1. Object-Oriented Programming
2. Advantage of OOPs over Procedure-oriented programming language
3. Difference between Object-oriented and Object-based programming language.

In this page, we will learn about the basics of OOPs. Object-Oriented Programming is a paradigm that provides many concepts, such as inheritance, data binding, polymorphism, etc.

Simula is considered the first object-oriented programming language. The programming paradigm where everything is represented as an object is known as a truly object-oriented programming language.

Smalltalk is considered the first truly object-oriented programming language.

The popular object-oriented languages are Java, C#, PHP, Python, C++, etc.

The main aim of object-oriented programming is to implement real-world entities, for example, object, classes, abstraction, inheritance, polymorphism, etc.

Object means a real-world entity such as a pen, chair, table, computer, watch, etc. Object-Oriented Programming is a methodology or paradigm to design a program using classes and objects. It simplifies software development and maintenance by providing some concepts:

• Object
• Class
• Inheritance
• Polymorphism
• Abstraction
• Encapsulation

Apart from these concepts, there are some other terms which are used in Object-Oriented design:

• Coupling
• Cohesion
• Association
• Aggregation
• Composition

Any entity that has state and behavior is known as an object. For example, a chair, pen, table, keyboard, bike, etc. It can be physical or logical.

An Object can be defined as an instance of a class. An object contains an address and takes up some space in memory. Objects can communicate without knowing the details of each other's data or code. The only necessary thing is the type of message accepted and the type of response returned by the objects.

Example: A dog is an object because it has states like color, name, breed, etc. as well as behaviors like wagging the tail, barking, eating, etc.

Collection of objects is called class. It is a logical entity.

A class can also be defined as a blueprint from which you can create an individual object. Class doesn't consume any space.

When one object acquires all the properties and behaviors of a parent object, it is known as inheritance. It provides code reusability. It is used to achieve runtime polymorphism.

If one task is performed in different ways, it is known as polymorphism. For example: to convince the customer differently, to draw something, for example, shape, triangle, rectangle, etc.

In Java, we use method overloading and method overriding to achieve polymorphism.

Another example can be to speak something; for example, a cat speaks meow, dog barks woof, etc.

Hiding internal details and showing functionality is known as abstraction. For example phone call, we don't know the internal processing.

In Java, we use abstract class and interface to achieve abstraction.

Binding (or wrapping) code and data together into a single unit are known as encapsulation. For example, a capsule, it is wrapped with different medicines.

A java class is the example of encapsulation. Java bean is the fully encapsulated class because all the data members are private here.

Coupling refers to the knowledge or information or dependency of another class. It arises when classes are aware of each other. If a class has the details information of another class, there is strong coupling. In Java, we use private, protected, and public modifiers to display the visibility level of a class, method, and field. You can use interfaces for the weaker coupling because there is no concrete implementation.

Cohesion refers to the level of a component which performs a single well-defined task. A single well-defined task is done by a highly cohesive method. The weakly cohesive method will split the task into separate parts. The java.io package is a highly cohesive package because it has I/O related classes and interface. However, the java.util package is a weakly cohesive package because it has unrelated classes and interfaces.

Association represents the relationship between the objects. Here, one object can be associated with one object or many objects. There can be four types of association between the objects:

• One to One
• One to Many
• Many to One, and
• Many to Many

Let's understand the relationship with real-time examples. For example, One country can have one prime minister (one to one), and a prime minister can have many ministers (one to many). Also, many MP's can have one prime minister (many to one), and many ministers can have many departments (many to many).

Association can be undirectional or bidirectional.

Aggregation is a way to achieve Association. Aggregation represents the relationship where one object contains other objects as a part of its state. It represents the weak relationship between objects. It is also termed as a has-a relationship in Java. Like, inheritance represents the is-a relationship. It is another way to reuse objects.

The composition is also a way to achieve Association. The composition represents the relationship where one object contains other objects as a part of its state. There is a strong relationship between the containing object and the dependent object. It is the state where containing objects do not have an independent existence. If you delete the parent object, all the child objects will be deleted automatically.

1) OOPs makes development and maintenance easier, whereas, in a procedure-oriented programming language, it is not easy to manage if code grows as project size increases.

2) OOPs provides data hiding, whereas, in a procedure-oriented programming language, global data can be accessed from anywhere.

Figure: Data Representation in Procedure-Oriented Programming

Figure: Data Representation in Object-Oriented Programming

3) OOPs provides the ability to simulate real-world event much more effectively. We can provide the solution of real word problem if we are using the Object-Oriented Programming language.

Object-based programming language follows all the features of OOPs except Inheritance. JavaScript and VBScript are examples of object-based programming languages.

Do You Know?

• Can we overload the main method?
• A Java Constructor returns a value but, what?
• Can we create a program without main method?
• What are the six ways to use this keyword?
• Why is multiple inheritance not supported in Java?
• Why use aggregation?
• Can we override the static method?
• What is the covariant return type?
• What are the three usages of Java super keyword?
• Why use instance initializer block?
• What is the usage of a blank final variable?
• What is a marker or tagged interface?
• What is runtime polymorphism or dynamic method dispatch?
• What is the difference between static and dynamic binding?
• How downcasting is possible in Java?
• What is the purpose of a private constructor?
• What is object cloning?

What will we learn in OOPs Concepts?

• Advantage of OOPs
• Naming Convention
• Object and class
• Method overloading
• Constructor
• static keyword
• this keyword with six usage
• Inheritance
• Aggregation
• Method Overriding
• Covariant Return Type
• super keyword
• Instance Initializer block
• final keyword
• Abstract class
• Interface
• Runtime Polymorphism
• Static and Dynamic Binding
• Downcasting with instanceof operator
• Package
• Access Modifiers
• Encapsulation
• Object Cloning

===========================================================================

Java naming convention is a rule to follow as you decide what to name your identifiers such as class, package, variable, constant, method, etc.

But, it is not forced to follow. So, it is known as convention not rule. These conventions are suggested by several Java communities such as Sun Microsystems and Netscape.

