How to use Java API Packages

Java contains many packages which are basically some predefined classes grouped into categories of related classes.

Java API stands for Java applications programming interface, also known as the Java class library.

In your code, import statements specify the classes required to compile a Java program.

For example when using:

import javax.awt.event;

you tell the compiler to load the event class from the javax.awt package.

One of the great things about Java is the large number of classes in the packages of the Java API that programmers can reuse.

We gathered a subset of the many packages in the Java API and tried to provide a brief description of each package.

When learning Java, you should spend time reading the descriptions of the packages and classes in the Java API documentation, just because of the large number and the variety of reusable components available in the Java API.

The core packages in J2SE 6.0 are:

• java.lang -- basic language functionality and fundamental types
• java.util -- a collection of data structure classes
• java.io -- file operations
• java.math -- multiprecision arithmetics
• java.nio -- the New I/O framework for Java
• java.net -- networking operations, sockets etc..
• java.security -- key generation, encryption and decryption
• java.sql -- Java database connectivity (JDBC) to access databases
• java.awt -- basic hierarchy of packages for native GUI components
• java.swing -- hierarchy of packages for platform-independent rich GUI components

Using packages

In Java source files, you specify the package you want to use with the 'package' keyword.

package java.awt.event;

 For us to use a package inside a Java program, we should first import the classes from the package with an 'import' statement.

By using the statement :

import java.awt.event.*;

 we tell the compiler to import all the classes from the java.awt.event package, while when using :

import java.awt.event.ActionEvent;

it only imports the ActionEvent class from the package. After either of these import statements, the ActionEvent class can be referenced using its simple class name:

ActionEvent myEvent = new ActionEvent();

Classes can also be used directly without an import statement by using the fully-qualified name of the class. For example,

java.awt.event.ActionEvent myEvent = new java.awt.event.ActionEvent();

 doesn't require a preceding import statement.