Java JDBC interview Question

What is the difference between Database and Database management system?
Database is a collection of interrelated data. Database management system is a software which can be used to manage the data by storing it on to the data base and by retrieving it from the data base.  And  DBMS is a collection of interrelated data and some set of programs to access the data.

      There are 3 types of Database Management Systems.

• Relational DataBase Management Systems(RDBMS):   It is a software system, which can be used to represents data in the form of tables. RDBMS will use SQL2 as a Queries  language.
• Object Oriented DataBase Management Systems(OODBMS):  It is a software system, which can be used to represent the data in the form of objects. This DBMS will use OQL as a Query language.
• Object Relational DataBase Management Systems(ORDBMS):  It is a DBMS which will represents some part of the data in the form of tables and some other part of the data in the form of objects. This management system will use SQL3 as a Query Language, it is a combination of  SQL2 and OQL.

How a query could be executed when we send a query to Database?
When we send an SQL Query from SQL prompt to the DataBaseEngine, then Database Engine will take the following steps.

• Query Tokenization:  This phase will take SQL query as an input and devide into stream of tokens.
• Query Parsing:   This phase will take stream of tokens as an input, with them it tried to construct a query tree. If query parser constructs query tree successfully then it was an indication that no grammatical mistakes in the taken SQL query. Otherwise there are some syntactical errors in the taken SQL query.
• Query Optimization:   This phase will take query tree as an input and performs number of query optimization mechanisms to reduce execution time and memory utilization.
• Query Execution:  This phase will take optimized query as an input and executes that SQL query by using interpreters internally as a result we will get some output on the SQL prompt.

What is  Driver? How many Drivers are available in JDBC? What are the types?

• It is a process of interacting with the database from a java application.
• In JDBC applications we must specify the complete database logic in java application as for the java API representations, later on we need to send java represented database logic to the database engine(DBE).
• DBE must execute the database logic but it was configured as per the java representations but DBE able to understand only Query Language representations.
• At the above situation if we want to execute our database logic, we need to use one interface in between java application and the database, that interface must convert java representations to query language representations and query language representations to java representations. Now this interface is called as a “Driver”.

Driver:

• It is a software or an interface existed in between a java application and database, which will map java api calls with query language api calls and vice versa.
• Initially sun Microsystems has provided “driver interface” to the market with this sun Microsystems has given an intimation to all the database vendors to have their own implementation as per their requirements for the Driver interface.
• As a response all the database vendors are providing their own implementation for the Driver interface inorder to interact with the respective databases from a java application.
• The users of the respective databases they must get the respective database provided Driver implementation from the database software and make use as part of the JDBC applications to interact with the respective databases form a java application.

Types of Drivers:
            There are 180+  number of Drivers in the market. But all these Drivers could be classified into the following 4 types.

• Type 1 Driver
• Type 2 Driver
• Type 3 Driver
• Type 4 Driver

Type 1 Driver:

• Type 1 Driver is also called as Jdbc-Odbc Driver or Bridge Driver.
• Jdbc-Odbc Driver is an implementation to Driver interface provided by the sun Microsystems along with the java software.
• Jdbc-Odbc Driver internally depends on the Microsoft product Odbc Driver.
• Odbc is nothing but open database connectivity. It is a open specification which can be used to interact with any type of databases.

Advantages:

• This Driver is already available with java software that’s why no need to bother about how to get the Driver implementation explicitily.
• Allmost all the databases could support this Driver.

Dis advantages:

• This Driver internally depends on Odbc Driver that’s why it is not suitable for internet or web applications or network applications.
• This Driver is a slower Driver, why because Jdbc-Odbc Driver will convert java calls to Odbc calls. Then Odbc Driver has to convert Odbc calls to query language calls.
• This driver is not portable Driver why because it was not complete the java implementations in Jdbc-Odbc Driver.
• It we want to use Jdbc-Odbc Driver in our jdbc applications then we must require to install Odbc-Native Library.