All the classes, interfaces, packages, methods and fields of Java programming language are given according to the Java naming convention. If you fail to follow these conventions, it may generate confusion or erroneous code.

By using standard Java naming conventions, you make your code easier to read for yourself and other programmers. Readability of Java program is very important. It indicates that less time is spent to figure out what the code does.

 =========================================================================

1. Object in Java
2. Class in Java
3. Instance Variable in Java
4. Method in Java
5. Example of Object and class that maintains the records of student
6. Anonymous Object

In this page, we will learn about Java objects and classes. In object-oriented programming technique, we design a program using objects and classes.

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, marker, 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 has three characteristics:

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• State: represents the data (value) of an object.
• Behavior: represents the behavior (functionality) of an object such as deposit, withdraw, etc.
• 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 white, known as its state. It is used to write, so writing is its behavior.

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.

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.

A class in Java can contain:

• Fields
• Methods
• Constructors
• Blocks
• Nested class and interface

1. class <class_name>{  
2.     field;  
3.     method;  
4. }  

A variable which is created inside the class but outside the method is known as an instance variable. 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.

In Java, a method is like a function which is used to expose the behavior of an object.

• Code Reusability
• Code Optimization
• 
  

The new keyword is used to allocate memory at runtime. All objects get memory in Heap memory area

In this example, we have created a Student class which has two data members id and name. We are creating the object of the Student class by new keyword and printing the object's value.

Here, we are creating a main() method inside the class.

File: Student.java

1. //Java Program to illustrate how to define a class and fields 
2. //Defining a Student class. 
3. class Student{  
4. //defining fields 
5. int id;//field or data member or instance variable 
6.  String name;  
7. //creating main method inside the Student class 
8. public static void main(String args[]){  
9. //Creating an object or instance 
10.   Student s1=new Student();//creating an object of Student 
11. //Printing values of the object 
12.   System.out.println(s1.id);//accessing member through reference variable 
13.   System.out.println(s1.name);  
14.  }  
15. }  

Test it Now

Output:

0 

null

In real time development, we create classes and use it from another class. It is a better approach than previous one. Let's see a simple example, where we are having main() method in another class.

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We can have multiple classes in different Java files or single Java file. If you define multiple classes in a single Java source file, it is a good idea to save the file name with the class name which has main() method.

File: TestStudent1.java

1. //Java Program to demonstrate having the main method in  
2. //another class 
3. //Creating Student class. 
4. class Student{  
5. int id;  
6.  String name;  
7. }  
8. //Creating another class TestStudent1 which contains the main method 
9. class TestStudent1{  
10. public static void main(String args[]){  
11.   Student s1=new Student();  
12.   System.out.println(s1.id);  
13.   System.out.println(s1.name);  
14.  }  
15. }  

Test it Now

Output:

0 

null

There are 3 ways to initialize object in Java.

1. By reference variable
2. By method
3. By constructor

Initializing an object means storing data into the object. Let's see a simple example where we are going to initialize the object through a reference variable.

File: TestStudent2.java

1. class Student{  
2. int id;  
3.  String name;  
4. }  
5. class TestStudent2{  
6. public static void main(String args[]){  
7.   Student s1=new Student();  
8.   s1.id=101;  
9.   s1.name="Sonoo";  
10.   System.out.println(s1.id+" "+s1.name);//printing members with a white space 
11.  }  
12. }  

Test it Now

Output:

101 Sonoo

We can also create multiple objects and store information in it through reference variable.

File: TestStudent3.java

1. class Student{  
2. int id;  
3.  String name;  
4. }  
5. class TestStudent3{  
6. public static void main(String args[]){  
7. //Creating objects 
8.   Student s1=new Student();  
9.   Student s2=new Student();  
10. //Initializing objects 
11.   s1.id=101;  
12.   s1.name="Sonoo";  
13.   s2.id=102;  
14.   s2.name="Amit";  
15. //Printing data 
16.   System.out.println(s1.id+" "+s1.name);  
17.   System.out.println(s2.id+" "+s2.name);  
18.  }  
19. }  

Test it Now

Output:

101 Sonoo

102 Amit

In this example, we are creating the two objects of Student class and initializing the value to these objects by invoking the insertRecord method. Here, we are displaying the state (data) of the objects by invoking the displayInformation() method.

File: TestStudent4.java

1. class Student{  
2. int rollno;  
3.  String name;  
4. void insertRecord(int r, String n){  
5.   rollno=r;  
6.   name=n;  
7.  }  
8. void displayInformation(){System.out.println(rollno+" "+name);}  
9. }  
10. class TestStudent4{  
11. public static void main(String args[]){  
12.   Student s1=new Student();  
13.   Student s2=new Student();  
14.   s1.insertRecord(111,"Karan");  
15.   s2.insertRecord(222,"Aryan");  
16.   s1.displayInformation();  
17.   s2.displayInformation();  
18.  }  
19. }  

Test it Now

Output:

111 Karan

222 Aryan

As you can see in the above figure, object gets the memory in heap memory area. The reference variable refers to the object allocated in the heap memory area. Here, s1 and s2 both are reference variables that refer to the objects allocated in memory.

We will learn about constructors in Java later.

Let's see an example where we are maintaining records of employees.

File: TestEmployee.java

1. class Employee{  
2. int id;  
3.     String name;  
4. float salary;  
5. void insert(int i, String n, float s) {  
6.         id=i;  
7.         name=n;  
8.         salary=s;  
9.     }  
10. void display(){System.out.println(id+" "+name+" "+salary);}  
11. }  
12. public class TestEmployee {  
13. public static void main(String[] args) {  
14.     Employee e1=new Employee();  
15.     Employee e2=new Employee();  
16.     Employee e3=new Employee();  
17.     e1.insert(101,"ajeet",45000);  
18.     e2.insert(102,"irfan",25000);  
19.     e3.insert(103,"nakul",55000);  
20.     e1.display();  
21.     e2.display();  
22.     e3.display();  
23. }  
24. }  

Test it Now

Output:

101 ajeet 45000.0

102 irfan 25000.0

103 nakul 55000.0

There is given another example that maintains the records of Rectangle class.

