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Java Training

Java Training. Session 2: Introduction to Object Oriented Programming. Objectives. Classes, Objects, Methods, Instance Variables Instantiating an Object of a Class Initializing Objects with Constructors Methods with Parameters Instance Variables, set Methods and get Methods

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Java Training

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  1. Java Training Session 2: Introduction to Object Oriented Programming

  2. Objectives • Classes, Objects, Methods, Instance Variables • Instantiating an Object of a Class • Initializing Objects with Constructors • Methods with Parameters • Instance Variables, set Methods and get Methods • Primitive Types vs. Reference Types • Static • Object Vs. Class

  3. Objects • Inspired by real life. • Look around you, everything is object. • Example: • Marker in my hand. • Computer • Table • Chair • etc…

  4. Object (Cont.) • So, What is an object? • Or what is the characteristics of an object? • In simplest possible term – An object is a collection of Fields/Properties/(State) and Method/(Behavior). • Let’s elaborate with example.

  5. Illustration Meto Methods Methods Fields Methods Methods

  6. A Pen • Fields • Height • Radius • Color • Method • Can open it’s cap • Can write • Can close it’s cap

  7. Car • Fields • Dimensions • Speed • Gear • Direction • Number of Wheels • Number of Seats • Number of Doors • Methods • Open door • Start car • Accelerate • Break

  8. Object in Action • So far we have understood that object has some fields and some methods. • To begin coding one more thing.. Constructor!! • Each java object have some special method. (we will elaborate on this in future, after understanding inheritance) • One of them is constructor. • It’s method that is called when a object is created( more java term is “instantiated” ). • Let’s see some codes now ..

  9. Class: GradeBook, File: GradeBook.java Instance Variables – Note the naming convention public class GradeBook { private String courseName; private String courseTeacher; public GradeBook(String name) { courseName = name; } public void setCourseName(String name) { courseName = name; } public String getCourseName() { return courseName; } public void displayMessage() { System.out.println("CourseName is: " + courseName); } public String getCourseTeacher() { return courseTeacher; } } Constructor Public method - Carefully note the naming convention

  10. Class: GradeBookTest, File: GradeBookTest.java public class GradeBookTest { public static void main(Stringargs[ ]) { // Instantiate or create two GradeBook objects GradeBook gb1 = new GradeBook(“OOPL”); GradeBook gb2 = new GradeBook(“DS”); System.out.printf(“gb1 course name is: %s\n”, gb1.getCourseName()); gb1.setCourseName(“DLD”); System.out.printf(“gb1 course name is: %s\n”, gb1.getCourseName()); System.out.printf(“gb1 course teacher is: %s\n”, gb1.getCourseTeacher()); // The above statement is trying to print something that is not initialized yet. } }

  11. Executing the Program • To compile the source files type • javacGradeBook.javaGradeBookTest.java • Or, javac *.java • It produces two class files named GradeBook.class and GradeBookTest.class • To run the program type • java GradeBookTest • We used GradeBookTest as the argument to java as class GradeBookTest contains the “main” method. • If we use “java GradeBook” then the JVM will throw an ERROR (more dangerous than an EXCEPTION) and will terminate immediately. • Exception in thread “main” java.lang.NoSuchMethodError: main

  12. Output of “java GradeBookTest” The String courseTeacher is “null”. But using a reference which is “null” will cause unexpected results or exceptions in many cases

  13. Some Words on Multiple Classes in the Same Directory • There is a special relationship between classes that are compiled in the same directory on disk. • By default, such classes are considered to be in the same package – known as the default package. • Classes in the same package are implicitly imported into the source code files of other classes in the same package. • Thus, an import declaration is not required when one class in a package uses another in the same package.

  14. Primitive Types vs. Reference Types • Data types in Java are divided into two categories • Primitive types • boolean, byte, char, short, int, long, float and double • By default, all primitive-type instance variables are initialized to 0 (except boolean which is initialized to “false”). • Local primitive-type variables are not initialized by default. • Reference types • All nonprimitive types e.g. class variables • By default, all reference-type instance variables are initialized to the special value “null”. • “null” is a keyword in java. • Local reference-type variables are not initialized by default. • Java does not permit using a non-initialized variable before assigning a value to that variable. • intn; // local variable of a method, not member of any class • System.out.printf(“Value if n is: %d\n”, n); // compiler error • (Example provided in LocalTest.java)

  15. Object Vs Class • Class is the definition • It’s just the definition • No real value is attach • Object is the instance • An object is the entity in ram that contains data. • Example • We all belongs to human class, but each of us is a different human object.

  16. Basic flow of class and object

  17. Static!! • Static fields and methods are unique to a class. • One class has one single memory allocation for a static field. • Static methods are accessed without instantiating the class. • Ex: System.out.println(“Hello Java”); • We will elaborate it later on. • Example File: StaticTest.java and StaticMainTest.java

  18. this Reference • In a class there is always a reference present name this • Which points to current object. • Can’t use this in a static method as static methods are not related to a object.

  19. Core Conception of OOP

  20. Encapsulation • The most important thing software engineers want to achieve. • It is a mechanism for restricting access to some of the object's components. • Provide user methods to interact but user directly can not change the fields.

  21. Encapsulation - Example • Say we have a class name Student which has a field name mark and a method name calculateGrade() • We will encapsulate the algorithm related to calculating grade depending on the method. • Example: StudentTest.java

  22. Let’s try some examples • Write a class of Triangle. • Fields • Base • Height • Methods • Constructor, Getter and Setters • calculateArea

  23. Let’s try some examples • Now take use input of base and height • Then create object depending on the user input.

  24. Let’s try points • Now write a class of Point • Fields • X • Y • Methods • Constructor, Getter and Setters • Print() // just print the coordinates • distance(Point) //calculate the distance between this point the provided point

  25. Back to triangle • Add a method name compare(Triangle) • That will compare the area of this triangle and the provided triangle.

  26. Few things • JRE – the package containing only JVM and related software. Can’t compile java codes with a jre. • JDK – the package containing software to run and compile java code. • Java Doc – All class function’s detail is provided in java doc format. • http://download.oracle.com/javase/6/docs/api/ • Java Tutorial – Official java tutorial from oracle.

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