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Java Training

Java Training. Session 3: More on Object Oriented Programming. static Methods, static Fields. static Methods Also called “class methods” Can be called without using any object (provided access is permitted) Calling syntax: ClassName.methodName([ parameters ]);

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Java Training

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  1. Java Training Session 3: More on Object Oriented Programming

  2. static Methods, static Fields • static Methods • Also called “class methods” • Can be called without using any object (provided access is permitted) • Calling syntax: ClassName.methodName([parameters]); • Can also be called using a reference to an object of the same class • ref.methodName([parameters]); • Can access only other static members of its class directly • Can be accessed by both static and non-static methods of the same class • For calling non-static methods of its class, must use an object of the same class • Is not associated with any object when called; hence does not have any “this” reference • Example: StaticTest.java, ObjectCount.java

  3. static Methods, static Fields (contd.) • static Fields • Also called “class member variables” • Can be accessed without using any object (provided access is permitted) • Is not associated with any object; only a single copy exists which is shared by all the objects of the class • Access syntax: ClassName.variableName • Can also be accessed using a reference to an object of the same class • ref.variableName • Can be accessed by both static and non-static methods of the same class • When its value is changed, the change visible to all current and future objects of the class • Example: ObjectCount.java

  4. static Methods, static Fields (contd.) • static methods are used to write those general purpose methods for which object creation is not necessary • They are intended to provide the flavor of global functions in Java • For example, all the methods of class Math (java.lang.Math) are static • Math.sqrt(900.0); • Math.pow(5, 2);

  5. static Methods, static Fields (contd.) • static member variables are used to perform some class level calculations • private static intobjCount = 0; • Example: ObjectCount.java • They are also used to define variables intended to hold constant values that should not be changed but can be easily used by others • public static final double EARTH_G = 9.8; • Math.E, Math.PI etc. • They give the flavor of #define style activities in Java • In this type of use, the convention is to capitalize every letter of the identifier and separate words with underscores • Example: PublicStaticFinal.java

  6. Static Example class SomeConstant { public static final double EARTH_G = 9.8; } public class PublicStaticFinal { public static void main(Stringargs[]) { System.out.println("EarthGravitaional const: " + SomeConstant.EARTH_G); // output = 9.8 //SomeConstant.EARTH_G = 10.0; // compiler error System.out.println("Value of PI is: " + Math.PI); // output = 3.141592653589793 System.out.println("Value of E is: " + Math.E); // output = 2.718281828459045 } }

  7. Let’s Try!! • Say Prime Bank is planning to start a new type of Account. And they are planning to give some rewards for the starting account holders. • For simplicity we consider some money rewards.

  8. Design • Write 2 classes. • Account • Fields • One static field for keeping track of number of accounts. • One field for name of the account • One field for awarded amount. • Methods • Constructor with account name parameter. • Getters for account name and reward amount • And a print() method which will print the account name and award amount • and AccountTest with a public main method.

  9. AccountTest public class AccountTest { public static void main(String[] args) { Account a1 = new Account("Shakib Al Hasan"); Account a2 = new Account("TamimIqbal"); Account a3 = new Account("Shafiul Islam"); Account a4 = new Account("MohammadAshraful"); Account a5 = new Account("AlokKapali"); a1.print(); a2.print(); a3.print(); a4.print(); a5.print(); } } Account Name: "Shakib Al Hasan", Rewared Amount: 10000 Account Name: "TamimIqbal", Rewared Amount: 5000 Account Name: "Shafiul Islam", Rewared Amount: 2000 Account Name: "Mohammad Ashraful", Rewared Amount: 0 Account Name: "AlokKapali", Rewared Amount: 0

  10. Package • Group of classes • Physically a folder having some classes. • Ensures hierarchy and easy reusability. • In a package all the class starts with package term. Ex. package com.primebank.sme; • If a class is not included in a package then it is in default package. • Class from default package can’t be imported from other project or package. So not reusable!!