File: TestRectangle1.java

1. class Rectangle{  
2. int length;  
3. int width;  
4. void insert(int l, int w){  
5.   length=l;  
6.   width=w;  
7.  }  
8. void calculateArea(){System.out.println(length*width);}  
9. }  
10. class TestRectangle1{  
11. public static void main(String args[]){  
12.   Rectangle r1=new Rectangle();  
13.   Rectangle r2=new Rectangle();  
14.   r1.insert(11,5);  
15.   r2.insert(3,15);  
16.   r1.calculateArea();  
17.   r2.calculateArea();  
18. }  
19. }  

Test it Now

Output:

55 

45     

There are many ways to create an object in java. They are:

• By new keyword
• By newInstance() method
• By clone() method
• By deserialization
• By factory method etc.

We will learn these ways to create object later.

Anonymous simply means nameless. An object which has no reference is known as an anonymous object. It can be used at the time of object creation only.

If you have to use an object only once, an anonymous object is a good approach. For example:

1. new Calculation();//anonymous object 

Calling method through a reference:

1. Calculation c=new Calculation();  
2. c.fact(5);  

Calling method through an anonymous object

1. new Calculation().fact(5);  

Let's see the full example of an anonymous object in Java.

1. class Calculation{  
2. void fact(int  n){  
3. int fact=1;  
4. for(int i=1;i<=n;i++){  
5.    fact=fact*i;  
6.   }  
7.  System.out.println("factorial is "+fact);  
8. }  
9. public static void main(String args[]){  
10. new Calculation().fact(5);//calling method with anonymous object 
11. }  
12. }  

Output:

Factorial is 120

We can create multiple objects by one type only as we do in case of primitives.

Initialization of primitive variables:

1. int a=10, b=20;  

Initialization of refernce variables:

1. Rectangle r1=new Rectangle(), r2=new Rectangle();//creating two objects 

Let's see the example:

1. //Java Program to illustrate the use of Rectangle class which 
2. //has length and width data members 
3. class Rectangle{  
4. int length;  
5. int width;  
6. void insert(int l,int w){  
7.   length=l;  
8.   width=w;  
9.  }  
10. void calculateArea(){System.out.println(length*width);}  
11. }  
12. class TestRectangle2{  
13. public static void main(String args[]){  
14.   Rectangle r1=new Rectangle(),r2=new Rectangle();//creating two objects 
15.   r1.insert(11,5);  
16.   r2.insert(3,15);  
17.   r1.calculateArea();  
18.   r2.calculateArea();  
19. }  
20. }  

Test it Now

Output:

55 

45     

File: TestAccount.java

1. //Java Program to demonstrate the working of a banking-system 
2. //where we deposit and withdraw amount from our account. 
3. //Creating an Account class which has deposit() and withdraw() methods 
4. class Account{  
5. int acc_no;  
6. String name;  
7. float amount;  
8. //Method to initialize object 
9. void insert(int a,String n,float amt){  
10. acc_no=a;  
11. name=n;  
12. amount=amt;  
13. }  
14. //deposit method 
15. void deposit(float amt){  
16. amount=amount+amt;  
17. System.out.println(amt+" deposited");  
18. }  
19. //withdraw method 
20. void withdraw(float amt){  
21. if(amount<amt){  
22. System.out.println("Insufficient Balance");  
23. }else{  
24. amount=amount-amt;  
25. System.out.println(amt+" withdrawn");  
26. }  
27. }  
28. //method to check the balance of the account 
29. void checkBalance(){System.out.println("Balance is: "+amount);}  
30. //method to display the values of an object 
31. void display(){System.out.println(acc_no+" "+name+" "+amount);}  
32. }  
33. //Creating a test class to deposit and withdraw amount 
34. class TestAccount{  
35. public static void main(String[] args){  
36. Account a1=new Account();  
37. a1.insert(832345,"Ankit",1000);  
38. a1.display();  
39. a1.checkBalance();  
40. a1.deposit(40000);  
41. a1.checkBalance();  
42. a1.withdraw(15000);  
43. a1.checkBalance();  
44. }}   

Test it Now

Output:

832345 Ankit 1000.0

Balance is: 1000.0

40000.0 deposited

Balance is: 41000.0

15000.0 withdrawn

Balance is: 26000.0

==============================================================================

In general, a method is a way to perform some task. Similarly, the method in Java is a collection of instructions that performs a specific task. It provides the reusability of code. We can also easily modify code using methods. In this section, we will learn what is a method in Java, types of methods, method declaration, and how to call a method in Java.

A method is a block of code or collection of statements or a set of code grouped together to perform a certain task or operation. It is used to achieve the reusability of code. We write a method once and use it many times. We do not require to write code again and again. It also provides the easy modification and readability of code, just by adding or removing a chunk of code. The method is executed only when we call or invoke it.

The most important method in Java is the main() method. If you want to read more about the main() method, go through the link https://www.javatpoint.com/java-main-method.

The method declaration provides information about method attributes, such as visibility, return-type, name, and arguments. It has six components that are known as method header, as we have shown in the following figure.

Method Signature: Every method has a method signature. It is a part of the method declaration. It includes the method name and parameter list.

Access Specifier: Access specifier or modifier is the access type of the method. It specifies the visibility of the method. Java provides four types of access specifier:

• Public: The method is accessible by all classes when we use public specifier in our application.
• Private: When we use a private access specifier, the method is accessible only in the classes in which it is defined.
• Protected: When we use protected access specifier, the method is accessible within the same package or subclasses in a different package.
• Default: When we do not use any access specifier in the method declaration, Java uses default access specifier by default. It is visible only from the same package only.

Return Type: Return type is a data type that the method returns. It may have a primitive data type, object, collection, void, etc. If the method does not return anything, we use void keyword.

