Dr. Rajeshree Khande : Java Inheritance
- 2. 2 Inheritance Inheritance allows a software developer to derive a new class from an existing one. The existing class is called the parent class or superclass The derived class is called the child class or subclass. Creates an is-a relationship The subclass is a more specific version of the Original. Book NovelDictionary Mystery RomanceRAJESHREE KHANDE
- 3. Inheritance The child class inherits the methods and data defined for the parent class The programmer can add new variables or methods, or can modify the inherited ones. Software reuse is at the heart of inheritance. By using existing software components to create new ones, we capitalize on all the effort that went into the design, implementation, and testing of the existing software 3RAJESHREE KHANDE
- 4. 4 Deriving Subclasses In Java, we use the reserved word extends to establish an inheritance relationship class Dictionary extends Book { variable declaration; method declaration; } Book NovelDictionary Mystery Romance RAJESHREE KHANDE
- 5. Form of Inheritance Single Inheritance Hierarchical Inheritance A B A DCB 5RAJESHREE KHANDE
- 6. Form of Inheritance B C A A B C Multilvel Inheritance Multiple Inheritance (Interface) 6RAJESHREE KHANDE
- 7. public class Book { protected int pages = 1500; public String message() { System.out.println(“Number of pages: ” + pages); } } public class Dictionary extends Book { private int def = 52500; public void defMessage() { System.out.println(“Number of definitions” + def ); System.out.println(“Def per page: ” + (def /pages)); } } Number of pages: 1500 Number of definitions: 52500 Definitions per page: 35 7RAJESHREE KHANDE
- 8. Some Inheritance Details An instance of a child class does not rely on an instance of a parent class Hence we could create a Dictionary object without having to create a Book object first Inheritance is a one-way street The Book class cannot use variables or methods declared explicitly in the Dictionary class 8RAJESHREE KHANDE
- 9. 9 The protected Modifier Visibility modifiers determine which class members are inherited and which are not Variables and methods declared with public visibility are inherited; those with private visibility are not. But public variables violate the principle of encapsulation There is a third visibility modifier that helps in inheritance situations: protected RAJESHREE KHANDE
- 10. 10 The protected Modifier The protected modifier allows a member of a base class to be inherited into a child Protected visibility provides more encapsulation than public visibility does the best possible encapsulation that permits inheritance RAJESHREE KHANDE
- 11. 11 The super Reference Constructors are not inherited, even though they have public visibility. Yet we often 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 often is used to invoke the parent's constructor. RAJESHREE KHANDE
- 12. 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 The super reference can also be used to reference other variables and methods defined in the parent’s class 12RAJESHREE KHANDE
- 13. public class Book { protected int pages; Book(int numPages) { pages = numPages; } } public class Dictionary { private int def; Dictionary(int numPages, int numDef ) { super(numPages); def = numDef ; } } 13RAJESHREE KHANDE
- 14. Multiple Inheritance Java supports single inheritance, meaning that a derived class can have only one parent class Multiple inheritance allows a class to be derived from two or more classes, inheriting the members of all parents Collisions, such as the same variable name in two parents, have to be resolved. Java does not support multiple inheritance. In most cases, the use of interfaces gives us aspects of multiple inheritance without the overhead 14RAJESHREE KHANDE
- 15. 15 Overriding Methods When a child class defines a method with the same name and signature as a method in the parent class, we say that the child’s version overrides the parent’s version in favor of its own. Signature: method’s name along with number, type, and order of its parameters The new method must have the same signature as the parent's method, but can have a different body The type of the object executing the method determines which version of the method is invoked RAJESHREE KHANDE
- 16. Overriding Methods A parent method can be invoked explicitly using the super reference If a method is declared with the final modifier, it cannot be overridden. 16RAJESHREE KHANDE
- 17. public class Book { protected int pages; Book(int numPages) { pages = numPages; } public void message() { System.out.println(“Number of pages: ” + pages); } } public class Dictionary extends Book { protected int definitions; Dictionary(int numPages, int numDefinitions) { super(numPages); definitions = numDefinitions; } public void message() { System.out.println(“Number of definitions” + definitions); System.out.println(“Definitions per page: ” + (definitions/pages)); super.message(); } } 17RAJESHREE KHANDE
