Interfaces in java with example

Interface concept included in java to overcome the Diamond problem of multiple inheritance in java. In other words, Interface was introduced in java to define objects outside the world through methods. For example, buttons on the front of the television set. That are the interfaces between you and the electrical set. You press the power button to turn the television to on or off.

An interface is a specification of method prototypes. Prototypes will be in the interface. All the methods of the interface are abstract. For the polymorphic interface inheritance, where the client wants to only deal with a type and does not care about the actual implementation use interfaces. If you need to change your design frequently, you should prefer using interface to abstract class.

Through interfaces we can achieve multiple inheritance in java. Because there is a possibility of inheriting more than one interface into java class by using the keyword called implements. When we try to inherit from one interface to another interface we must use extends keyword like classes.

In interfaces we have to define the method declaration only. It will not allow to implement methods inside the interfaces. The method implementation/declaration should be present in the class which has implements the interface. The class which contains method implementation called as subclass / child class of the interface.

Here is the example interface with the name called Vehicle with few of the unimplemented methods. 
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
package com.javatbrains.interfaces;

public interface Vehicle {
    
     void changeGear(int newValue);

     void speedUp(int increment);

     void applyBrakes(int decrement);
}

To implement the interface methods we have to create a class which will implements the interface. To implement the interface, will use the implements keyword in java. The below is the Car class which will implements the Vehicle interface. The methods in the Car class will contains @Override annotation on top of the method. Those methods are called overridden methods. Along with overridden methods, subclass will contains it's own methods. Those methods are not declared with the @Override annotation. 
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
package com.javatbrains.interfaces;

public class Car implements Vehicle {
     int speed = 0;
     int gear = 1;

     @Override
     public void changeGear(int newValue) {
          gear = newValue;
     }

     @Override
     public void speedUp(int increment) {
          speed = speed + increment;
     }

     @Override
     public void applyBrakes(int decrement) {
          speed = speed - decrement;
     }

     void printStates() {
        System.out.println(" speed: " +
            speed + " gear: " + gear);
   }

}

Implementing an interface allows a class to become more formal about the behaviour it promises to provide. When you are implementing an interface into a class that class should override the interface methods into implementing class. Otherwise compiler will complain.

Key points in interfaces:
  • An interface is a specification of method prototypes.
  • An interface contains 0 or more abstract methods.
  • All the methods of the interface are public and abstract by default.
  • An interface may have variables, which are public static final by default. In other words, interfaces can declare only constants, not instance variables.
  • We can not create an object for interfaces.
  • But we can create a reference of interface type.
  • All the abstract methods of the interface should be implemented in its implementation class.
  • I nterface reference can be used to refer to the objects of implementation classes.
  • If any method is not implemented in an implementation class, class should be declared as "abstract".
  • Once an interface is written, any third party vendor can implement it.
  • An interface cannot implement another interface.
  • An interface can extends another interface.
  • We can write a class inside an interface.
  • A class can implements multiple interfaces.
  • All the methods are incomplete only.
  • Interface methods must not be static. Because interface methods are abstract, they can not be marked final, strictfp, native.
  • Interface does not have constructors. Interfaces are not part of an object's inheritance tree.
Here is the another example of one interface is implementing more than one class. 
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
package com.javatbrains.interfaces;

interface DbConnection {
     void connect(); // public abstract
}

class OracleConnection implements DbConnection {

     @Override
     public void connect() {
          System.out.println("Connecting to Oracle database");
     }

}

class SybaseConnection implements DbConnection {

     @Override
     public void connect() {
          System.out.println("Connecting to Sybase database");
     }

}

class MySQLConnection implements DbConnection {

     @Override
     public void connect() {
          System.out.println("Connecting to MySQL database");
     }

}

public class Database {

     public static void main(String[] args) throws Exception {
          // Accepts the database name and store it in object c
          Class c = Class.forName(args[0]);
          // Create an object to the class whose name is in c
          DbConnection connection = (DbConnection) c.newInstance();
          connection.connect();
     }

}

What is marker interface?
          Marker interface is an interface, which doesn't contains any of the methods. But, still that type of interfaces are doing their functionality. For example, Serializable, Cloneable..etc.