Method Name: It is a unique name that is used to define the name of a method. It must be corresponding to the functionality of the method. Suppose, if we are creating a method for subtraction of two numbers, the method name must be subtraction(). A method is invoked by its name.

Parameter List: It is the list of parameters separated by a comma and enclosed in the pair of parentheses. It contains the data type and variable name. If the method has no parameter, left the parentheses blank.

Method Body: It is a part of the method declaration. It contains all the actions to be performed. It is enclosed within the pair of curly braces.

While defining a method, remember that the method name must be a verb and start with a lowercase letter. If the method name has more than two words, the first name must be a verb followed by adjective or noun. In the multi-word method name, the first letter of each word must be in uppercase except the first word. For example:

Single-word method name: sum(), area()

Multi-word method name: areaOfCircle(), stringComparision()

It is also possible that a method has the same name as another method name in the same class, it is known as method overloading.

There are two types of methods in Java:

• Predefined Method
• User-defined Method

In Java, predefined methods are the method that is already defined in the Java class libraries is known as predefined methods. It is also known as the standard library method or built-in method. We can directly use these methods just by calling them in the program at any point. Some pre-defined methods are length(), equals(), compareTo(), sqrt(), etc. When we call any of the predefined methods in our program, a series of codes related to the corresponding method runs in the background that is already stored in the library.

Each and every predefined method is defined inside a class. Such as print() method is defined in the java.io.PrintStream class. It prints the statement that we write inside the method. For example, print("Java"), it prints Java on the console.

Let's see an example of the predefined method.

Demo.java

1. public class Demo   
2. {  
3. public static void main(String[] args)   
4. {  
5. // using the max() method of Math class 
6. System.out.print("The maximum number is: " + Math.max(9,7));  
7. }  
8. }  

Output:

The maximum number is: 9

In the above example, we have used three predefined methods main(), print(), and max(). We have used these methods directly without declaration because they are predefined. The print() method is a method of PrintStream class that prints the result on the console. The max() method is a method of the Math class that returns the greater of two numbers.

We can also see the method signature of any predefined method by using the link https://docs.oracle.com/. When we go through the link and see the max() method signature, we find the following:

In the above method signature, we see that the method signature has access specifier public, non-access modifier static, return type int, method name max(), parameter list (int a, int b). In the above example, instead of defining the method, we have just invoked the method. This is the advantage of a predefined method. It makes programming less complicated.

Similarly, we can also see the method signature of the print() method.

The method written by the user or programmer is known as a user-defined method. These methods are modified according to the requirement.

Let's create a user defined method that checks the number is even or odd. First, we will define the method.

1. //user defined method 
2. public static void findEvenOdd(int num)  
3. {  
4. //method body 
5. if(num%2==0)   
6. System.out.println(num+" is even");   
7. else 
8. System.out.println(num+" is odd");  
9. }  

We have defined the above method named findevenodd(). It has a parameter num of type int. The method does not return any value that's why we have used void. The method body contains the steps to check the number is even or odd. If the number is even, it prints the number is even, else prints the number is odd.

Once we have defined a method, it should be called. The calling of a method in a program is simple. When we call or invoke a user-defined method, the program control transfer to the called method.

1. import java.util.Scanner;  
2. public class EvenOdd  
3. {  
4. public static void main (String args[])  
5. {  
6. //creating Scanner class object    
7. Scanner scan=new Scanner(System.in);  
8. System.out.print("Enter the number: ");  
9. //reading value from the user 
10. int num=scan.nextInt();  
11. //method calling 
12. findEvenOdd(num);  
13. }  

In the above code snippet, as soon as the compiler reaches at line findEvenOdd(num), the control transfer to the method and gives the output accordingly.

Let's combine both snippets of codes in a single program and execute it.

EvenOdd.java

1. import java.util.Scanner;  
2. public class EvenOdd  
3. {  
4. public static void main (String args[])  
5. {  
6. //creating Scanner class object    
7. Scanner scan=new Scanner(System.in);  
8. System.out.print("Enter the number: ");  
9. //reading value from user 
10. int num=scan.nextInt();  
11. //method calling 
12. findEvenOdd(num);  
13. }  
14. //user defined method 
15. public static void findEvenOdd(int num)  
16. {  
17. //method body 
18. if(num%2==0)   
19. System.out.println(num+" is even");   
20. else 
21. System.out.println(num+" is odd");  
22. }  
23. }  

Output 1:

Enter the number: 12

12 is even

Output 2:

Enter the number: 99

99 is odd

Let's see another program that return a value to the calling method.

In the following program, we have defined a method named add() that sum up the two numbers. It has two parameters n1 and n2 of integer type. The values of n1 and n2 correspond to the value of a and b, respectively. Therefore, the method adds the value of a and b and store it in the variable s and returns the sum.

Addition.java

1. public class Addition   
2. {  
3. public static void main(String[] args)   
4. {  
5. int a = 19;  
6. int b = 5;  
7. //method calling 
8. int c = add(a, b);   //a and b are actual parameters 
9. System.out.println("The sum of a and b is= " + c);  
10. }  
11. //user defined method 
12. public static int add(int n1, int n2)   //n1 and n2 are formal parameters 
13. {  
14. int s;  
15. s=n1+n2;  
16. return s; //returning the sum 
17. }  
18. }  

Output:

The sum of a and b is= 24

A method that has static keyword is known as static method. In other words, a method that belongs to a class rather than an instance of a class is known as a static method. We can also create a static method by using the keyword static before the method name.

The main advantage of a static method is that we can call it without creating an object. It can access static data members and also change the value of it. It is used to create an instance method. It is invoked by using the class name. The best example of a static method is the main() method.

Display.java

1. public class Display  
2. {  
3. public static void main(String[] args)   
4. {  
5. show();  
6. }  
7. static void show()   
8. {  
9. System.out.println("It is an example of static method.");  
10. }  
11. }  

Output:

It is an example of a static method.

