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Oop lecture3 Oop lecture3 Presentation Transcript

  • Lecture 3Concepts of OOP Object Oriented Programming Eastern University, Dhaka Md. Raihan Kibria
  • Difference between a Class and an Object A class A class is a template Definition of a class is only given during design/coding time A class can be compared to a struct in c An object An object is an instance of a class An object is made only during run time An object is made from the definition of a class
  • An exampleClass definition:public class A{ String code; String name;}Code to make a new instance:A a = new A();
  • Object/instance So far we have defined a class and written code to instantiate it. But we still have not created the object. Compile: javac A.java Run: java A.class Only now an object or instance of class A is created. JVM creates the instance or object based on class definition found in the byte-code
  • Some rules for creating class in java A class can be defined in any file. However, a public class must be in a file name that matches the class name. Example of a public class:public class Student{ String code; String name;}file name: Student.java
  • Some rules for creating class in java Example of non-public class:class A{ String name; String code;}Class B{ int age; String program;}Fliename could be anything, such as Example.java
  • Some rules for creating class in java Please note that many non-public classes may reside in one file. But a file can have only one public class. A file can have one public class and many non-public classes.
  • Packages Packages are folders, roughly speaking. Typically, “package” is the first line in any java program. Many java files can be placed under a package. Package is a way to organize codes.
  • Examplepackage com.abc;public class A{ String ...}This means the file A.java is under folder com/abc
  • Examplepublic class B{ String ...}This means the file B.java is not under any folder
  • Compiling packaged classesSuppoer A.java is under com folder.The first line of A.java is:package com; To compile issue commandjavac com/A.java The output A.class file will be produced under com folder To run issue command:java com.A
  • Compiling non-packaged classes Suppose B.java has no package definition To compile issue command:javac B.javaTo run issue commandjava B
  • Compiling a package To compile all files under package issue command:javac com/*.java To compile multiple packages issue command:javac com/*.java com/abc/*.java