  11. Package Naming Convention • Usually package names are in all small letter. • Ex: java.net, java.sql, java.io, com.mysql.jdbc. • Another common practice is to have a hierarchy like • com.mysql.jdbc • org.springframework.jdbc • First part indicates nature of project. Type like TLD (Top Level Domain) • com for commercial project • org for organizational project • edu for educational project. • Second part indicates name of the corporation or organization. • Third part is the name of the project.

  12. MySql provided Classes

  13. Core Concepts of OOP

  14. Inheritance • Inheritance in oop is similar to real life inheritance. • Receiving something from ancestors. • One class can reuse fields and methods previously declared in other class. • This increases reusability and reduce reinventing the while.

  15. Extends • In Java a class inherits other class by extends key word. public classScienceStudentextends Student SuperClass SubClass

  16. Example • Student, ScienceStudent and CommerceStudent example. • Let’s try – Extend ScienceStudent class to create BiologyStudent and ComputerStudent. • BiologyStudent have one more field naming biology. • ComputerStudent have one more field naming computer.

  17. protected Members • Inherited by subclasses just like public members • Can be accessed by subclasses • For non-subclasses, they act as private members of the class

  18. Constructors in Subclasses • Every constructor of a class calls its superclass constructor explicitly or implicitly • A class’s constructor can call its superclass’s constructor explicitly using the “super” keyword • Its must be the first statement of the constructor • Can supply arguments to call specific versions of superclass constructors • If a class’s constructor does not explicitly call its superclass’s constructor, then “javac” implicitly tries to call the no-argument constructor of the superclass • If the superclass does not have a no-argument constructor, then “javac” reports a compiler error

  19. Superclasses and Subclasses • Superclassesrepresent a general category • Subclasses are built upon the superclasses to handle specific details • Subclasses have a “IS-A” relation with superclasses • Example: • class A { } • class B extends A { }, class C extends A { } • B “is-a” A, C “is-a” A, but B “is not a” C • A, B, C all “are” Object. (who is this Object :-?)

  20. Object!! • Object is a very special class in Java. • Object is super class for all classes in java. • Any class is a sub class of Object.

  21. Object Class Methods • Object class don’t have any members. But it has the following methods. • booleanequals(Objectobj) • String toString() • protected Object clone() • protected void finalize() • Class<?>getClass() • inthashCode() • void notify() • voidnotifyAll() • void wait() • voidwait(long timeout) • voidwait(long timeout, intnanos)

  22. Polymorphism • One name, many methods (tasks) • Program in the general, rather than program in the specific • We already have used that !! • The print() method in our student class is a very good example of polymorphism.

  23. Polymorphism • Another type of polymorphism is also possible. Called Method Overloading. • Same method having same name but different number of parameters. • Let’s revisit the Triangle we wrote in previous class. • Example: • Triangle.compare(Triangle t1, Triangle t2) • t1.compare(Triangle t2)

  24. Method Overriding • When a subclass defines a method whose signature is identical to a method present in the superclass then the new method overrides the method present in the superclass. • If the signatures differ in the parameter types or parameter counts, then it simply becomes a overloaded method.

  25. Logical Operators • Conditional Operators • Conditional AND (&&) • Conditional OR (||) • Perform Short-Circuit evaluation • Short-Circuit Evaluation • The parts of an expression are evaluated only until it is known whether the condition is true or false • Example: LogicalTest.java

  26. Logical Operators (contd.) • Boolean Logical Operators • Boolean Logical AND (&) • Boolean Logical Inclusive OR (|) • Boolean Logical Exclusive OR [XOR] (^) • Do not perform Short-Circuit evaluation; always evaluate both of their operands • Example: LogicalTest.java

  27. Logical Operators (contd.) • Logical Negation Operator (!) • Also called logical NOT or logical complement • Reverses the meaning of a condition • boolean b = !true; // b = false

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