The method of the class is known as an instance method. It is a non-static method defined in the class. Before calling or invoking the instance method, it is necessary to create an object of its class. Let's see an example of an instance method.

InstanceMethodExample.java

1. public class InstanceMethodExample  
2. {  
3. public static void main(String [] args)  
4. {  
5. //Creating an object of the class 
6. InstanceMethodExample obj = new InstanceMethodExample();  
7. //invoking instance method  
8. System.out.println("The sum is: "+obj.add(12, 13));  
9. }  
10. int s;  
11. //user-defined method because we have not used static keyword 
12. public int add(int a, int b)  
13. {  
14. s = a+b;  
15. //returning the sum 
16. return s;  
17. }  
18. } 

Output:

The sum is: 25

There are two types of instance method:

• Accessor Method
• Mutator Method

Accessor Method: The method(s) that reads the instance variable(s) is known as the accessor method. We can easily identify it because the method is prefixed with the word get. It is also known as getters. It returns the value of the private field. It is used to get the value of the private field.

Example

1. public int getId()    
2. {    
3. return Id;    
4. } 

Mutator Method: The method(s) read the instance variable(s) and also modify the values. We can easily identify it because the method is prefixed with the word set. It is also known as setters or modifiers. It does not return anything. It accepts a parameter of the same data type that depends on the field. It is used to set the value of the private field.

Example

1. public void setRoll(int roll)   
2. {  
3. this.roll = roll;  
4. }  

Student.java

1. public class Student   
2. {  
3. private int roll;  
4. private String name;  
5. public int getRoll()    //accessor method 
6. {  
7. return roll;  
8. }  
9. public void setRoll(int roll) //mutator method 
10. {  
11. this.roll = roll;  
12. }  
13. public String getName()   
14. {  
15. return name;  
16. }  
17. public void setName(String name)   
18. {  
19. this.name = name;  
20. }  
21. public void display()  
22. {  
23. System.out.println("Roll no.: "+roll);  
24. System.out.println("Student name: "+name);  
25. }  
26. }  

The method that does not has method body is known as abstract method. In other words, without an implementation is known as abstract method. It always declares in the abstract class. It means the class itself must be abstract if it has abstract method. To create an abstract method, we use the keyword abstract.

Syntax

1. abstract void method_name();  
2. 
   

Demo.java

1. abstract class Demo //abstract class 
2. {  
3. //abstract method declaration 
4. abstract void display();  
5. }  
6. public class MyClass extends Demo  
7. {  
8. //method impelmentation 
9. void display()  
10. {  
11. System.out.println("Abstract method?");  
12. }  
13. public static void main(String args[])  
14. {  
15. //creating object of abstract class 
16. Demo obj = new MyClass();  
17. //invoking abstract method 
18. obj.display();  
19. }  
20. }  

Output:

Abstract method...

It is a method that returns an object to the class to which it belongs. All static methods are factory methods. For example, NumberFormat obj = NumberFormat.getNumberInstance();

=========================================================================

1. Types of constructors
   1. Default Constructor
   2. Parameterized Constructor
2. Constructor Overloading
3. Does constructor return any value?
4. Copying the values of one object into another
5. Does constructor perform other tasks instead of the initialization

In Java, a constructor is a block of codes similar to the method. It is called when an instance of the class is created. At the time of calling constructor, memory for the object is allocated in the memory.

It is a special type of method which is used to initialize the object.

Every time an object is created using the new() keyword, at least one constructor is called.

It calls a default constructor if there is no constructor available in the class. In such case, Java compiler provides a default constructor by default.

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There are two types of constructors in Java: no-arg constructor, and parameterized constructor.

Note: It is called constructor because it constructs the values at the time of object creation. It is not necessary to write a constructor for a class. It is because java compiler creates a default constructor if your class doesn't have any.

There are two rules defined for the constructor.

1. Constructor name must be the same as its class name
2. A Constructor must have no explicit return type
3. A Java constructor cannot be abstract, static, final, and synchronized

There are two types of constructors in Java:

1. Default constructor (no-arg constructor)
2. Parameterized constructor

A constructor is called "Default Constructor" when it doesn't have any parameter.

1. <class_name>(){}  

In this example, we are creating the no-arg constructor in the Bike class. It will be invoked at the time of object creation.

1. //Java Program to create and call a default constructor 
2. class Bike1{  
3. //creating a default constructor 
4. Bike1(){System.out.println("Bike is created");}  
5. //main method 
6. public static void main(String args[]){  
7. //calling a default constructor 
8. Bike1 b=new Bike1();  
9. }  
10. }  

Test it Now

Output:

Bike is created

The default constructor is used to provide the default values to the object like 0, null, etc., depending on the type.

1. //Let us see another example of default constructor 
2. //which displays the default values 
3. class Student3{  
4. int id;  
5. String name;  
6. //method to display the value of id and name 
7. void display(){System.out.println(id+" "+name);}  
8. public static void main(String args[]){  
9. //creating objects 
10. Student3 s1=new Student3();  
11. Student3 s2=new Student3();  
12. //displaying values of the object 
13. s1.display();  
14. s2.display();  
15. }  
16. }  

Test it Now

Output:

0 null

0 null

Explanation:In the above class,you are not creating any constructor so compiler provides you a default constructor. Here 0 and null values are provided by default constructor.

A constructor which has a specific number of parameters is called a parameterized constructor.

The parameterized constructor is used to provide different values to distinct objects. However, you can provide the same values also

.

In this example, we have created the constructor of Student class that have two parameters. We can have any number of parameters in the constructor.

1. //Java Program to demonstrate the use of the parameterized constructor. 
2. class Student4{  
3. int id;  
4.     String name;  
5. //creating a parameterized constructor 
6.     Student4(int i,String n){  
7.     id = i;  
8.     name = n;  
9.     }  
10. //method to display the values 
11. void display(){System.out.println(id+" "+name);}  
12. public static void main(String args[]){  
13. //creating objects and passing values 
14.     Student4 s1 = new Student4(111,"Karan");  
15.     Student4 s2 = new Student4(222,"Aryan");  
16. //calling method to display the values of object 
17.     s1.display();  
18.     s2.display();  
19.    }  
20. }  

Test it Now

Output:

111 Karan

222 Aryan

In Java, a constructor is just like a method but without return type. It can also be overloaded like Java methods.