Type 2 Driver:

            Type 2 Driver is also called as “part java part native Driver”. i.e., this Driver was designed by using some part of the java implementations and some other part of the database vendor provided native implementations. This Driver is also called as “native driver”.

Advantages:
            When compared to Type 1 driver it is efficient driver why because Type 2 driver directly will convert java api calls to database vendor api calls.

Dis advantages:

• If we want to use Type 2 Driver in our Jdbc applications then we must require to install database vendor native api.
• It is a costful Driver.
• It is not suitable for web applicadtions, distributed applications and web applications.
• Type 2 Driver performance is low when compared to Type 3 and Type 4 drivers.
• This driver is not portable driver. Why because this driver was not designed completely in java technology.

Type 3 Driver: 

• It is also called as middleware database access server driver.
• This driver could be used to interact with multiple databases from the multiple clients.
• This driver could be used in collaboration with application server.
• This driver is suggestable for network applications.

Advantages:

• It is a fastest driver among all the drivers available in the market.
• To use Type 3 driver in our jdbc applications it is not required to install odbc native library and database native library.
• It is very much suitable for network applications.

Dis advantages:

• This driver is not suitable for simple jdbc applications.
• This driver requires minimum 3-Tier Architecture.
• When compared to Type1 and Type2 drivers.. Type3 driver is efficient and portable. But when compared to Type4 driver, Type3 driver is not portable.

Type 4 Driver:

• This driver is also called as pure java driver i.e, this driver was completely implemented by using java technology.
• When compared to Type1, Type2 and Type3 drivers.. Type4 driver is portable driver.
• Type4 driver can be used for any kind of applications.
• Type4 driver is a cheapest driver when compared to all the drivers that’s why it is frequently used driver.

What is JDBC and What are the steps to write a JDBC application?

           
The process of interacting with the database from a java application is called as JDBC(Java Database Connectivity)
To interact with the database from a java application we will use the following five   steps.

1. load and register the driver.
2. Establish a connection between java application and the database.
3. prepare either statement object or PreparedStatement object or CallebleStatement object as per the application requirements.
4. write and executer the sql queries.
5. terminate the connection which we have established.

How to load a JDBC driver?

• In general sun Microsystems  has provided Driver interface for this all the database vendors has provided their own implementation.
• If we want to use the database vendor provided Driver implementation to our jdbc application, first we need to make the availability of the respective Driver’s  .class file to JVM, for this we need to set class path environment variable to the location where we have the driver implementation.
• Sun Microsystems is also provided an implementation to the Driver interface in the form of JdbcOdbcDriver class as part of the java software.
• If we want to use JdbcOdbcDriver in our jdbc applications no need to set class path environment variable. Why because it was already available in the java software’s pre-defined library.
• JdbcOdbcDriver internally depends on the mocrosoft product Odbc driver. If we want to use the JdbcOdbcDriver in our jdbc applications first we must configure Odbc driver, for this we will use the following path.

Start/ conrlol panel / performance and maintenance / administrative tools / data source(Odbc)/ user dsn / click on Add / select microsofr Odbc for oracle / finish / provide data source name only / click on ok / ok.

• To load the driver’s class byte code to the memory we will use the following method.

public void forName(String class name)

Eg:   

Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);

      Where forName() is a static method, which can be used to load the respective driver class byte code to the memory.

• Each and every driver class has already a static block at the time of loading the respective driver class byte code to the memory automatically the available static block could be executed, by this 

  DriverManager.registerDriver(….)
  

  method will be executed as part of the static block.
• By the execution of the

  registerDriver(….)
  

  method automatically the specified driver will be register to the jdbc application.
• If you want to design any jdbc application, we need to use some pre-defined library, which was provided by the Jdbc API in the form of java.sql package, that’s why we need to import java.sql package in our jdbc application.

Note:-   The best alternative for

Class.forName(..)

is 

DriverManagre.registerDriver(new sun.jdbc.odbc.JdbcOdbcDriver());

 To register the driver.

How to establish a Database connection between java application and Database?

If we want to establish a connection between java application and the database we will the following piece of code.

