Animal class has the method move. Then, at the runtime, it runs the method specific for that object.

Consider the following example :

class Animal{

public void move(){
System.out.println("Animals can move");
}
}

class Dog extends Animal{

public void move(){
System.out.println("Dogs can walk and run");
}
public void bark(){
System.out.println("Dogs can bark");
}
}

public class TestDog{

public static void main(String args[]){
Animal a = new Animal(); // Animal reference and object
Animal b = new Dog(); // Animal reference but Dog object

a.move();// runs the method in Animal class
b.move();//Runs the method in Dog class
b.bark();
}
}

This would produce the following result:

TestDog.java:30: cannot find symbol
symbol  : method bark()
location: class Animal
b.bark();
^

This program will throw a compile time error since b's reference type Animal doesn't have a method by the name of bark.

Using the super keyword:

When invoking a superclass version of an overridden method the super keyword is used.

class Animal{

public void move(){
System.out.println("Animals can move");
}
}

class Dog extends Animal{

public void move(){
super.move(); // invokes the super class method
System.out.println("Dogs can walk and run");
}
}

public class TestDog{

public static void main(String args[]){

Animal b = new Dog(); // Animal reference but Dog object
b.move(); //Runs the method in Dog class

}
}

This would produce the following result:

Animals can move
Dogs can walk and run

15. Java Polymorphism

Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

Any Java object that can pass more than one IS-A test is considered to be polymorphic. In Java, all Java objects are polymorphic since any object will pass the IS-A test for their own type and for the class Object.

It is important to know that the only possible way to access an object is through a reference variable. A reference variable can be of only one type. Once declared, the type of a reference variable cannot be changed.

The reference variable can be reassigned to other objects provided that it is not declared final. The type of the reference variable would determine the methods that it can invoke on the object.

A reference variable can refer to any object of its declared type or any subtype of its declared type. A reference variable can be declared as a class or interface type.

Example:

Let us look at an example.

public interface Vegetarian{}
public class Animal{}
public class Deer extends Animal implements Vegetarian{}

Now, the Deer class is considered to be polymorphic since this has multiple inheritance. Following are true for the above example:

A Deer IS-A Animal

A Deer IS-A Vegetarian

A Deer IS-A Deer

A Deer IS-A Object

When we apply the reference variable facts to a Deer object reference, the following declarations are legal:

Deer d = new Deer();
Animal a = d;
Vegetarian v = d;
Object o = d;

All the reference variables d,a,v,o refer to the same Deer object in the heap.

Virtual Methods:

In this section, I will show you how the behavior of overridden methods in Java allows you to take advantage of polymorphism when designing your classes.

We already have discussed method overriding, where a child class can override a method in its parent. An overridden method is essentially hidden in the parent class, and is not invoked unless the child class uses the super keyword within the overriding method.
public class Employee {
private String name;
private String address;
private int number;
public Employee(String name, String address, int number)
{
System.out.println("Constructing an Employee.");
this.name = name;
this.address = address;
this.number = number;
}
public void mailCheck()
{
System.out.println("Mailing a check to " + this.name + " " + this.address);
}
public String toString()
{
return name + " " + address + " " + number;
}
public String getName()
{
return name;
}
public String getAddress()
{
return address;
}
public int getNumber()
{
return number;
}
public void setAddress(String newAddress)
{
address = newAddress;
}

}

public class Salary extends Employee {
private double salary;
public Salary(String name, String address, int number, double salary)
{
super(name, address, number);
this.salary = salary;
}
public void mailCheck()
{
System.out.println("Within mailCheck of salary class ");
System.out.println("Mailing check to " + getName() + "with salary " + salary);
}
public double getSalary()
{
return salary;
}
public void setSalary(double newSalary)
{
if (newSalary > 0.0)
salary = newSalary;
else
throw new IndexOutOfBoundsException("Argument illegal!");
}
public double computePay()
{
System.out.println("Computing salary pay for " + getName());
return salary/52;
}

}

Now, you study the following program carefully and try to determine its output:

public class Test{
public static void main(String args[])
{
Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP", 3, 3600.00);
   Employee e = new Salary("John Adams", "Boston, MA", 2, 2400.00);
   System.out.println("Call mailCheck using Salary reference - - ");
   s.mailCheck();
   System.out.println("\nCall mailCheck using Employee reference --");
   e.mailCheck();
}

}

Here, we instantiate two Salary objects . one using a Salary reference s, and the other using an Employee reference e.

While invoking s.mailCheck() the compiler sees mailCheck() in the Salary class at compile time, and the JVM invokes mailCheck() in the Salary class at run time.

Invoking mailCheck() on e is quite different because e is an Employee reference. When the compiler sees e.mailCheck(), the compiler sees the mailCheck() method in the Employee class.

