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Lecture 3 Object Oriented Programming in Java

Lecture 3 Object Oriented Programming in Java. Language Basics Classes, Interfaces and Packages. Assignment B. Primitive objects and primitive object wrapper classes throwing exceptions. Today’s Lecture. Trail: Learning the Java Language Lesson: Classes, Interfaces, and Packages.

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Lecture 3 Object Oriented Programming in Java

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  1. Lecture 3Object Oriented Programming in Java Language Basics Classes, Interfaces and Packages Object Oriented Programming in Java (95-707)Java Language Basics

  2. Assignment B • Primitive objects and primitive object wrapper classes • throwing exceptions Object Oriented Programming in Java (95-707)Java Language Basics

  3. Today’s Lecture • Trail: Learning the Java Language • Lesson: Classes, Interfaces, and Packages Object Oriented Programming in Java (95-707)Java Language Basics

  4. What makes up a class: • class declaration • class body • constructor for the class • variables • methods Object Oriented Programming in Java (95-707)Java Language Basics

  5. Inheritance Tree • An object that is instantiated from a class contains all the variables and methods defined for that that class and all its ancestors. • However, the methods may have been modified (overridden) somewhere along the way. Also, access to those variables and methods may have been restricted through use of the public, private, and protected keywords Object Oriented Programming in Java (95-707)Java Language Basics

  6. The parents rule • Java does not involve a specification of an access specifier, public, private, or protected at the inheritance interface. In other words, inheritance access control cannot be used in Java to modify the access control assigned to a member of the parent class: • in other words (english) once the parent defines the access to its methods and variables, the children cannot modify them. Object Oriented Programming in Java (95-707)Java Language Basics

  7. Inheritance • extends keyword • Default superclass: Object • Inheriting the methods of the Object class • inheriting the variables of the Object class • class A inherits from class B • class A inherits the variables from class B • class A inherits the methods from class B • Inheritance is restricted to the non-private methods and variables Object Oriented Programming in Java (95-707)Java Language Basics

  8. Examples of inheritance Object Oriented Programming in Java (95-707)Java Language Basics

  9. Public, Final & Abstract classes • Classes can be declared to be either public, abstract, or final, or some combination of the three. • However, before we can make any sense out of the public keyword, we need to know a little about how classes are grouped into packages. Object Oriented Programming in Java (95-707)Java Language Basics

  10. Packages • We will see packages later on in more details. • In simple words, packages are a way to group classes together. • A class defines its package by adding the keyword “package” at the beginning of the file. • Example: • package com.mellon.aPackageName Object Oriented Programming in Java (95-707)Java Language Basics

  11. Public classes • Explicitly define the public keyword otherwise your class will be visible only within its own package Object Oriented Programming in Java (95-707)Java Language Basics

  12. Abstract Classes • An abstract class is a class that is designed to be inherited from(subclassed). It is not intended to be a class from which objects are instantiated (you cannot do a “new” on the class) • Note that the methods themselves can be abstract • Why create abstract classes? Object Oriented Programming in Java (95-707)Java Language Basics

  13. Final Classes • The opposite of an abstract class is a final class. A final class cannot be subclassed. • Why use final classes? • Can a class be both abstract and final? • Examples: • abstract and final • final class A is extended by class B Object Oriented Programming in Java (95-707)Java Language Basics

  14. Examples of abstract and final classes Object Oriented Programming in Java (95-707)Java Language Basics

  15. Interfaces • Interfaces are like the roles that a class can play • A class that implements an interface must define the methods that are specified by the interface • Whenever a class claims to implement an interface, you can be assured that it provides a definition for all the methods declared within that interface. Otherwise, the class cannot be compiled. Object Oriented Programming in Java (95-707)Java Language Basics

  16. Implementing Interfaces • A class may implement one or more interfaces using the keyword implements and a comma-delimited list of interface names as shown below:class MyClassName extends MySuperClass implements MyInterface, YourInterface { //body of class } Object Oriented Programming in Java (95-707)Java Language Basics

  17. Examples with interfaces Object Oriented Programming in Java (95-707)Java Language Basics

  18. Constructors • Constructors are used to instantiate a class • Even if you don’t explicitly define a constructor the compiler creates one for you • It is safer to explicitly define the constructor(s) • Constructor(s) are methods that have the same name has the class name and return void Object Oriented Programming in Java (95-707)Java Language Basics

