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Class Diagram for Words
Book
# pages : int
+ pageMessage() : void
Dictionary
- definitions : int
+ definitionMessage() : void
Words
+ main (args : String[]) : void](https://image.slidesharecdn.com/inheritanceanditsnecessityinjava-240220151239-fb8b302a/85/Inheritance-and-its-necessity-in-java-ppt-11-320.jpg)
Inheritance and its necessity in java.ppt
- 2. 2 Inheritance Review of class relationships Uses – One class uses the services of another class, either by making objects of that class or by using static functions of the class. Has A – One class’s attributes includes one or more object of another class.
- 3. 3 Inheritance Is A – Describes that one class is a more specific form of another class. For example, Triangle is a Shape, Prius is a HybridCar. We cannot say Shape is a Triangle, nor can we say Triangle is a Rectangle.
- 4. 4 Inheritance Inheritance is a fundamental object- oriented design technique used to create and organize reusable classes Reference is Chapter 8 in the book.
- 5. 5 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, or base class The derived class is called the child class or subclass As the name implies, the child inherits characteristics of the parent That is, the child class inherits the methods and data defined by the parent class
- 6. 6 Inheritance Inheritance relationships are shown in a UML class diagram using a solid arrow with an unfilled triangular arrowhead pointing to the parent class Car Prius Proper inheritance creates an is-a relationship, meaning the child is a more specific version of the parent
- 7. 7 Inheritance A programmer can tailor a derived class as needed by adding new variables or methods, or by modifying the inherited ones Software reuse is a fundamental benefit 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
- 8. 8 Deriving Subclasses In Java, we use the reserved word extends to establish an inheritance relationship See Words.java (page 440) See Book.java (page 441) See Dictionary.java (page 442) class Prius extends Car { // class contents }
- 9. 9 The protected Modifier Visibility modifiers affect the way that class members can be used in a child class Variables and methods declared with private visibility cannot be referenced by name in a child class They can be referenced in the child class if they are declared with public visibility -- but public variables violate the principle of encapsulation There is a third visibility modifier that helps in inheritance situations: protected
- 10. 10 The protected Modifier The protected modifier allows a child class to reference a variable or method directly in the child class It provides more encapsulation than public visibility, but is not as tightly encapsulated as private visibility A protected variable is visible to any class in the same package as the parent class The details of all Java modifiers are discussed in Appendix E Protected variables and methods can be shown with a # symbol preceding them in UML diagrams
- 11. 11 Class Diagram for Words Book # pages : int + pageMessage() : void Dictionary - definitions : int + definitionMessage() : void Words + main (args : String[]) : void
- 12. 12 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 See Words2.java (page 445) See Book2.java (page 446) See Dictionary2.java (page 447)
- 13. 13 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
- 14. 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
- 15. 15 Class Hierarchies A child class of one parent can be the parent of another child, forming a class hierarchy Business KMart Macys ServiceBusiness Kinkos RetailBusiness
- 16. 16 Class Hierarchies Two children of the same parent are called siblings Common features should be put as high in the hierarchy as is reasonable An inherited member is passed continually down the line Therefore, a child class inherits from all its ancestor classes There is no single class hierarchy that is appropriate for all situations
- 17. 17 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
- 18. 18 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 define the toString method, we are actually overriding an inherited definition The toString method in the Object class is defined to return a string that contains the name of the object’s class along with some other information
- 19. 19 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 As we've seen, the String class defines the equals method to return true if two String objects contain the same characters The designers of the String class have overridden the equals method inherited from Object in favor of a more useful version