Constructor overloading in Java is a technique of having more than one constructor with different parameter lists. They are arranged in a way that each constructor performs a different task. They are differentiated by the compiler by the number of parameters in the list and their types.

1. //Java program to overload constructors 
2. class Student5{  
3. int id;  
4.     String name;  
5. int age;  
6. //creating two arg constructor 
7.     Student5(int i,String n){  
8.     id = i;  
9.     name = n;  
10.     }  
11. //creating three arg constructor 
12.     Student5(int i,String n,int a){  
13.     id = i;  
14.     name = n;  
15.     age=a;  
16.     }  
17. void display(){System.out.println(id+" "+name+" "+age);}  
18. public static void main(String args[]){  
19.     Student5 s1 = new Student5(111,"Karan");  
20.     Student5 s2 = new Student5(222,"Aryan",25);  
21.     s1.display();  
22.     s2.display();  
23.    }  
24. } 

Test it Now

Output:

111 Karan 0

222 Aryan 25

There are many differences between constructors and methods. They are given below.

Java ConstructorJava MethodA constructor is used to initialize the state of an object.A method is used to expose the behavior of an object.A constructor must not have a return type.A method must have a return type.The constructor is invoked implicitly.The method is invoked explicitly.The Java compiler provides a default constructor if you don't have any constructor in a class.The method is not provided by the compiler in any case.The constructor name must be same as the class name.The method name may or may not be same as the class name.

There is no copy constructor in Java. However, we can copy the values from one object to another like copy constructor in C++.

There are many ways to copy the values of one object into another in Java. They are:

• By constructor
• By assigning the values of one object into another
• By clone() method of Object class

In this example, we are going to copy the values of one object into another using Java constructor.

1. //Java program to initialize the values from one object to another object. 
2. class Student6{  
3. int id;  
4.     String name;  
5. //constructor to initialize integer and string 
6.     Student6(int i,String n){  
7.     id = i;  
8.     name = n;  
9.     }  
10. //constructor to initialize another object 
11.     Student6(Student6 s){  
12.     id = s.id;  
13.     name =s.name;  
14.     }  
15. void display(){System.out.println(id+" "+name);}  
16. public static void main(String args[]){  
17.     Student6 s1 = new Student6(111,"Karan");  
18.     Student6 s2 = new Student6(s1);  
19.     s1.display();  
20.     s2.display();  
21.    }  
22. } 

Test it Now

Output:

111 Karan

111 Karan

We can copy the values of one object into another by assigning the objects values to another object. In this case, there is no need to create the constructor.

1. class Student7{  
2. int id;  
3.     String name;  
4.     Student7(int i,String n){  
5.     id = i;  
6.     name = n;  
7.     }  
8.     Student7(){}  
9. void display(){System.out.println(id+" "+name);}  
10. public static void main(String args[]){  
11.     Student7 s1 = new Student7(111,"Karan");  
12.     Student7 s2 = new Student7();  
13.     s2.id=s1.id;  
14.     s2.name=s1.name;  
15.     s1.display();  
16.     s2.display();  
17.    }  
18. }  

Test it Now

Output:

111 Karan

111 Karan

Yes, it is the current class instance (You cannot use return type yet it returns a value).

Yes, like object creation, starting a thread, calling a method, etc. You can perform any operation in the constructor as you perform in the method.

Yes.

Java provides a Constructor class which can be used to get the internal information of a constructor in the class. It is found in the java.lang.reflect package.

=========================================================================

1. Static variable
2. Program of the counter without static variable
3. Program of the counter with static variable
4. Static method
5. Restrictions for the static method
6. Why is the main method static?
7. Static block
8. Can we execute a program without main method?

The static keyword in Java is used for memory management mainly. We can apply static keyword with variables, methods, blocks and nested classes. The static keyword belongs to the class than an instance of the class.

The static can be:

1. Variable (also known as a class variable)
2. Method (also known as a class method)
3. Block
4. Nested class

If you declare any variable as static, it is known as a static variable.

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• The static variable can be used to refer to the common property of all objects (which is not unique for each object), for example, the company name of employees, college name of students, etc.
• The static variable gets memory only once in the class area at the time of class loading.

It makes your program memory efficient (i.e., it saves memory).

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1. class Student{  
2. int rollno;  
3.      String name;  
4.      String college="ITS";  
5. }  

Suppose there are 500 students in my college, now all instance data members will get memory each time when the object is created. All students have its unique rollno and name, so instance data member is good in such case. Here, "college" refers to the common property of all objects. If we make it static, this field will get the memory only once.

1. //Java Program to demonstrate the use of static variable 
2. class Student{  
3. int rollno;//instance variable 
4.    String name;  
5. static String college ="ITS";//static variable 
6. //constructor 
7.    Student(int r, String n){  
8.    rollno = r;  
9.    name = n;  
10.    }  
11. //method to display the values 
12. void display (){System.out.println(rollno+" "+name+" "+college);}  
13. }  
14. //Test class to show the values of objects 
15. public class TestStaticVariable1{  
16. public static void main(String args[]){  
17.  Student s1 = new Student(111,"Karan");  
18.  Student s2 = new Student(222,"Aryan");  
19. //we can change the college of all objects by the single line of code 
20. //Student.college="BBDIT"; 
21.  s1.display();  
22.  s2.display();  
23.  }  
24. }  

Test it Now

Output:

111 Karan ITS

222 Aryan ITS

In this example, we have created an instance variable named count which is incremented in the constructor. Since instance variable gets the memory at the time of object creation, each object will have the copy of the instance variable. If it is incremented, it won't reflect other objects. So each object will have the value 1 in the count variable.

