123 JAVA CLASSES, OBJECTS AND METHODS.ppt

123 JAVA CLASSES, OBJECTS AND METHODS.ppt

• 1.
  CLASSES, OBJECTS AND METHODS
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  What is Class? •A class is a blueprint for the object. • It determines how an object will behave and what the object will contain. • In other words, it is a blueprint or a set of instruction to build a specific type of object. Syntax class <class_name> { field; method; }
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  What is anObject? • An object is a self-contained component which consists of methods and properties to make a particular type of data useful. • Object determines the behavior of the class. When you send a message to an object, you are asking the object to invoke or execute one of its methods. Syntax ClassName ReferenceVariable = new ClassName();
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  What is theDifference Between Object & Class? • A class is a blueprint or prototype that defines the variables and the methods (functions) common to all objects of a certain kind. • An object is a specimen of a class. Software objects are often used to model real-world objects you find in everyday life.
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  • Understand theconcept of Java Classes and Objects with an example. • Let's take an example of developing a pet management system, specially meant for dogs. • You will need various information about the dogs like different breeds of the dogs, the age, size, etc. • You need to model real-life beings, i.e., dogs into software entities.
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  • You cansee the picture of three different breeds of dogs below. • Some of the differences you might have listed out maybe breed, age, size, color, etc. • If you think for a minute, these differences are also some common characteristics shared by these dogs. • These characteristics (breed, age, size, color) can form a data members for your object.
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  • Next, listout the common behaviors of these dogs like sleep, sit, eat, etc. • So these will be the actions of our software objects.
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  • Class -Dogs • Data members - size, age, color, breed, etc. • Methods- eat, sleep, sit and run.
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  • Now, fordifferent values of data members (breed size, age, and color) in Java class, you will get different dog objects. • You can design any program using this OOPs approach.
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  Example Code: Classand Object // Class Declaration public class Dog { // Instance Variables String breed; String size; int age; String color; // method 1 public String getInfo() { return ("Breed is: "+breed+" Size is:"+size+" Age is:"+age+" color is: "+color); } public static void main(String[] args) { Dog maltese = new Dog(); maltese.breed="Maltese"; maltese.size="Small"; maltese.age=2; maltese.color="white"; System.out.println(maltese.getInfo()); } } OUTPUT Breed is: Maltese Size is:Small Age is:2 color is: white
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  Object and ClassExample: main outside class In previous program, we are creating main() method inside the class. Now, we create classes and define main() method in another class. This is a better way than previous one. // Class Declaration class Dog { // Instance Variables String breed; String size; int age; String color; // method 1 public String getInfo() { return ("Breed is: "+breed+" Size is:"+size+" Age is:"+age+" color is: "+color); } } public class Execute{ public static void main(String[] args) { Dog maltese = new Dog(); maltese.breed="Maltese"; maltese.size="Small"; maltese.age=2; maltese.color="white"; System.out.println(maltese.getInfo()); } }
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  A class inJava can contain: – fields – methods – constructors – blocks – nested class and interface
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  Declaration of Class •A class is declared by use of the class keyword. • The class body is enclosed between curly braces { and }. • The data or variables, defined within a class are called instance variables. • The code is contained within methods. Collectively, the methods and variables defined within a class are called members of the class.
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  Declaration of InstanceVariables • Variables defined within a class are called instance variables because each instance of the class (that is, each object of the class) contains its own copy of these variables. • An instance variable can be declared public or private or default (no modifier). • When we do not want our variable’s value to be changed out-side our class we should declare them private. • public variables can be accessed and changed from outside of the class..
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  Java - Methods •A Java method is a collection of statements that are grouped together to perform an operation. • Methods are bound to a class and they define the behavior of a class. • When you call the System.out.println() method, for example, the system actually executes several statements in order to display a message on the console. Creating Method Considering the following example to explain the syntax of a method − Syntax public int addition(int a, int b) { return a+b; } Here, public − modifier int − return type addition − name of the method a, b − formal parameters int a, int b − list of parameters
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  Types of Javamethods • Depending on whether a method is defined by the user, or available in standard library, there are two types of methods: – Standard Library Methods – User-defined Methods Standard Library Methods • The standard library methods are built-in methods in Java that are readily available for use. • These standard libraries come along with the Java Class Library (JCL) in a Java archive (*.jar) file with JVM and JRE. For example, • print() is a method of java.io.PrintSteam. The print("...") prints the string inside quotation marks. • sqrt() is a method of Math class. It returns square root of a number.
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  User-defined Method • Youcan also define methods inside a class as per your wish. Such methods are called user-defined methods. The complete syntax for defining a Java method is: modifier static returnType nameOfMethod (Parameter List) { // method body }
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  How to calla Java Method? • Now you defined a method, you need to use it. For that, you have to call the method. Here's how: myFunction(); • This statement calls the myFunction(); method that was declared earlier. • While Java is executing the program code, it encounters myFunction(); in the code. • The execution then branches to the myFunction() method, and executes code inside the body of the method. • After the codes execution inside the method body is completed, the program returns to the original state and executes the next statement.
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  Complete Program ofJava Method class Main { public static void main(String[] args) { System.out.println("About to encounter a method."); // method call myMethod(); System.out.println("Method was executed successfully!"); } // method definition private static void myMethod() { System.out.println("Printing from inside myMethod()!"); } } Output: About to encounter a method. Printing from inside myMethod(). Method was executed successfully! • The method myMethod() in the above program doesn't accept any arguments. Also, the method doesn't return any value (return type is void). • Note that, we called the method without creating object of the class. It was possible because myMethod() is static.
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  What are theadvantages of using methods? • The main advantage is code reusability. You can write a method once, and use it multiple times. You do not have to rewrite the entire code each time. Think of it as, "write once, reuse multiple times." • Methods make code more readable and easier to debug. https://www.programiz.com/java-programming/methods