Connection con= DriverManager.getConnection(“jdbc:odbc:abc”,”abc”,”system”,”manager”);

Where getConnectin() is a static method from DriverManager class, which can be used to return connection object.

Basically Connection is an interface, how getConnection() will create an object for
Connection interface?

Connection is an interface from java.sql package, for which

getConnection(_)

  was return an anonymous inner class object of the Connection interface.

Note:-  Anonymous inner class is a nameless inner class, which can be sued to provide an implementation either for the interfaces or for abstract classes.

Eg:  

interface I
         {
                void ai();
          }
         Class Outer
         {
                     I  i = new I(){
                      public void a1()
                       {
                                             
                       }
                       public void a2()
                       {

                       }
            }            
     Outer o = new Outer();             
     o.i.a1();  // à  correct            
     o.i.a2();  // à  wrong

 getConnection(_) is a static method from DriverManager class, which will call internally  connect() method, this connect() will establish a virtual socket connection in between the java application and the database.

What is the requirement to use Statement object?

• After establishing a connection between java application and the database we need to write the sql queries and we need to execute them.
• To execute the sql queries we will use some pre-defined library, which was defined in the form of Statement object,  PreparedStattement object and CallableStatement object.
• As per the application requirements we need to create either Statement object or CallableStatement object and PreparedStatement object.
• To create Statement object we will use the following method from connection object.

           

public  Statement createStatement()

Eg:    

Statement st = con.createStatement();

What are the differences between executeQuery(…), executeUpdate(…) and execute(…)  methods?

where executeQuery() can be used to execute selection group sql queries to fetch the data from database.
When we use selection group sql query with executeQuery() then JVM will send that sql query to the database engine, database engine will execute it, by this database engine(DBE) will fetch the data from database and send back to the java application.
Java is a purely object oriented technology. That’s why the jdbc application will maintain the fetched data from database, in the form of an object at heap memory, called as ResultSet object.

           

public ResultSet executeQuery(String sqlquery);

            where executeUpdate() can be used to execute updation group sql query to update the database. When we provide updation group sql query as a parameter to executeUpdate(), then JVM will send that sql query to DBE, here DBE will execute it and perform updations on the database, by this DBE will identify the number of records got updated value called as “records updated count” and return back to the java application.

           

public int executeUpdate(String sqlquery)

            where execute() can be used to execute either selection group sql queries or updation group queries.
            When we use selection group sql query with the execute() then we will get ResultSet object at heap memory with the fetched data. But execute() will return “true” as a Boolean value.
            When we use updation group sql query with execute() then we will get “ records updated count value” at jdbc application. But execute() will return “false” as a Boolean value.

            public 

boolean execute(String sqlquery)

What is  ResultSet object and How to Fetch the Data from Database?

ResultSet is an Object which can be used to maintain the fetched data from database in the JDBC applications
           
When we execute a selection group sql query, either with executeQuety()  or with execute() automatically a ResultSet object will be created at heap memory with the fetched data from database.

• To get the ResultSet object reference directly we will use executeQuery(..).
• When we create a ResultSet object automatically a cursor will be create called as “ResultSet cursor” to read the data from ResultSet object.
• When we create the ResultSet object by default ResultSet cursor will be created before the first record.
• If we want to read the data from ResultSet object every time we need to check whether the next record is available or not. If the next record is available automatically we need to move that ResultSet cursor to next record position.
• To perform this work we will use the following method from ResultSet interface.  

  public boolean next()
  

• 
  
After getting ResultSet cursor to a record position then we need to get the data from respective fields of the particular record, for this we will use following method,

public xxx getXxx(int fno)
            //(or)
public xxx getXxx(String fname)
//where xxx is byte, shor, int, long, float, //double, char.

 Ingeneral execute() method can be used to execute selection group SQl queries for getting the data from Database , but execute() return a boolean value true so here how it possible to fetch the data from database?

