What is Class?

A Class is a blue print or Concept or idea of an Object

Class is a prototype of an object that models the attributes and methods of an object

Class can not be used directly; at first, an object is to be created. The process of creating object is known as instantiation or object creation.

What are Object?

Object is an instance of a Class

Every object contains some properties and methods

Properties are attributes that an object holds and methods are the actions or operations those can be performed on those attributes of that object.

If the value of an attribute is changed, according to OOP it is said that the State of an object is changed. Generally, the methods are responsible to change those object states.

The Principal of OOP/ Characteristics of OOP:

1. Abstraction
2. Encapsulation
3. Inheritance
4. Polymorphism

Abstraction

• Combining the related attributes and methods into one class is known as abstraction
• The objective of the abstraction is to hide the attributes and methods from other class
• Creating Class is the example of Abstraction

Encapsulation

• Accessing the attributes through the methods is known as encapsulation.
• The objective of the encapsulation is to protect the attributes of a class from direct access.
• Declaring methods to use attributes is the example of Encapsulation

Inheritance

Polymorphism

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

• Coupling
• Cohesion
• Association
• Aggregation
• Composition

Coupling

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

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

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

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.

Composition

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.

Advantage of OOPs over Procedure-oriented programming language

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.

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.

Constructor in Java

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.

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.

Rules for creating Java constructor

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

Note: We can use access modifiers while declaring a constructor. It controls the object creation. In other words, we can have private, protected, public or default constructor in Java.

Types of Java constructors

There are two types of constructors in Java:

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

Java Default Constructor

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

Syntax of default constructor:

    //Java Program to create and call a default constructor  
    class Bike1{  
    //creating a default constructor  
    Bike1(){System.out.println("Bike is created");}  
    //main method  
    public static void main(String args[]){  
    //calling a default constructor  
    Bike1 b=new Bike1();  
    }  
    } 

Output:

Bike is created
	

       //Let us see another example of default constructor  
    //which displays the default values  
    class Student3{  
    int id;  
    String name;  
    //method to display the value of id and name  
    void display(){System.out.println(id+" "+name);}  
      
    public static void main(String args[]){  
    //creating objects  
    Student3 s1=new Student3();  
    Student3 s2=new Student3();  
    //displaying values of the object  
    s1.display();  
    s2.display();  
    }  
    }  
	
	Output:

0 null
0 null

Java Parameterized Constructor

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

Why use the 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.

      //Java Program to demonstrate the use of the parameterized constructor.  
    class Student4{  
        int id;  
        String name;  
        //creating a parameterized constructor  
        Student4(int i,String n){  
        id = i;  
        name = n;  
        }  
        //method to display the values  
        void display(){System.out.println(id+" "+name);}  
       
        public static void main(String args[]){  
        //creating objects and passing values  
        Student4 s1 = new Student4(111,"Karan");  
        Student4 s2 = new Student4(222,"Aryan");  
        //calling method to display the values of object  
        s1.display();  
        s2.display();  
       }  
    }  
	
	Output:

111 Karan
222 Aryan

Constructor Overloading in Java

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.

         //Java program to overload constructors  
    class Student5{  
        int id;  
        String name;  
        int age;  
        //creating two arg constructor  
        Student5(int i,String n){  
        id = i;  
        name = n;  
        }  
        //creating three arg constructor  
        Student5(int i,String n,int a){  
        id = i;  
        name = n;  
        age=a;  
        }  
        void display(){System.out.println(id+" "+name+" "+age);}  
       
        public static void main(String args[]){  
        Student5 s1 = new Student5(111,"Karan");  
        Student5 s2 = new Student5(222,"Aryan",25);  
        s1.display();  
        s2.display();  
       }  
    }  

Test it Now

Output:

111 Karan 0
222 Aryan 25

Difference between constructor and method in Java

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

Java Copy Constructor

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.

    //Java program to initialize the values from one object to another object.  
    class Student6{  
        int id;  
        String name;  
        //constructor to initialize integer and string  
        Student6(int i,String n){  
        id = i;  
        name = n;  
        }  
        //constructor to initialize another object  
        Student6(Student6 s){  
        id = s.id;  
        name =s.name;  
        }  
        void display(){System.out.println(id+" "+name);}  
       
        public static void main(String args[]){  
        Student6 s1 = new Student6(111,"Karan");  
        Student6 s2 = new Student6(s1);  
        s1.display();  
        s2.display();  
       }  
    }  

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.

        class Student7{  
        int id;  
        String name;  
        Student7(int i,String n){  
        id = i;  
        name = n;  
        }  
        Student7(){}  
        void display(){System.out.println(id+" "+name);}  
       
        public static void main(String args[]){  
        Student7 s1 = new Student7(111,"Karan");  
        Student7 s2 = new Student7();  
        s2.id=s1.id;  
        s2.name=s1.name;  
        s1.display();  
        s2.display();  
       }  
    } 

Output:

111 Karan
111 Karan
	

Q) Does constructor return any value?

