Chapter 9 java
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The document discusses inheritance in Java. It defines inheritance as deriving a new subclass from an existing superclass, where the subclass inherits properties from the superclass. The key points made include: - A subclass inherits all non-private fields and methods from its superclass. - A subclass constructor must call the superclass constructor using super(). - Inheritance allows subclasses to override or hide superclass methods. - Abstract classes define common structures for subclasses to implement, and subclasses must override all abstract methods. - The final keyword can be used to prevent overriding of methods or preventing inheritance of a class.
Chapter 9 java
- 2. Inheritance It is always nice if we could reuse something that already exist rather than creating the same all over again, Java supports this concept, Java classes can be used in several ways, this is basically done by creating new classes, reusing the properties of existing once
- 3. Continue.. The mechanism of deriving a new class from an old one is called inheritance, the old class is known as the base class, superclass or parent class and the new one is called the sub class, derived class or child class. To inherit a class, you simply incorporate the definition of one class into another by using the extends keyword.
- 4. Continue.. You can only specify one superclass for any subclass that you create. Java does not support the inheritance of multiple superclasses into a single subclass. You can, as stated, create a hierarchy of inheritance in which a subclass becomes a superclass of another subclass. However, no class can be a superclass of itself.
- 5. Member Access and Inheritance Although a subclass includes all of the members of its superclass, it cannot access those members of the superclass that have been declared as private.
- 6. Using super to Call Superclass Constructors A subclass can call a constructor defined by its superclass by use of the following form of super: super(arg-list); Here, arg-list specifies any arguments needed by the constructor in the superclass. super( ) must always be the first statement executed inside a subclass’ constructor.
- 7. Continue.. The second form of super acts somewhat like this, except that it always refers to the superclass of the subclass in which it is used. This usage has the following general form: super.member Here, member can be either a method or an instance variable.
- 8. Continue.. This second form of super is most applicable to situations in which member names of a subclass hide members by the same name in the superclass.
- 9. Continue.. Note : A Superclass Variable Can Reference a Subclass Object A reference variable of a superclass can be assigned a reference to any subclass derived from that superclass. You will find this aspect of inheritance quite useful in a variety of situations
- 10. Types of Inheritance 1. Single Inheritance (one super class) 2. Multiple Inheritance (Several Super classes) 3. Multi-level Inheritance (Derived from a derived class. 4. Hierarchical Inheritance (one super class many sub class.
- 11. Method Overriding In a class hierarchy, when a method in a subclass has the same name and type signature as a method in its superclass, then the method in the subclass is said to override the method in the superclass.
- 12. Continue.. When an overridden method is called from within its subclass, it will always refer to the version of that method defined by the subclass. The version of the method defined by the superclass will be hidden.
- 13. Abstract Class There are situations in which you will want to define a superclass that declares the structure of a given abstraction without providing a complete implementation of every method. That is, sometimes you will want to create a superclass that only defines a generalized form that will be shared by all of its subclasses, leaving it to each subclass to fill in the details.
- 14. Continue.. Such a class determines the nature of the methods that the subclasses must implement. You can require that certain methods be overridden by subclasses by specifying the abstract type modifier. These methods are sometimes referred to as subclasser responsibility because they have no implementation specified in the superclass.
- 15. Continue.. Thus, a subclass must override them—it cannot simply use the version defined in the superclass. To declare an abstract method, use this general form: abstract type name(parameter-list); As you can see, no method body is present. Any class that contains one or more abstract methods must also be declared abstract.
- 16. Continue.. To declare a class abstract, you simply use the abstract keyword in front of the class keyword at the beginning of the class declaration. There can be no objects of an abstract class. That is, an abstract class cannot be directly instantiated with the new operator. Such objects would be useless, because an abstract class is not fully defined.
- 17. Continue.. Also, you cannot declare abstract constructors, or abstract static methods. Any subclass of an abstract class must either implement all of the abstract methods in the superclass, or be declared abstract itself.
- 18. Using Final to Prevent Overriding While method overriding is one of Java’s most powerful features, there will be times when you will want to prevent it from occurring. To disallow a method from being overridden, specify final as a modifier at the start of its declaration. Methods declared as final cannot be overridden.
- 19. Using Final to prevent Inheritance Sometimes you will want to prevent a class from being inherited. To do this, precede the class declaration with final. Declaring a class as final implicitly declares all of its methods as final, too.
- 20. Thank you