Packages

Packages are used to group related classes and interfaces together. The package keyword is used to group a class or interface into a package.

Java Common Packages

Packages in the Java language itself begin with the java. or javax.. The Java Platform classes are organized into packages, here are the some common packages :

java.lang

This package provides the classes fundamental to the design of the Java programming language. The classes in this package are automatically available. There's no need to import them. All primitive types are defined in this package. Here are some classes defined in the java.lang package :

Boolean  
Byte  
Character  
Double   
Float  
Integer  
Long  
Math  
Number  
Object  
String  
Thread

java.io

The package provides for system input and output through data stream , serialization and the file system. Some classes in the package are :

 BufferedInputStream 
 BufferedOutputStream 
 BufferedReader 
 BufferedWriter
 Console
 File
 fileReader
 FileWriter
 InputStream
 InputStreamReader
 OutputStream
 OutputStreamWriter
 Reader
 StringReader
 Writer

java.util

Contains the collection 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).

Some classes and interfaces in the package are :

 Collection<E>
 Comparator<T>
 List<E>
 Map<K,V>
 Queue<E>
 Set<E>
 Arrays
 ArrayList<E>
 Date
 Collections
 Dictionary<K,V>
 LinkedList<E>
 Scanner
 UUID

java.text

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

Some classes in the package are :

Annotation
DateFormat
DecimalFormat
NumberFormat
SimpleDateFormat