- 18. 18 Overloading vs. Overriding Overloading deals with multiple methods in the same class with the same name but different signatures Overloading lets you define a similar operation in different ways for different data Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature Overriding lets you define a similar operation in different ways for different object types RAJESHREE KHANDE
- 19. 19 Overloading vs. Overriding Overloading deals with multiple methods with the same name in the same class, but with 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 RAJESHREE KHANDE
- 20. 20 Class Hierarchies A child class of one parent can be the parent of another child, forming a class hierarchy Book NovelDictionary Mystery Romance RAJESHREE KHANDE
- 21. 21 The Object Class A class called Object is defined in the java.lang package of the Java standard class library. All classes are derived from the Object class If a class is not explicitly defined to be the child of an existing class, it is assumed to be the child of the Object class Therefore, the Object class is the ultimate root of all class hierarchies RAJESHREE KHANDE
- 22. The Object Class The Object class contains a few useful methods, which are inherited by all classes For example, the toString method is defined in the Object class Every time we have defined toString, we have actually been overriding an existing definition The toString method in the Object class is defined to return a string that contains the name of the object’s class together along with some other information All objects are guaranteed to have a toString method via inheritance, thus the println method can call toString for any object that is passed to it 22RAJESHREE KHANDE
- 23. The Object Class The equals method of the Object class returns true if two references are aliases. We can override equals in any class to define equality in some more appropriate way The String class (as we've seen) defines the equals method to return true if two String objects contain the same characters Therefore the String class has overridden the equals method inherited from Object in favor of its own version 23RAJESHREE KHANDE
- 24. Abstract Class Sometimes, a class that you define represents an abstract concept and, should not be instantiated. Take, for example, food in the real world. Have you ever seen an instance of food? No. What you see instead are instances of carrot, apple, and chocolate. Food represents the abstract concept of things that we can eat. It doesn't make sense for an instance of food to exist. Similarly in object-oriented programming, you may want to model abstract concepts but you don't want to be able to create an instance of it. 24RAJESHREE KHANDE
- 25. Abstract Class For example, the Number class in the java.lang package represents the abstract concept of numbers. It makes sense to model numbers in a program, but it doesn't make sense to create a generic number object. Instead, the Number class makes sense only as a superclass to classes like Integer and Float which implement specific kinds of numbers. Classes such as Number, which implement abstract concepts and should not be instantiated, are called abstract classes. 25RAJESHREE KHANDE
- 26. Abstract Class An abstract class is a class that can only be subclassed--it cannot be instantiated. To declare that your class is an abstract class, use the keyword abstract before the class keyword in your class declaration: Syntax abstract class Number { . . . } If you attempt to instantiate an abstract class, the compiler will display an error similar to the following and refuse to compile your program: 26RAJESHREE KHANDE
- 27. Abstract Method An abstract class may contain abstract methods. i.e methods with no implementation An abstract class can define a complete programming interface, thereby providing its subclasses with the method declarations for all of the methods necessary to implement that programming interface. The abstract class can leave some or all of the implementation details of those methods up to its subclasses 27RAJESHREE KHANDE
- 28. Abstract Class :Example 28RAJESHREE KHANDE
- 29. Each non-abstract subclass of GraphicObject, such as Circle and Rectangle, would have to provide an implementation for the draw method abstract class GraphicObject { int x, y; . . . abstract void draw(); } class Circle extends GraphicObject { void draw() { . . . } } class Rectangle extends GraphicObject { void draw() { . . . } } Abstract Class :Example 29RAJESHREE KHANDE
- 30. Abstract Class An abstract class is not required to have an abstract method in it. But any class that has an abstract method in it or that does not provide an implementation for any abstract methods declared in its superclasses must be declared as an abstract class. 30RAJESHREE KHANDE