Do you Know?
Location: Bengaluru, Karnataka 560017, India

Comments

Post a Comment

Popular posts from this blog

Hibernate auto increment with example

In previous example we learn how to map java object with database table for manually(not using auto increment) assigned all the values.  To create primary key auto increment application by using hibernate mapping(hbm.xml) or using annotations continue reading this article.  W e need to create a database and create an employee table by using the below query. Table: CREATE table EMPLOYEE( eid integer ( 10 ) primary key not null , firstname varchar ( 30 ), lastname varchar ( 30 ), designation varchar ( 30 ), salary double( 10 , 2 )); Here, we will create a bean/POJO (Plain Old Java Object) class of employee. Employee.java: package com . javatbrains . hibernate ; import java.io.Serializable ; /*This is the POJO class. It contains only Getter and Setter methods*/ public class Employee implements Serializable { private static final long serialVersionUID = - 75885815725314443L ; private int eid ; ...
Read more

how to count the page views by using JSP

To count no.of page's are viewed by user in your application is a challenge in real time application. In this page we will see how to count the page views when user has viewed the page.           Here, we will do one example how many times user has visited each page in our application. In this post we will come to know, how to count the total application page views as well as individually each pages count.           Let us assume my web application name is "Page Counter". For creating the web application in eclipse IDE, Click here . This Page Counter contains three pages, assume that are the different pages in an application. But, there is the link between each other files for navigating from one page to another. Also it will show the total pages view count and individual page view count in all the pages. Let us see, below is my first jsp file,         ...
Read more

Multithreading in java with example

Multithreading  is one of the most important concept in core java. In this article we will learn what is multithreading? , what is the use of it? and What is the use of Synchronization and when to use it?  with detailed examples. At a time, two or more threads are accessing the same object is called as Multithreading  in Java .  First, we will create two threads for two objects. It is also possible to run two or more threads on a single class object. In this case, there is a possibility to get unreliable results. If the two threads are perform same task, then they need same object to be executed each time. For your better understanding, take an example of any reservations like, railway, movie ticket booking,etc. Let us think only one berth is available in a train and two passengers are asking for that berth. The first person has sent a request to allocate that ticket/berth to him. At the same time, the second person also sent a request to allocate that ...
Read more

How to retrieve data from table by using JDBC with example

To retrieve data from mysql data table by using jdbc, we need to confirm weather which table data we need to retrieve. To confirm open your mysql and view all database names by using the ' show databases ' commands in mysql. Then select which is your database name from showing list of database names, by using the ' use <database name> ' command in mysql. Then we need to see all your existing tables by executing the ' show tables ' command. This command gives you the list of existing tables in your selected database. You need to use existing table only you can follow with that. But, here we will see with entirely new which is not existing in mysql database. Now, We will create simple mysql table with minimal columns. create table emp(eno integer(10), ename varchar(30), sal float); The above command creates a new table, we will move further to insert records into emp table by executing the insert which we mentioned in the below. ...
Read more

Prime, Fibonacci and Factorial number with example in java

Prime number, Fibonacci series and Factorial number programs are most commonly asked questions in interview. Read this article to know what is and how to write programs for prime number, fibonacci series and factorial number. Prime Number: prime number is natural number greater than 1 that has no positive divisor other than 1 and itself. A natural number greater than 1 is not a prime number, is called Composite number . For example, 7 is a prime number. Because it can divide with 1 and 7 only. Where as 8 is composite number. Since it has the divisor 2 and 4 in addition to the 1 and 8. The below example represents the finding the passing number is prime number or not. If the passing number is prime number it will print true otherwise it will print false. package com . javatbrains . practice ; public class PrimeNumber { public boolean isPrimeNumber ( int number ) { if ( number <= 1 ) return false ; // There's only one ...
Read more

How to insert images into database using JDBC?