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

Can constructor perform other tasks instead of initialization?

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.

Is there Constructor class in Java?

Yes.

What is the purpose of Constructor class?

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.

Java static keyword

The static keyword in Java is used for memory management mainly. We can apply java static keyword with variables, methods, blocks and nested class. 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

1) Java static variable

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

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.

Advantages of static variable It makes your program memory efficient (i.e., it saves memory).

Understanding the problem without static variable

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.

    //Java Program to demonstrate the use of static variable  
    class Student{  
       int rollno;//instance variable  
       String name;  
       static String college ="ITS";//static variable  
       //constructor  
       Student(int r, String n){  
       rollno = r;  
       name = n;  
       }  
       //method to display the values  
       void display (){System.out.println(rollno+" "+name+" "+college);}  
    }  
    //Test class to show the values of objects  
    public class TestStaticVariable1{  
     public static void main(String args[]){  
     Student s1 = new Student(111,"Karan");  
     Student s2 = new Student(222,"Aryan");  
     //we can change the college of all objects by the single line of code  
     //Student.college="BBDIT";  
     s1.display();  
     s2.display();  
     }  
    }  

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.

        //Java Program to demonstrate the use of an instance variable  
    //which get memory each time when we create an object of the class.  
    class Counter{  
    int count=0;//will get memory each time when the instance is created  
      
    Counter(){  
    count++;//incrementing value  
    System.out.println(count);  
    }  
      
    public static void main(String args[]){  
    //Creating objects  
    Counter c1=new Counter();  
    Counter c2=new Counter();  
    Counter c3=new Counter();  
    }  
    }  

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.

            //Java Program to illustrate the use of static variable which  
    //is shared with all objects.  
    class Counter2{  
    static int count=0;//will get memory only once and retain its value  
      
    Counter2(){  
    count++;//incrementing the value of static variable  
    System.out.println(count);  
    }  
      
    public static void main(String args[]){  
    //creating objects  
    Counter2 c1=new Counter2();  
    Counter2 c2=new Counter2();  
    Counter2 c3=new Counter2();  
    }  
    }  

Test it Now

Output:

1
2
3

2) Java static method

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.

  
    //Java Program to demonstrate the use of a static method.  
    class Student{  
         int rollno;  
         String name;  
         static String college = "ITS";  
         //static method to change the value of static variable  
         static void change(){  
         college = "BBDIT";  
         }  
         //constructor to initialize the variable  
         Student(int r, String n){  
         rollno = r;  
         name = n;  
         }  
         //method to display values  
         void display(){System.out.println(rollno+" "+name+" "+college);}  
    }  
    //Test class to create and display the values of object  
    public class TestStaticMethod{  
        public static void main(String args[]){  
        Student.change();//calling change method  
        //creating objects  
        Student s1 = new Student(111,"Karan");  
        Student s2 = new Student(222,"Aryan");  
        Student s3 = new Student(333,"Sonoo");  
        //calling display method  
        s1.display();  
        s2.display();  
        s3.display();  
        }  
    }  

Test it Now

Output:111 Karan BBDIT
       222 Aryan BBDIT
       333 Sonoo BBDIT

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

Test it Now

Output:125

Restrictions for the static method

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.

  
    class A{  
     int a=40;//non static  
       
     public static void main(String args[]){  
      System.out.println(a);  
     }  
    }        

Test it Now

Output:Compile Time Error

Q) Why is the Java main method static?

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.

3) Java static block

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

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

Test it Now

Output:static block is invoked
       Hello main

Q) Can we execute a program without main() method?

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.

    class A3{  
      static{  
      System.out.println("static block is invoked");  
      System.exit(0);  
      }  
    }  

Test it Now
Output:
static block is invoked

this keyword in java

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

Usage of java this keyword

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 usage of this keyword

1) this: to refer current class instance variable 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:

    class Student{  
    int rollno;  
    String name;  
    float fee;  
    Student(int rollno,String name,float fee){  
    rollno=rollno;  
    name=name;  
    fee=fee;  
    }  
    void display(){System.out.println(rollno+" "+name+" "+fee);}  
    }  
    class TestThis1{  
    public static void main(String args[]){  
    Student s1=new Student(111,"ankit",5000f);  
    Student s2=new Student(112,"sumit",6000f);  
    s1.display();  
    s2.display();  
    }}  

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.

    class Student{  
    int rollno;  
    String name;  
    float fee;  
    Student(int rollno,String name,float fee){  
    this.rollno=rollno;  
    this.name=name;  
    this.fee=fee;  
    }  
    void display(){System.out.println(rollno+" "+name+" "+fee);}  
    }  
      
    class TestThis2{  
    public static void main(String args[]){  
    Student s1=new Student(111,"ankit",5000f);  
    Student s2=new Student(112,"sumit",6000f);  
    s1.display();  
    s2.display();  
    }}  