• Execute() can be used to execute both selection group sql query and updation group sql query.
• If we use execute() to execute a selection group sql query then DBE(Database engine) will execute that sql query and  send back the fetched data from database to java application. Now java application will prepare a ResultSet object with the fetched data but execute() will return “true” as a Boolean value.
• At this situation to get the reference of the ResultSet object explicitily, we will use the following method from Statement object.

public ResultSet getResultSet();
//eg
       boolean b = st.execute(“select * from emp1”);
                    System.out.println(b);
                    ResultSet rs = st.getResultSet();

If we use selection group SQL query to executeUpdate() ,what happened?

• If we use selection group sql query as a parameter to executeUpdate(…) then JVM will send that sql query to the DBE, DBE will fetch the data and send  back to the java application here java application will store the fetched data in the form of ResultSet object. But executeUpdate() is expecting records  updated count value.

 Due to this contradiction JVM will rise an exception like java.lang.SQLException.

If we handle the above exception properly then we will get ResultSet abject and we will get the data from Database.

 What is ResultSet and What are the types of ResultSets are available in JDBC application?

            In jdbc applications ResultSets could be classified in the following two ways.

• On the basis of ResultSet privilizations - 

There are 2 types of ResultSets.

• Read only ResultSet
• Updatable ResultSet

Read only ResultSet:-   It is a ResultSet, which will allow the users to read the        data only. To refer this ResultSet, we will use the following constant from ResultSet interface.

                       

public static final int CONCUR_READ_ONLY;

 Updatable ResultSet:-  If is a ResultSet object, which will allow users to perform some updations on its content. To refer this ResultSet we will use the following constant from ResultSet interface.

                       

public static final int CONCUR_UPDATABLE;

2)On the  basis of  the ResultSet cursor movement:-

There are 2 types of ResultSets.

• Forward  only ResultSet
• Scrollable ResultSet

Forward only ResultSet:-  It is a ResultSet object, which will allow the users to iterate the data in any forward direction. To refer this ResultSet object we will use the following  constant from ResultSet interface.

                       

public static final int TYPE_FORWARD_ONLY;

Scrollable ResultSet:-  These are the ResultSet objects, which will allow the users to iterate the data in both forward and backward directions.
            There are 2 types of Scrollable ResultSets.

• Scroll sensitive ResultSets
• Scroll in sensitive ResultSets.

What is the difference between ScrollSensitive ResultSet  and ScrollInsensitive  ResultSets?

Scroll sensitive ResultSet is a ResultSet object, which will allow the later updations from database automatically after creating it. To refer this ResultSet we will use the following constant.
           

public static final int TYPE_SCROLL_SENSITIVE;

            Scroll insensitive ResultSet is a ResultSet object, which will not allow later updations from database after creating it. To refer this ResultSet we will use the following constant from ResultSet interface.

• public static final int TYPE_SCROLL_INSENSITIVE;
  

What is the default ResultSet type in JDBC application and How it is possible to create a specific type of ResultSet object?

• The default ResultSet type in the jdbc applications is Read only and forward only.
• In jdbc applications we are able to specify the following types of the ResultSet combination to any particular ResultSet. 

• read-only, forward only
• read-only, scroll sensitive
• read-only, scroll insensitive
• updatable, forward only
• updatable, scroll sensitive
• updatable, scroll insensitive

• if we want to specity a particular type to the ResultSet object then we should use either of the above constants combination as a parameter to createStatement() method, for this we will use the following method.

public Statement createStatement(int forward / ScrollSensitive / ScrollInsensitive, int  readonly / updatable)

Eg:

Statement st = con. createSensitive(ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);
ResultSet rs = con.executeQuery(….);

How to iterate the data from Scrollable ResultSet objuect in both forward and backword direction?

• to iterate the data in forward direction from a ResultSet object we will use the following 2 methods.

         

public Boolean next()
public xxx getXxx(int fieldno.)

         

    Where xxx may be byte, short, char, int, long, float, double.

• To iterate the data in backward direction from Scrollable ResultSet object we will use the following 2 methods.

           

public Boolean previous()

           

public xxx getXxx(int fieldno)

            Where previous() is a Boolean method, which can be used to check whether the previous record is available or not, if it is available then cursor will be moved to previous record position.