1. //Java Program to demonstrate the use of an instance variable 
2. //which get memory each time when we create an object of the class. 
3. class Counter{  
4. int count=0;//will get memory each time when the instance is created 
5. Counter(){  
6. count++;//incrementing value 
7. System.out.println(count);  
8. }  
9. public static void main(String args[]){  
10. //Creating objects 
11. Counter c1=new Counter();  
12. Counter c2=new Counter();  
13. Counter c3=new Counter();  
14. }  
15. }  

Test it Now

Output:

1

1

1

As we have mentioned above, static variable will get the memory only once, if any object changes the value of the static variable, it will retain its value.

1. //Java Program to illustrate the use of static variable which 
2. //is shared with all objects. 
3. class Counter2{  
4. static int count=0;//will get memory only once and retain its value 
5. Counter2(){  
6. count++;//incrementing the value of static variable 
7. System.out.println(count);  
8. }  
9. public static void main(String args[]){  
10. //creating objects 
11. Counter2 c1=new Counter2();  
12. Counter2 c2=new Counter2();  
13. Counter2 c3=new Counter2();  
14. }  
15. } 

Test it Now

Output:

1

2

3

If you apply static keyword with any method, it is known as static method.

• A static method belongs to the class rather than the object of a class.
• A static method can be invoked without the need for creating an instance of a class.
• A static method can access static data member and can change the value of it.

1. //Java Program to demonstrate the use of a static method. 
2. class Student{  
3. int rollno;  
4.      String name;  
5. static String college = "ITS";  
6. //static method to change the value of static variable 
7. static void change(){  
8.      college = "BBDIT";  
9.      }  
10. //constructor to initialize the variable 
11.      Student(int r, String n){  
12.      rollno = r;  
13.      name = n;  
14.      }  
15. //method to display values 
16. void display(){System.out.println(rollno+" "+name+" "+college);}  
17. }  
18. //Test class to create and display the values of object 
19. public class TestStaticMethod{  
20. public static void main(String args[]){  
21.     Student.change();//calling change method 
22. //creating objects 
23.     Student s1 = new Student(111,"Karan");  
24.     Student s2 = new Student(222,"Aryan");  
25.     Student s3 = new Student(333,"Sonoo");  
26. //calling display method 
27.     s1.display();  
28.     s2.display();  
29.     s3.display();  
30.     }  
31. }  

Test it Now

Output:111 Karan BBDIT

       222 Aryan BBDIT

       333 Sonoo BBDIT

1. //Java Program to get the cube of a given number using the static method 
2. class Calculate{  
3. static int cube(int x){  
4. return x*x*x;  
5.   }  
6. public static void main(String args[]){  
7. int result=Calculate.cube(5);  
8.   System.out.println(result);  
9.   }  
10. }  

Test it Now

Output:125

There are two main restrictions for the static method. They are:

1. The static method can not use non static data member or call non-static method directly.
2. this and super cannot be used in static context.
3. class A{  
4. int a=40;//non static 
5. public static void main(String args[]){  
6.   System.out.println(a);  
7.  }  
8. }    

Test it Now

Output:Compile Time Error

Ans) It is because the object is not required to call a static method. If it were a non-static method, JVM creates an object first then call main() method that will lead the problem of extra memory allocation.

• Is used to initialize the static data member.
• It is executed before the main method at the time of classloading.
• 
  

1. class A2{  
2. static{System.out.println("static block is invoked");}  
3. public static void main(String args[]){  
4.    System.out.println("Hello main");  
5.   }  
6. }  

Test it Now

Output:static block is invoked

       Hello main

Ans) No, one of the ways was the static block, but it was possible till JDK 1.6. Since JDK 1.7, it is not possible to execute a Java class without the main method.

1. class A3{  
2. static{  
3.   System.out.println("static block is invoked");  
4.   System.exit(0);  
5.   }  
6. }  

Test it Now

Output:

static block is invoked

Since JDK 1.7 and above, output would be:

Error: Main method not found in class A3, please define the main method as:

   public static void main(String[] args)

or a JavaFX application class must extend javafx.application.Application

================================================================================

There can be a lot of usage of Java this keyword. In Java, this is a reference variable that refers to the current object.

Here is given the 6 usage of java this keyword.

1. this can be used to refer current class instance variable.
2. this can be used to invoke current class method (implicitly)
3. this() can be used to invoke current class constructor.
4. this can be passed as an argument in the method call.
5. this can be passed as argument in the constructor call.
6. this can be used to return the current class instance from the method.

Suggestion: If you are beginner to java, lookup only three usages of this keyword.

The this keyword can be used to refer current class instance variable. If there is ambiguity between the instance variables and parameters, this keyword resolves the problem of ambiguity.

Let's understand the problem if we don't use this keyword by the example given below:

1. class Student{  
2. int rollno;  
3. String name;  
4. float fee;  
5. Student(int rollno,String name,float fee){  
6. rollno=rollno;  
7. name=name;  
8. fee=fee;  
9. }  
10. void display(){System.out.println(rollno+" "+name+" "+fee);}  
11. }  
12. class TestThis1{  
13. public static void main(String args[]){  
14. Student s1=new Student(111,"ankit",5000f);  
15. Student s2=new Student(112,"sumit",6000f);  
16. s1.display();  
17. s2.display();  
18. }}  

Test it Now

Output:

0 null 0.0

0 null 0.0

In the above example, parameters (formal arguments) and instance variables are same. So, we are using this keyword to distinguish local variable and instance variable.