Test it Now

Output:

111 ankit 5000
112 sumit 6000

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

    class Student{  
    int rollno;  
    String name;  
    float fee;  
    Student(int r,String n,float f){  
    rollno=r;  
    name=n;  
    fee=f;  
    }  
    void display(){System.out.println(rollno+" "+name+" "+fee);}  
    }  
      
    class TestThis3{  
    public static void main(String args[]){  
    Student s1=new Student(111,"ankit",5000f);  
    Student s2=new Student(112,"sumit",6000f);  
    s1.display();  
    s2.display();  
    }}  

Test it Now

Output:

111 ankit 5000
112 sumit 6000

2) this: to invoke current class method

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

    class A{  
    void m(){System.out.println("hello m");}  
    void n(){  
    System.out.println("hello n");  
    //m();//same as this.m()  
    this.m();  
    }  
    }  
    class TestThis4{  
    public static void main(String args[]){  
    A a=new A();  
    a.n();  
    }}  

Test it Now

Output:

hello n
hello m

3) this() : to invoke current class constructor

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:

    class A{  
    A(){System.out.println("hello a");}  
    A(int x){  
    this();  
    System.out.println(x);  
    }  
    }  
    class TestThis5{  
    public static void main(String args[]){  
    A a=new A(10);  
    }}  

Test it Now

Output:

hello a
10

Calling parameterized constructor from default constructor:

    class A{  
    A(){  
    this(5);  
    System.out.println("hello a");  
    }  
    A(int x){  
    System.out.println(x);  
    }  
    }  
    class TestThis6{  
    public static void main(String args[]){  
    A a=new A();  
    }}  

Test it Now

Output:

5
hello a

Real usage of this() constructor call

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.

    class Student{  
    int rollno;  
    String name,course;  
    float fee;  
    Student(int rollno,String name,String course){  
    this.rollno=rollno;  
    this.name=name;  
    this.course=course;  
    }  
    Student(int rollno,String name,String course,float fee){  
    this(rollno,name,course);//reusing constructor  
    this.fee=fee;  
    }  
    void display(){System.out.println(rollno+" "+name+" "+course+" "+fee);}  
    }  
    class TestThis7{  
    public static void main(String args[]){  
    Student s1=new Student(111,"ankit","java");  
    Student s2=new Student(112,"sumit","java",6000f);  
    s1.display();  
    s2.display();  
    }} 

Output:

111 ankit java null
112 sumit java 6000



    class Student{  
    int rollno;  
    String name,course;  
    float fee;  
    Student(int rollno,String name,String course){  
    this.rollno=rollno;  
    this.name=name;  
    this.course=course;  
    }  
    Student(int rollno,String name,String course,float fee){  
    this.fee=fee;  
    this(rollno,name,course);//C.T.Error  
    }  
    void display(){System.out.println(rollno+" "+name+" "+course+" "+fee);}  
    }  
    class TestThis8{  
    public static void main(String args[]){  
    Student s1=new Student(111,"ankit","java");  
    Student s2=new Student(112,"sumit","java",6000f);  
    s1.display();  
    s2.display();  
    }}  

Test it Now

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

	

4) this: to pass as an argument in the method

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:

    class S2{  
      void m(S2 obj){  
      System.out.println("method is invoked");  
      }  
      void p(){  
      m(this);  
      }  
      public static void main(String args[]){  
      S2 s1 = new S2();  
      s1.p();  
      }  
    }  

Test it Now

Output:

method is invoked

Application of this that can be passed as an argument:

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.

5) this: to pass as argument in the constructor call

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:

    class B{  
      A4 obj;  
      B(A4 obj){  
        this.obj=obj;  
      }  
      void display(){  
        System.out.println(obj.data);//using data member of A4 class  
      }  
    }  
      
    class A4{  
      int data=10;  
      A4(){  
       B b=new B(this);  
       b.display();  
      }  
      public static void main(String args[]){  
       A4 a=new A4();  
      }  
    }  

Test it Now

Output:10

6) this keyword can be used to return current class instance

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:

Syntax of this that can be returned as a statement

    return_type method_name(){  
    return this;  
    }  

Example of this keyword that you return as a statement from the method

    class A{  
    A getA(){  
    return this;  
    }  
    void msg(){System.out.println("Hello java");}  
    }  
    class Test1{  
    public static void main(String args[]){  
    new A().getA().msg();  
    }  
    }  
	
	Output:

Hello java

Proving this keyword

    class A5{  
    void m(){  
    System.out.println(this);//prints same reference ID  
    }  
    public static void main(String args[]){  
    A5 obj=new A5();  
    System.out.println(obj);//prints the reference ID  
    obj.m();  
    }  
    }  

Test it Now

Output:

A5@22b3ea59
A5@22b3ea59