JAVA Program to demonstrate use of Dynamic Method Dispatch

class A {
void callme() {
System.out.println("Inside A's callme method");
}
}
class B extends A {
// override callme()
void callme() {
System.out.println("Inside B's callme method");
}
}
class C extends A {
// override callme()
void callme() {
System.out.println("Inside C's callme method");
}
}
class Dispatch {
public static void main(String args[]) {
A a = new A(); // object of type A
B b = new B(); // object of type B
C c = new C(); // object of type C
A r; // obtain a reference of type A
r = a; // r refers to an A object
r.callme(); // calls A's version of callme
r = b; // r refers to a B object
r.callme(); // calls B's version of callme
THE JAVA LANGUAGE
r = c; // r refers to a C object
r.callme(); // calls C's version of callme
}
}

JAVA Program to implement an Abstract class

abstract class Figure {
double dim1;
double dim2;
Figure(double a, double b) {
dim1 = a;
dim2 = b;
}
// area is now an abstract method
abstract double area();
}
class Rectangle extends Figure {
Rectangle(double a, double b) {
super(a, b);
}
// override area for rectangle
double area() {
System.out.println("Inside Area for Rectangle.");
return dim1 * dim2;
}
}
class Triangle extends Figure {
Triangle(double a, double b) {
super(a, b);
}
// override area for right triangle
double area() {
System.out.println("Inside Area for Triangle.");
return dim1 * dim2 / 2;
}
}
class AbstractAreas {
public static void main(String args[]) {
// Figure f = new Figure(10, 10); // illegal now
Rectangle r = new Rectangle(9, 5);
Triangle t = new Triangle(10, 8);
Figure figref; // this is OK, no object is created
figref = r;
System.out.println("Area is " + figref.area());
figref = t;
System.out.println("Area i is " + figref.area());
}
}

JAVA Program to implement multi level inheritance .

class A {
String getName(){
return "Gagan";
}
}
class B extends A {
int getAge() {
return 20;
}}
class C extends B {
String getAddress(){
return "A/1,India";
}}
class D extends C {
void print() {
System.out.println(getName());
System.out.println(getAge());
System.out.println(getAddress());
}}
class main{
public static void main(String args[]){
D obj1=new D();
obj1.print();
}}

JDBC (Java Database Connectivity)

What is JDBC Driver ?

JDBC drivers implement the defined interfaces in the JDBC API for interacting with your database server.
For example, using JDBC drivers enable you to open database connections and to interact with it by sending SQL or database commands then receiving results with Java. The Java.sql package that ships with JDK contains various classes with their behaviours defined and their actual implementaions are done in third-party drivers. Third party vendors implements the java.sql.Driver interface in their database driver.

Understanding the JDBC Architecture

JDBC is an API specification developed bySun Microsystems that defines a uniform interface for accessing various relational databases. JDBC is a core part of the Java platform and is included in the standard JDK distribution.
The primary function of the JDBC API is to provide a means for the developer to issue SQL statements and process the results in a consistent, database-independent manner. JDBC provides rich, object-oriented access to databases by defining classes and interfaces that represent objects such as:
1. Database connections
2. SQL statements
3. Result Set
4. Database metadata
5. Prepared statements
6. Binary Large Objects (BLOBs)
7. Character Large Objects (CLOBs)
8. Callable statements
9. Database drivers
10. Driver manager
The JDBC API uses a Driver Manager and database-specific drivers to provide transparent connectivity to heterogeneous databases. The JDBC driver manager ensures that the correct driver is used to access each data source. The Driver Manager is capable of supporting multiple concurrent drivers connected to multiple heterogeneous databases.

Layers of the JDBC Architecture:

A JDBC driver translates standard JDBC calls into a network or database protocol or into a database library API call that facilitates communication with the database. This translation layer provides JDBC applications with database independence. If the back-end database changes, only the JDBC driver need be replaced with few code modifications required.

JDBC Drivers Types:

JDBC driver implementations vary because of the wide variety of operating systems and hardware platforms in which Java operates. Sun has divided the implementation types into four categories, Types 1, 2, 3, and 4, which is explained below:

Type 1: JDBC-ODBC Bridge driver (Bridge)
Type 2: Native-API/partly Java driver (Native)
Type 3: AllJava/Net-protocol driver (Middleware)
Type 4: All Java/Native-protocol driver (Pure)

Type 1 JDBC Driver (JDBC-ODBC Bridge driver):

The Type 1 driver translates all JDBC calls into ODBC calls and sends them to the ODBC driver. ODBC is a generic API. The JDBC-ODBC Bridge driver is recommended only for experimental use or when no other alternative is available.

Type 1: JDBC-ODBC Bridge
Advantage:
The JDBC-ODBC Bridge allows access to almost any database, since the database's ODBC drivers are already available.
Disadvantages:
1. Since the Bridge driver is not written fully in Java, Type 1 drivers are not portable.
2. A performance issue is seen as a JDBC call goes through the bridge to the ODBC driver, then to the database, and this applies even in the reverse process. They are the slowest of all driver types.
3. The client system requires the ODBC Installation to use the driver.
4. Not good for the Web.

Type 2 JDBC Driver (Native-API/partly Java driver):

The distinctive characteristic of type 2 jdbc drivers are that Type 2 drivers convert JDBC calls into database-specific calls i.e. this driver is specific to a particular database. Some distinctive characteristic of type 2 jdbc drivers are shown below. Example: Oracle will have oracle native api.

Type 2: Native api/ Partly Java Driver
Advantage:
The distinctive characteristic of type 2 jdbc drivers are that they are typically offer better performance than the JDBC-ODBC Bridge as the layers of communication (tiers) are less than that of Type 1 and also it uses Native api which is Database specific.
Disadvantage:
1. Native API must be installed in the Client System and hence type 2 drivers cannot be used for the Internet.
2. Like Type 1 drivers, it’s not written in Java Language which forms a portability issue.
3. If we change the Database we have to change the native api as it is specific to a database
4. Mostly obsolete now
5. Usually not thread safe.

Type 3 JDBC Driver (All Java/Net-protocol driver):

Type 3 database requests are passed through the network to the middle-tier server. The middle-tier then translates the request to the database. If the middle-tier server can in turn use Type1, Type 2 or Type 4 drivers.

Type 3: All Java/ Net-Protocol Driver
Advantage:
1. This driver is server-based, so there is no need for any vendor database library to be present on client machines.
2. This driver is fully written in Java and hence Portable. It is suitable for the web.
3. There are many opportunities to optimize portability, performance, and scalability.
4. The net protocol can be designed to make the client JDBC driver very small and fast to load.
5. The type 3 driver typically provides support for features such as caching (connections, query results, and so on), load balancing, and advanced system administration such as logging and auditing.
6. This driver is very flexible allows access to multiple databases using one driver.
7. They are the most efficient amongst all driver types.
Disadvantage:
It requires another server application to install and maintain. Traversing the recordset may take longer, since the data comes through the backend server.

Type 4 JDBC Driver (Native-protocol/all-Java driver):

The Type 4 uses java networking libraries to communicate directly with the database server.

Type 4: Native-protocol/all-Java driver
Advantage:
1. The major benefit of using a type 4 jdbc drivers are that they are completely written in Java to achieve platform independence and eliminate deployment administration issues. It is most suitable for the web.
2. Number of translation layers is very less i.e. type 4 JDBC drivers don't have to translate database requests to ODBC or a native connectivity interface or to pass the request on to another server, performance is typically quite good.
3. You don’t need to install special software on the client or server. Further, these drivers can be downloaded dynamically.
Disadvantage:
With type 4 drivers, the user needs a different driver for each database.