1. class Student{  
2. int rollno;  
3. String name;  
4. float fee;  
5. Student(int rollno,String name,float fee){  
6. this.rollno=rollno;  
7. this.name=name;  
8. this.fee=fee;  
9. }  
10. void display(){System.out.println(rollno+" "+name+" "+fee);}  
11. }  
12. class TestThis2{  
13. public static void main(String args[]){  
14. Student s1=new Student(111,"ankit",5000f);  
15. Student s2=new Student(112,"sumit",6000f);  
16. s1.display();  
17. s2.display();  
18. }}  

Test it Now

Output:

111 ankit 5000.0

112 sumit 6000.0

If local variables(formal arguments) and instance variables are different, there is no need to use this keyword like in the following program:

1. class Student{  
2. int rollno;  
3. String name;  
4. float fee;  
5. Student(int r,String n,float f){  
6. rollno=r;  
7. name=n;  
8. fee=f;  
9. }  
10. void display(){System.out.println(rollno+" "+name+" "+fee);}  
11. }  
12. class TestThis3{  
13. public static void main(String args[]){  
14. Student s1=new Student(111,"ankit",5000f);  
15. Student s2=new Student(112,"sumit",6000f);  
16. s1.display();  
17. s2.display();  
18. }} 

Test it Now

Output:

111 ankit 5000.0

112 sumit 6000.0

You may invoke the method of the current class by using the this keyword. If you don't use the this keyword, compiler automatically adds this keyword while invoking the method. Let's see the example

1. class A{  
2. void m(){System.out.println("hello m");}  
3. void n(){  
4. System.out.println("hello n");  
5. //m();//same as this.m() 
6. this.m();  
7. }  
8. }  
9. class TestThis4{  
10. public static void main(String args[]){  
11. A a=new A();  
12. a.n();  
13. }}  

Test it Now

Output:

hello n

hello m

The this() constructor call can be used to invoke the current class constructor. It is used to reuse the constructor. In other words, it is used for constructor chaining.

Calling default constructor from parameterized constructor:

1. class A{  
2. A(){System.out.println("hello a");}  
3. A(int x){  
4. this();  
5. System.out.println(x);  
6. }  
7. }  
8. class TestThis5{  
9. public static void main(String args[]){  
10. A a=new A(10);  
11. }}  

Test it Now

Output:

hello a

10

Calling parameterized constructor from default constructor:

1. class A{  
2. A(){  
3. this(5);  
4. System.out.println("hello a");  
5. }  
6. A(int x){  
7. System.out.println(x);  
8. }  
9. }  
10. class TestThis6{  
11. public static void main(String args[]){  
12. A a=new A();  
13. }} 

Test it Now

Output:

5

hello a

The this() constructor call should be used to reuse the constructor from the constructor. It maintains the chain between the constructors i.e. it is used for constructor chaining. Let's see the example given below that displays the actual use of this keyword.

1. class Student{  
2. int rollno;  
3. String name,course;  
4. float fee;  
5. Student(int rollno,String name,String course){  
6. this.rollno=rollno;  
7. this.name=name;  
8. this.course=course;  
9. }  
10. Student(int rollno,String name,String course,float fee){  
11. this(rollno,name,course);//reusing constructor 
12. this.fee=fee;  
13. }  
14. void display(){System.out.println(rollno+" "+name+" "+course+" "+fee);}  
15. }  
16. class TestThis7{  
17. public static void main(String args[]){  
18. Student s1=new Student(111,"ankit","java");  
19. Student s2=new Student(112,"sumit","java",6000f);  
20. s1.display();  
21. s2.display();  
22. }}  

Test it Now

Output:

111 ankit java 0.0

112 sumit java 6000.0

1. class Student{  
2. int rollno;  
3. String name,course;  
4. float fee;  
5. Student(int rollno,String name,String course){  
6. this.rollno=rollno;  
7. this.name=name;  
8. this.course=course;  
9. }  
10. Student(int rollno,String name,String course,float fee){  
11. this.fee=fee;  
12. this(rollno,name,course);//C.T.Error 
13. }  
14. void display(){System.out.println(rollno+" "+name+" "+course+" "+fee);}  
15. }  
16. class TestThis8{  
17. public static void main(String args[]){  
18. Student s1=new Student(111,"ankit","java");  
19. Student s2=new Student(112,"sumit","java",6000f);  
20. s1.display();  
21. s2.display();  
22. }}  

Test it Now

Output:

Compile Time Error: Call to this must be first statement in constructor

The this keyword can also be passed as an argument in the method. It is mainly used in the event handling. Let's see the example:

1. class S2{  
2. void m(S2 obj){  
3.   System.out.println("method is invoked");  
4.   }  
5. void p(){  
6.   m(this);  
7.   }  
8. public static void main(String args[]){  
9.   S2 s1 = new S2();  
10.   s1.p();  
11.   }  
12. }  

Test it Now

Output:

method is invoked

In event handling (or) in a situation where we have to provide reference of a class to another one. It is used to reuse one object in many methods.

We can pass the this keyword in the constructor also. It is useful if we have to use one object in multiple classes. Let's see the example:

1. class B{  
2.   A4 obj;  
3.   B(A4 obj){  
4. this.obj=obj;  
5.   }  
6. void display(){  
7.     System.out.println(obj.data);//using data member of A4 class 
8.   }  
9. }  
10. class A4{  
11. int data=10;  
12.   A4(){  
13.    B b=new B(this);  
14.    b.display();  
15.   }  
16. public static void main(String args[]){  
17.    A4 a=new A4();  
18.   }  
19. }

Test it Now

Output:10

We can return this keyword as an statement from the method. In such case, return type of the method must be the class type (non-primitive). Let's see the example:

1. return_type method_name(){  
2. return this;  
3. }  

1. class A{  
2. A getA(){  
3. return this;  
4. }  
5. void msg(){System.out.println("Hello java");}  
6. }  
7. class Test1{  
8. public static void main(String args[]){  
9. new A().getA().msg();  
10. }  
11. }  

Test it Now

Output:

Hello java

Let's prove that this keyword refers to the current class instance variable. In this program, we are printing the reference variable and this, output of both variables are same.

1. class A5{  
2. void m(){  
3. System.out.println(this);//prints same reference ID 
4. }  
5. public static void main(String args[]){  
6. A5 obj=new A5();  
7. System.out.println(obj);//prints the reference ID 
8. obj.m();  
9. }  
10. }  

Test it Now

Output:

A5@22b3ea59

A5@22b3ea59