Here, at compile time, the compiler used mailCheck() in Employee to validate this statement.
At run time, however, the JVM invokes mailCheck() in the Salary class. //在run time时候，使用的是Salary里的mailCheck()

This behavior is referred to as virtual method invocation, and the methods are referred to as virtual methods. All methods in Java behave in this manner, whereby
an overridden method is invoked at run time, 在run time，总是重载的方法被使用，no matter what data type the reference is that was used in the source code at compile time.

16. Java Abstraction

Abstraction refers to the ability to make a class abstract in OOP. An abstract class is one that cannot be instantiated. All other functionality of the class still exists, and its fields, methods, and constructors are all accessed in the same manner. You just
cannot create an instance of the abstract class.

If a class is abstract and cannot be instantiated, the class does not have much use unless it is subclass. Abstract class往往是没有用的，除非他是子类，（其实应该是父类），这样的话我们只是使用子类。This is typically how abstract classes come about during the design phase. A parent
class contains the common functionality of a collection of child classes, but the parent class itself is too abstract to be used on its own.

Abstract Class:

Use the abstract keyword to declare a class abstract. The keyword appears in the class declaration somewhere before the class keyword.

/* File name : Employee.java */
public abstract class Employee
{
private String name;
private String address;
private int number;
public Employee(String name, String address, int number)
{
System.out.println("Constructing an Employee");
this.name = name;
this.address = address;
this.number = number;
}
public double computePay()
{
System.out.println("Inside Employee computePay");
return 0.0;
}
public void mailCheck()
{
System.out.println("Mailing a check to " + this.name
+ " " + this.address);
}
public String toString()
{
return name + " " + address + " " + number;
}
public String getName()
{
return name;
}
public String getAddress()
{
return address;
}
public void setAddress(String newAddress)
{
address = newAddress;
}
public int getNumber()
{
return number;
}
}

Notice that nothing is different in this Employee class. The class is now abstract, but it still has three fields, seven methods, and one constructor.

Now if you would try as follows:

/* File name : AbstractDemo.java */
public class AbstractDemo
{
public static void main(String [] args)
{
/* Following is not allowed and would raise error */
Employee e = new Employee("George W.", "Houston, TX", 43);

System.out.println("\n Call mailCheck using Employee reference--");
e.mailCheck();
}
}

When you would compile above class then you would get the following error:

Employee.java:46: Employee is abstract; cannot be instantiated
Employee e = new Employee("George W.", "Houston, TX", 43);
^
1 error

Extending Abstract Class:

We can extend Employee class in normal way as follows:

/* File name : Salary.java */
public class Salary extends Employee
{
private double salary; //Annual salary
public Salary(String name, String address, int number, double
salary)
{
super(name, address, number);
setSalary(salary);
}
public void mailCheck()
{
System.out.println("Within mailCheck of Salary class ");
System.out.println("Mailing check to " + getName()
+ " with salary " + salary);
}
public double getSalary()
{
return salary;
}
public void setSalary(double newSalary)
{
if(newSalary >= 0.0)
{
salary = newSalary;
}
}
public double computePay()
{
System.out.println("Computing salary pay for " + getName());
return salary/52;
}
}

Here, we cannot instantiate a new Employee, but if we instantiate a new Salary object, the Salary object will inherit the three fields and seven methods from Employee.

/* File name : AbstractDemo.java */
public class AbstractDemo
{
public static void main(String [] args)
{
Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP", 3, 3600.00);
Employee e = new Salary("John Adams", "Boston, MA", 2, 2400.00);

System.out.println("Call mailCheck using Salary reference --");
s.mailCheck();

System.out.println("\n Call mailCheck using Employee reference--");
e.mailCheck();
}
}

This would produce the following result:

Constructing an Employee
Constructing an Employee
Call mailCheck using  Salary reference --
Within mailCheck of Salary class
Mailing check to Mohd Mohtashim with salary 3600.0

Call mailCheck using Employee reference--
Within mailCheck of Salary class
Mailing check to John Adams with salary 2400.

Abstract Methods:

If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes,希望method是通过子类来完成。 you can declare the method in the parent class as abstract.

The abstract keyword is also used to declare a method as abstract. An abstract method consists of a method signature, but no method body.

Abstract method would have no definition, and its signature is followed by a semicolon, not curly braces as follows:

public abstract class Employee
{
private String name;
private String address;
private int number;

public abstract double computePay();

//Remainder of class definition
}

Declaring a method as abstract has two results:

The class must also be declared abstract. If a class contains an abstract method, the class must be abstract as well.

Any child class must either override the abstract method or declare itself abstract.

A child class that inherits an abstract method must override it. If they do not, they must be abstract and any of their children must override it.

Eventually, a descendant class has to implement the abstract method; otherwise, you would have a hierarchy of abstract classes that cannot be instantiated.

If Salary is extending Employee class, then it is required to implement computePay() method as follows:

/* File name : Salary.java */
public class Salary extends Employee
{
private double salary; // Annual salary

public double computePay()
{
System.out.println("Computing salary pay for " + getName());
return salary/52;
}

//Remainder of class definition
}