Five commonly used Java-api package:

• java.lang. *
  　　offer classes that are fundamental Java programming language programming. The most important class is Object (which is the root of the class hierarchy) and Class (which represents an instance of an application running in the class)
• Classes in java.util. *
  　　Collection class contains the collections framework, legacy, event model, date and time facilities, internationalization, and miscellaneous utility classes (a string tokenizer, a random number generator, and a bit array, the date of the Date class, stack Stack class, etc. vector vector). Collections, the time processing mode, date and time, and other commonly used tool kit
• * java.io.
  　　the Java core libraries java.io provides a comprehensive IO interface. Comprising: a file reading and writing, the standard output device and the like. Java is an IO stream input and output, based on all serialized data is written to the output stream, or read from the input stream
• java.net. *
  　　Not all systems supporting IPv6 protocol, and when the Java networking stack attempts to detect it and use it transparently when available, can also be used to disable the system properties it. In a case where IPv6 is not available or is explicitly disabled, Inet6Address most network connections are no longer valid operating parameters. Although it is possible to ensure that when a host name lookup method java.net.InetAddress.getByName like not return Inet6Address, but still possible to create such an object by passing literal. In this case, most of the methods in the use of Inet6Address calls will throw an exception
• java.sql. *
  　　Providing using JavaTM programming language API for accessing and processing the data stored in the data source (usually a relational database). This API includes a frame, this frame may be dynamically with different drivers installed to access different data sources