Java Abstract class and interface Interview Questions

Below are the some question which are generally asked in Java interview question from Abstract class and interface.

Question-Can abstract class have constructor?

Yes,An abstract class can contain a constructor. An abstract class constructor is used to initialize fields of the abstract class. You could provide a constructor for an abstract class if you want to initialize specific fields of the abstract class before the instantiation of a child-class.We cannot create an instance of an abstract class, But we can create instances of classes those are derived from the abstract class. So, when an instance of derived class is created, the parent abstract class constructor will automatically get called.

Question-Can abstract class implements interface in                         Java? Is it necessary to implement all                                 methods?

     Yes, abstract class can implement the interface. When we implement an interface to an abstract class, its means that the abstract class inherited all the methods of the interface.

  Its depend upon the class which is implementing the interface. When we implement an interface to an abstract class, then the abstract class inherited all the methods of the interface. As, it is not important to implement all the method in abstract class however it comes to abstract class , so the abstract class can left some of the method in interface without implementation. In concrete class we need to implement all the inherited method.

Question-Can we declare abstract class as final in Java?

     No a class cannot be declare as "abstract" and "final" both. Declaring a class as "abstract" means it contains method definition and hence depends on its subclasses to provide concrete implementation.

     Making a class "final" means that no other class can extend it and hence abstract classes cannot be marked final.

     In short An abstract class must be sub-classed, whereas a final class must not be sub-classed.

      An abstract class without being inherited is of no use and hence will result in compile time error.  

Question-Can an abstract class have static methods in                Java?

     Yes, an abstract class can have static method. You cannot create an abstract static method, you can create a non abstract static method.

    Reason is you do not need a object instance to access a static method because that method can be called directly, even if you do not have an instance of the abstract class.(to invoke static method you do not need to build a class instance (i.e object))

Question-is it possible to create a instance of abstract                class in java?

     No, its never possible to instantiate an abstract class. But might be interviewer can say “YES WE CAN” at that time as below,

    abstract class myClass {

        public void  m1()

        {

           System.out.print(“Inside Abstract class");

        }

     }

    class  implmentmyClass {         public static void main(String a[]) {         myClass m = new myClass () {};
            m.m1();        }
    }

  The given code block in last page instantiates an anonymous inner class which is a subclass of the myClass abstract class. It is not same as instantiating the abstract class itself. we can use new keyword to create an anonymous class instance by just adding {} as implement body at the end of the abstract class. so its not the abstract class instantiation its anonymous class.

Question- Is it necessary for abstract class to have                       abstract method?

  No, we can have an abstract class without Abstract Methods as both are independent. Declaring a class abstract means that it can not be instantiated on its own and can only be sub classed. Declaring a method abstract means that Method will be defined in the subclass. So you can easily define an abstract class without any abstract method.

Question- Can we define main method in abstract class? 

     Yes, we can define main method inside the abstract class, Loading a class is not the same as creating an instance/object of the class(using new operator). And there's no need to create an instance of the class to call main(), because it's static method. So there's no problem if we will define the main method inside an abstract class.

Question- Can you use abstract and final both with a                   method.

     No, we can not declare abstract method as final. We have to provide implementation to abstract methods in subclasses, and if we will mark it as final then we can’t provide the method reimplementation for that final method in it’s derived classes.

     final abstract int add(int n1,int n2);

   Here we can’t provide the implementation for the add method as it is declared as final. But at the same time you declare it as abstract. Here we can’t provide the implementation for the add method. Even we described to provide implementation but we cant.

Question- Can you declare an interface method as static ?

    NO, interface only contains abstract method.

    Suppose we have a Test class with a static        method add. we can call this as

    Test.add(2, 3);

     If Test were an interface instead of a class, it could not have any defined functions. As such, calling something like Test.add(2, 3) doesn't make any sense.

Question- Can we declare an Interface be final ?

