Classes objects in java

In this you learnt about
How to Create classes
How to Create objects in Java
Command line Arguments in Java

1. JAVA Creating Classes & Objects Command line arguments P.Prathibha

2. Topics for Today’s Session About JAVA Creating Classes and Objects Command line arguments How to Create class and Object

3. Java Java is a computing platform for application development and an object-oriented, Java is Class-based and Concurrent programming language  It means the code can be executed by multiple processes at the same time. Java can  r un on all platforms and free to access. Java is Simple, Secure, Robust, Complete Object oriented and Platform Independent High level Language It is Portable and Multi-thread technology gives High Performance .

4. Objects and Classes in Java An object in Java is the physical as well as a logical entity, whereas, a class in Java is a logical entity only. An entity that has state and behavior is known as an object e.g., chair, bike, pen, table, car, etc. It can be physical or logical (tangible and intangible). The example of an intangible object is the banking system. An object consists of : State : It is represented by attributes of an object. It also reflects the properties of an object. Behavior : It is represented by methods of an object. It also reflects the response of an object with other objects. Identity : An object identity is typically implemented via a unique ID. The value of the ID is not visible to the external user. However, it is used internally by the JVM to identify each object uniquely. For Example, Pen is an object. Its name is Reynolds; color is Blue, known as its state. It is used to write, so writing is its behavior. Identity Name of Dog State / Attributes Breed Color Age Behaviours Bark Eat Sleep Bite

5. An object is an instance of a class. A class is a template or blueprint from which objects are created. So, an object is the instance(result) of a class. Object Definitions: An object is a real-world entity. An object is a runtime entity. The object is an entity which has state and behavior. The object is an instance of a class.

6. Class A class is a group of objects which have common properties. It is a template or blueprint from which objects are created. It is a logical entity. It can't be physical. Class is a a collection of the data items and functions. The data items are called as fields and function are called as methods. Java is said to be true object oriented programming language because without class we cannot do anything. A class in Java can contain: Fields Methods Constructors Blocks Nested class and interface

7. Create a Class To create a class, use the keyword class : A class defines the state and behavior of the program components known as objects. Classes create objects and objects use methods to communicate between them. Methods are just like functions. The instance of the class is said to be object. Class variable is said to be object. A class is a user-defined data type serves to define properties. Once a class is defined we can create variables of that type using declarations. Example: Create a class named "MyClass" with a variable a: Class classname [extends superclassname] { [variable/field declaration]; [methods declaration]; } class MyClass { int a = 5; } Here
Everything inside the brackets is optional. classname and superclassname are any valid java identifiers.
The keyword extends indicate that the properties of the superclass name are extended to the classname class.
This concept is known as Inheritance.

8. Fields declaration: Data is encapsulated in a class by placing data fields in the class. These variables are called “instance variables”. These are declared just as local variables; Methods Declaration: A class with only data fields has no life. Such classes cannot respond to any messages. We must add methods that are useful for manipulating the data of the class. Methods for a class are declared inside the body of the class but immediately after the declaration of the variables. Type methodname (parameter list)
{
method_body;
} Method declaration having four parts: 1. The name of the method ( method name ) (It is any valid identifier)
2. The type of the value of the method returns ( type ) (it is any valid data type, it is void when it does not return any value.)
3. A list of parameters ( parameter list ) (This list contains variable names and types of all the values we want to send as input, when no input data are required, we must use empty parentheses)
4. The body of the method. It describes the operations to be performed on the data.
Example: : int sum (int a, int b)
{
c=a+b;
return c;
} Method in Java In Java, a method is like a function which is used to expose the behavior of an object. Advantage of Method Code Reusability Code Optimization

9. A class can contain any of the following variable types. Local variables − Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed. Instance variable s − Instance variables are variables within a class but outside any method. These variables are initialized when the class is instantiated. Instance variables can be accessed from inside any method, constructor or blocks of that particular class. Instance variable doesn't get memory at compile time. It gets memory at runtime when an object or instance is created. That is why it is known as an instance variable. Class variable s − Class variables are variables declared within a class, outside any method, with the static keyword .

10. Example class Dog { String breed; int age; String color; void barking() { } void hungry() { } void sleeping() { } } Creating Objects: An object is a block of memory that contains space to store all the instance variables. An object is a software entity (unit) that combines a set of data with set of operations to manipulate that data. A class defines a type of object. i.e., each object belongs to some class object is also called as an instance. Java has an operator new to create objects. Creating an object is also referred to as instantiating an object. Objects in java can be created by using the syntax. Syntax: Classname objname;
Objname = new classname(); The first statement declares a variable to hold the object reference and second one actually assigns the object reference to the variable.
Ex triangle tri1; // declaring the object) Tri1 = new triangle //instantiating the object) The above two statements are combined into a single statement as Triangle tri1 = new triangle();

11. Accessing class Members: every object contains its own set of variables. We should assign values to these variables in order to use them in our program. When we are outside the class we cannot access the instance variables and methods directly. For this we use dot operator. objectname.varaiblename = value;
objectname.methodname ( parameter list ); Example : tri1.length = 15;
tri2.length = 20;
tri1.getData( 20,30) public class MyClass { int a = 5; public static void main(String[] args) { MyClass myObj = new MyClass(); System.out.println(myObj.a); } }

12. Command line Arguments The java command-line argument is an argument i.e. passed at the time of running the java program. A Java application can accept any number of arguments from the command line. The String array stores all the arguments passed through the command line. Arguments are always stored as strings and always separated by white-space. // Example using Commandline arguments c lass Cmdargs { public static void main(String[] args) { System.out.println("Command-Line arguments are"); // loop through all arguments for(String str: args) { System.out.println(str); } } } 1. To compile the code javac Cmdargs.java 2. To run the code java Cmdargs Now suppose we want to pass some arguments while running the program, we can pass the arguments after the class name. For example, java Main apple ball cat output. Command-Line arguments are Apple Ball Cat

13. // Program to check for command line arguments class Hello { public static void main(String[] args) { // check if length of args array is greater than 0 if (args.length > 0) { System.out.println("The command line"+ " arguments are:"); / / iterating the args array and printing the command line arguments for (String s:args) System.out.println(s); } else System.out.println("No command line "+ "arguments found."); } } javac Hello.java java Hello Lion Dog Cat Elephant Output: The Command-Line arguments are Lion Dog Cat Elephant javac Hello.java java Hello Output No Command-Line arguments foun

14. Summary In this lesson you learnt about What is class and Object? How to create Class in java? How to creabj te O ects in java Command line arguments