Important packages in core java

java.io

Provides for system input and output through data streams, serialization and the file system.

java.lang

Provides classes that are fundamental to the design of the Java programming language.

java.util

Contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes (a string tokenizer, a random-number generator, and a bit array).

java.sql

Provides the API for accessing and processing data stored in a data source (usually a relational database) using the Java programming language.

java.awt

Contains all of the classes for creating user interfaces and for painting graphics and images.

java.applet

Provides the classes necessary to create an applet and the classes an applet uses to communicate with its applet context.

java.beans

Contains classes related to developing beans -- components based on the JavaBeansTM architecture.

java.math

Provides classes for performing arbitrary-precision integer arithmetic (BigInteger) and arbitrary-precision decimal arithmetic (BigDecimal).

java.net

Provides the classes for implementing networking applications.

java.nio

Defines buffers, which are containers for data, and provides an overview of the other NIO packages.

java.security

Provides the classes and interfaces for the security framework.

java.text

Provides classes and interfaces for handling text, dates, numbers, and messages in a manner independent of natural languages.