  19. Multiple Constructors • A class can have multiple constructors • The only difference between them is the list of parameters that comes with the constructor • Why are they parameters to the constructors? Object Oriented Programming in Java (95-707)Java Language Basics

  20. A “super” keyword • The super keyword allows a constructor to call the constructor of a parent class • If present in the constructor the super keyword must appear first in the body of the method • Why must super appear first? Object Oriented Programming in Java (95-707)Java Language Basics

  21. Access control to the constructors • A constructor can be private, protected, or public • Always explicitly specify the access • If all the constructors are private how do I instantiate the class Object Oriented Programming in Java (95-707)Java Language Basics

  22. Examples of constructors Object Oriented Programming in Java (95-707)Java Language Basics

  23. Variables • Types of variables • Member variables • class variables • Instance variables • Method variables • local variables Object Oriented Programming in Java (95-707)Java Language Basics

  24. The “static” keyword • The keyword static determines whether the variable is a class variable or an instance variable • To access a static variable you must qualify it with the class name (NOT the instance name) • for example:ClassName.theVariableName = aValue; • Why should we use static variable? Object Oriented Programming in Java (95-707)Java Language Basics

  25. Final Variables • Determines whether the variable is a constant or not • Example: • declaring a final object and trying to assign a new value to it Object Oriented Programming in Java (95-707)Java Language Basics

  26. Controlling access to member variables • Classes can protect their member variables from being access by other objects Object Oriented Programming in Java (95-707)Java Language Basics

  27. Subtleties of the private keywords • It is applied at the class level. In other words, objects of the same types have access to each other’s private variables Object Oriented Programming in Java (95-707)Java Language Basics

  28. Subtleties of the protected keyword • The protected member variable of a super class is accessible from a subclass only if the subclass and the superclass are in the same package Object Oriented Programming in Java (95-707)Java Language Basics

  29. Subtleties of the public keyword • None! • Everyone can see a public member variable. Object Oriented Programming in Java (95-707)Java Language Basics

  30. Subtleties of the package keyword • You get package level access if you don’t specify the access level Object Oriented Programming in Java (95-707)Java Language Basics

  31. Examples of access levels on member variables Object Oriented Programming in Java (95-707)Java Language Basics

  32. Access level to member methods • Works just like member variables access level Object Oriented Programming in Java (95-707)Java Language Basics

  33. Instance Vs. Class Members(methods and variables) • “instance” member by default • use “static” to override the default and it the member becomes a “class” member Object Oriented Programming in Java (95-707)Java Language Basics

  34. Class Members • All instances of a class share the same copy of the class member • class variables can be accessed via the instance (use the instance name) or via the class itself (use the class name) Object Oriented Programming in Java (95-707)Java Language Basics

  35. Instance Members • By default all members are instance members • Every instantiated class carries its own copy of the instance variable. In other words, instance variables are not shared across instances of the class Object Oriented Programming in Java (95-707)Java Language Basics

  36. Examples of instance and class members Object Oriented Programming in Java (95-707)Java Language Basics

  37. Packages • Purpose: • grouping relevant classes together • avoiding name clashes among developers • Notation: package mypackage public class AnyClass { …. } • Scope of the package statement: • entire source file • If you don’t declare a package: • the compiler puts your class in the default package Object Oriented Programming in Java (95-707)Java Language Basics

  38. Package name conventions • Use reversed Internet domain name: • com.company.package • example: • com.mellon.lineofbusiness.packagename • com.sun... • edu.cmu… Object Oriented Programming in Java (95-707)Java Language Basics

  39. Using a package • Strategy 1: • use the packagename.classname approach • example: • com.mellon.aClassName s = new com.mellon.aClassName(); • Strategy 2 • import the package • put import statement at the beginning of the file and after the package statement: import com.mellon.*; … aClasName s = new aClassName(); • Use wildcards where necessary • java.lang.* is always imported by default Object Oriented Programming in Java (95-707)Java Language Basics

  40. Compiler, Run-time and Packages • Compiler (javac): • creates directories that reflect the package hierarchy • compiles the files and places them in their respective directory hierarchy • uses the “classpath” option on the command line to find the source files within the hierarchy of directories. • Example: • javac -classpath .;c:\myclasses;c:\jdk\lib\classes.zip *.java • Run-time (java): • uses the “classpath” option on the command line to find classes within the hierarchy of directories. • Example: • java -classpath .;c:\myclasses;c:\jdk\lib\classes.zip Object Oriented Programming in Java (95-707)Java Language Basics

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