     No. We can not instantiate interfaces, so in order to make interfaces useful we must create subclasses. When the final keyword appears in a class declaration, it means that the class may never be subclassed or overridden. Interface represent behaviour, rather than implementation, therefore it makes no sense for it to be final.

Question- Can we define private and protected modifiers             for variables in interfaces.

    No we cant define the variable with private, protected in interface. only public, static & final are permitted with variable inside an interface. An interface provide a way for the client to interact with the object. If variables will not be public, the clients would not have access to them. that is why variable are public.

Question-Can we use “abstract” keyword with                              constructor?

     No, we cant declare the constructor as abstract because abstract means we must have to override it in non abstract child class and you can not override a constructor. Constructors are not inherited thus we cannot override them.

Question-Can we declare abstract methods as private?

     No. Abstract methods can not be private. we cant inherit the private method, so its doesn't make a sense to declare a private method abstract. Declaring a abstract method means we need to override and implement it in child/subclass, but we cant override private methods so we cant make it  abstract. You can declare it protected instead of private. 

Question-We can’t instantiate an abstract class. Then                  why constructors are allowed in abstract                      class?

    The use of constructor to initialize the instance of classes. We use a constructor to create new object often with parameter specifying the initial value or other important information about the object. Suppose inside a class we have some fields we always want them to initialize, to do this we create a constructor and initialize them. If we have two different subclasses of abstract class when we instantiate them their constructor will get called and then parent constructor will get called and field will get initialized. Every constructor invokes the constructor of its super class with an implicit call to super. 

Question-Abstract classes can be nested. True or false?

    True, Abstract classes can be nested. A abstract nested classes are the inner classes marked with abstract modifier. Inner class acts as a member of the enclosing class and can have any access modifiers: abstract, final, public, protected, private, static. For more details please dig the Inner Class in java. 

Question-Can we declare abstract methods as                              synchronized?

    Yes we can declare the abstract method as synchronized but it would be useless to do so since synchronization is an aspect of the implementation, not the declaration, and abstract methods do not have an implementation. You can use the 'synchronized' modifier on a concrete implementation of an abstract method, which is where it belongs. 

Question-Can abstract method declaration include                       throws clause?

Yes. We can define an abstract class with throws clause.

Question-What are the valid and invalid keywords or                     modifier with abstract class?

Valid - public,  protected and default.

Invalid - static,  final and private. 

Abstract Class vs Interface in Java

This is very frequent question asked in Interview.Today we will try to understand the Abstract class and Interface in Java.

Abstract Class vs Interface in Java

Abstract Class:-

• A class that is preceded with keyword “abstract” is known as abstract class.
• It extended by other class and its method implemented.
• It can’t be instantiated means we cant create the object of abstract class.
• it may or may not include abstract methods.
• An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), as below,

          abstract returnType methodName(argument1 a, argument2 b);

• If a class contain abstract methods, then the class itself must be declared abstract.
• When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract.
• abstract class may contain non-final and final variables. 

     abstract classes can be use in below situation,
                1-If need to share code among closely related classes.

               2-classes that extend your abstract class have many common methods or fields,                       or require access modifiers other than public.

               3-declare non-static or non-final fields. It enables us to define methods that can                        access and modify the state of the object to which they belong. 

Interface :-

• An interface is basically a contract -- it doesn't have any implementation. 
• An interface can contain only method declaration; it cannot contain method definition.
• In interface all fields are automatically public, static, and final and all methods (as default methods) are public.
• The class that implements the interface should implement all the members of an interface. Like abstract classes, an interface cannot be instantiated.
• interface is used to "implements“ whereas abstract class is used to "extends".
• interface can be used to achieve multiple inheritance.

interface is more flexible than abstract class because one class can only "extends" one super class, but "implements" multiple interfaces.

interface can be use in below situation,

     1-we expect that unrelated classes will implement interface(the interfaces Comparable             and Cloneable are implemented by many unrelated classes)

       2-Need to specify the behaviour of a particular data type, but not concerned about who             implements its behaviour.

       3-Need to take advantage of multiple inheritance.