How to insert images into database table by using JDBC? To know about these topic follow the below lines,           Before going to write a JDBC program for inserting images into database table, we need to create a specific table for inserting images column datatype. For creating the database table, execute the below command in MySQL command prompt. 1 2 3 4 CREATE TABLE ` images ` ( ` image_id ` int ( 11 ) NOT NULL AUTO_INCREMENT, ` image_flag ` int ( 11 ) NOT NULL , ` image ` longblob NOT NULL , PRIMARY KEY ( ` image_id ` ));           After executing the above query in MySQL command prompt, execute the below query to know weather the above table query is created or not. 1 Show tables;            When you have executed the above query that will show you the list of table which all exists in the current database. Current tab...
Read more

How to sort list of objects in java with examples

We could achieve list of objects sorting in java by using sort() method which is available under Collections class as well as by using Comparable and Comparator  interfaces. Comparable and Comparator are two interfaces in java used to sort list of objects. These interfaces are available under java.util package along with Collections  class and other classes and interfaces. Comparable interface contains the method of       public int compareTo(Object obj) Comparator interface contains the method of       public int compare(Object obj1, Object obj2) The compare() and compareTo() methods will return negative integer(-1), or zero(0) or positive integer(1). When the first obj1 is less than obj2 method will return negative integer. When obj1 is equals to obj2 will return zero and when obj1 is grater than obj2 it will return positive integer. If it's objects are stored in any Collection classes like ArrayList, H...
Read more

String interview questions and answers

String interview questions and answers are placed the current post as of my experience in interviews. This will helps who are trying to attend interview as fresher as well as experience. If any one is facing any other questions which are not present in the current page, please drop me an email to " subbareddynallamachu@gmail.com ".  Q1:  String is a class or data type? A: String is a class in java.lang package. But, in java, all classes are also are considered as data types. So we can take String as data type also. Q2:  What is the difference between == and equals() methods in java? A: == operator compares the references of the String object. It does not compare the content of the object. equals() method compares the contents. While comparing the Strings, equals method should be used as it yields the correct result. Q3:  What is String constant pool? A: String constant pool is a separate block of memory where the String objects are held by JVM....
Read more

Exception in thread "main" java.lang.NoClassDefFoundError: javax/transaction/SystemException

Exception in thread "main" java.lang.NoClassDefFoundError: javax/transaction/SystemException error we got in Hibernate 4.3 to work with my MySQL database. We are using hibernate code generation tool in Eclipse and we are able to connect to the database using the Hibernate configuration file. When we try to run code in my Main class application is connecting to the database, but transaction is failing and we got the following error: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Exception in thread "main" java . lang . NoClassDefFoundError : javax / transaction / SystemException at java . lang . Class . forName0 ( Native Method ) at java . lang . Class . forName ( Unknown Source ) at org . jboss . logging . Logger . getMessageLogger ( Logger . java : 2248 ) at org . jboss . logging . Logger . getMessageLogger ( Logger . java : 2214 ) at org . hibernate . cfg . Configuration .< clinit >( Configuration . java : 184 ) at be . comp . permanenti...
Read more

Store file into table by using JDBC with example

To store large data files or text files into database table, we will use CLOB (Charecter Large Object) datatype of SQL . By using CLOB it is possible to save entire text book or a file or resume into a column of data table. If your using MySQL database instead of CLOB use LONGTEXT to save file into datatable. For saving files into a data table, we need to create one table which will contains a column type as CLOB to store files. Here, is the query to create a data table. 1 2 3 4 create table myfiles( file_id int ( 10 ), file_name varchar ( 30 ), file_type LONGTEXT); Once table creation completes, these table contains the 3 columns one is file_id represents the file id, file_name represents the name of the file and finally file_type will get save entire file into the table column. Now, we will move to write a JDBC program which will saves the file into the newly creating table. Here is the JDBC program, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16...
Read more