Classes objects in java

Classes objects in java

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  JAVA P.Prathibha
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  Topics for Today’sSession About JAVA Creating Classes and Objects Command line arguments How to Create class and Object
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  Java  Java isa 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 run 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.
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  Objects and Classesin 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 State / Attributes Breed Color Age Behaviours Bark Eat Sleep Bite
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   An objectis 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.
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  Class  A classis 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.    A class in Java can contain:  Fields  Methods  Constructors  Blocks  Nested class and interface
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  Create a Class       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.
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  Fields declaration:  Datais 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
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  Local variables Instance variable     Classvariables .
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  Example class Dog { Stringbreed; 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();
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  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); } }
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  Ex: A javaprogram to demonstrate application ofclasses, objects and methods.(Pra.Prog. 17) class triangle { int length, width; void getData(int x, int y) { length = x; width= y; } int triarea( ) { int area = (length * width)/2; return(area); } } class triarea { public static void main (String args[ ]) { int area1, area2; triangle tri1 = new triangle(); triangle tri2 = new triangle(); tri1.length= 20; tri1.width = 30; area1 =( tri1.length * tri1.width)/2; tri2.getData( 10, 15); area2 = tri2.triarea(); System.out.println( "area1 = " + area1); System.out.println( "area2 = " + area2); } } Instance variables declared in a single line getData is a method, it does not return any value so its type is void, we are passing two integer values and they are assigned to length and width triarea is another method, it returns a value of data type int. We are not passing values, so the parameter list is empty class with main method Creating objects Accessing variables Accessing Methods
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  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 class 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
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  // Program tocheck 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
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