• Lets discuss about what are the common functionalities everyone needs when developing a application and what are the classes we require mostly to rectify common problems.

Exceptions:

•  Exception classes are very important in every java project,
• Finding the places where the chances of occurring exception and placing in try and in catch block assigning to corresponding exception class or super class of all exceptions Exception.

1. Checked Exceptions
2. Unchecked Exceptions
3. Errors

Basic IO:

•  Basic input and output operations covered by java platform classes focuses on IO streams and serialization.

1.     Byte Streams are provided to handle I/O of raw binary data.
2.     Character Streams handles I/O of character data
3.     Buffered Streams optimize input and output by reducing the number of calls to the native API.
4.     Scanning and Formatting allows a program to read and write the formatted text.
5.     I/O from the Command Line describes the Standard Streams and the Console object.
6.     Data Streams will handle the binary I/O Strings and primitive data types
7.     Object Streams provided to handle binary I/O of all objects.

 Concurrency:

• Will Explains how to build applications which perform multiple tasks simultaneously.
• Java Platform having full support to concurrent programming 
• In JDK 5 Java introduces high level concurrent programming APIs.
• Like java.util.concurrent packages. 

  The Platform Environment 

• To examine and configure java environment java provides classes and libraries.
• JVM and Java class libraries
• Configuration Utilities describes APIs used to access configuration data supplied when the application is deployed, or by the application's user.
• System Utilities describes miscellaneous APIs defined in the System and Runtime classes.
• PATH and CLASSPATH describes environment variables used to configure JDK development tools and other applications.

• Here the List of  commonly used java classes

1. java.lang.Exception
2. java.lang.System
3. java.lang.String
4. java.util.ArrayList
5. java.util.HashMap
6. java.lang.Object
7. java.lang.Thread
8. java.lang.Class
9. java.util.Date
10. java.util.Iterator