Inheritance

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Inheritance

Inheritance
Swing Forms Hierarchy
The Object class
The equals( ) method
The finalaize( ) method
Methods of the Object class
Member Access Modifiers
An annotation for overriding a method
Case Study: Product Database
Data Attributes and Class Candidates
The syntax for creating subclasses
The Class class
Access to object's type
The equals( ) method
The equals( ) method override
Abstract Classes
An abstract Product class
Concrete class
The final modifier
A final class example
A final method example
A final parameter example

1. Inheritance

2. Swing Forms Hierarchy

The inheritance hierarchy for Swing forms and controls

3. The Object class

Every Java object inherits several methods of the Object class
- (or original methods can be overridden in a subclass of the Object class.)
Two of the most common methods are
- toString( )
- equals( )
The toString method returns the class name of the object followed by its memory location.
It has been overridden in the String class.

4. The equals( ) method

The default equals( ) method returns true if
- the object for which the method is invoked, and
- the object passed as equals( ) parameter
share the same memory location.
The equals( ) method is overridden in the String class, and in the ArrayList class.

5. The finalaize( ) method

The finalaize( ) method provides "last minute" chance for processing an object which is about to be destroyed by the java garbage collector.

6. Methods of the Object class


    java.lang.Object

    toString()
    equals( Object )
    getClass()
    clone()
    hashCode()
    finalize()

7. Member Access Modifiers


    private

    public

    protected

    //default

8. An annotation for overriding a method

Java annotations provide metadata about a particular construct in the program.
```
    @Override
    // method declaration goes here
```
Annotation's role is similar to program comments. Annotations can be useful to the compiler, various software tools, and program documentation.

9. Case Study: Product Database

Possible areas of use:
- DVD burning/production
- Bookstore
- Game shop
- Shipping warehouse

10. Data Attributes and Class Candidates


  _______________________________________________________
 |             |_____________Useful classes______________|
 | Data        |         |      |     | Software |       |
 | Attribute   | Product | Book | DVD | Download | Game  |
 |-------------+---------+------+-----+----------+-------|
 | code        |    x
 | description |    x
 | price       |    x
 | weight      |            x
 | author      |            x
 | publisher   |            x
 | version     |                           x
 | releaseDate |                           x
 `-------------+------------------------------------------

11. The syntax for creating subclasses

To declare a subclass


    public class SubclassName extends SuperClassName{}

To call a superclass constructor
```
    super( argumentList )
```
To call a superclass method
```
    super.methodName( argumentList )
```

12. The Class class

```
    java.lang.Class
```
Common method
```
    getName()
```

13. Access to object's type

Code that tests an object's type:


    Product p = new Book();  // create a Book object
    if ( p.getClass().getName().equals("Book") ) {
        System.out.println("This is a Book object");
    }

The console output:


    This is a Book object

An easier way to test an object's type:


    Product p = new Book();  // create a Book object
    if ( p instanceof Book ) {
        System.out.println("This is a Book object");
    }

The console output:
```
    This is a Book object
```

14. The equals( ) method

How the equals method of the Object class works:

(a) Both variables refer to the same object:


    Product product1 = new Product();
    Product product2 = product1;
    if ( product1.equals( product2 ) ) // true

(b) Both variables refer to different objects that store the same data:


    Product product1 = new Product();
    Product product2 = new Product();
    if ( product1.equals( product2 ) ) //false

15. The equals( ) method override

To override the equals() method of the Object class, add your own equals() method to the class.

For example, the Product class could implement the method as follows:


    @Override
    public boolean equals( Object object )
    {
        if ( object instanceof Product )
        {
            Product product2 = (Product) object;
            if (
                code.equals( product2.getCode() )
                &&
                description.equals( product2.getDescription() )
                &&
                price == product2.getPrice()
            )
            {
                return true;
            }
        }
        return false;
    }//equals

16. Abstract Classes

The abstract keyword
- restricts class of objects by preventing their instantiation
- forces abstract methods to be implemented in the subclasses
If a class contains an abstract method (with no body) then the class must be declared as an abstract class.
To allow object instantiation, all abstract methods must be implemented in the subclass which inherits from the abstract superclass.

17. An abstract Product class


public abstract class Product
{
    private String code;
    private String description;
    private double price;

    // regular constructors and methods for instance 
    // variables

    @Override
    public String toString()
    {
        return "Code:        " + code + "\n" +
               "Description: " + description + "\n" +
               "Price:       " + this.getFormattedPrice()
                               + "\n";
    }

    // an abstract method
    abstract String getDisplayText(); 
}//class Product

18. Concrete class

A class that inherits the abstract Product class


    public class Book extends Product
    {
        private String author;

        // regular constructor and methods for the Book class

        // implement the abstract method
        @Override
        public String getDisplayText() 
        {
            return super.toString() +
                "Author:      " + author + "\n";
        }
    }//class Book

19. The final modifier

The final modifier can
- prevent a class from being sub classed (i.e. inherited from)
- prevent a method from being overridden in a subclass
- prevent a parameter passed into a method from being modified

20. A final class example


    public final class Book extends Product
    {
        // all methods in the class are automatically final
    }//class Book

21. A final method example


    public final String getVersion()
    {
        return version;
    }

22. A final parameter example


    public void setVersion( final String version )
    {
        // version = "new value"; // not allowed
        this.version = version;
    }