In most modern operating systems, a large body of reusable code is provided to simplify the programmers job. This code is typically provided as a set of [[Library (computer science)#Dynamic linking|dynamically loadable libraries]] that applications can call at runtime. Because the Java platform is not dependant on any specific operating system, applications cannot rely of any of the existing libraries. Instead, the Java platform provides a comprehensive set of standard class libraries, much of the same reusable functions commonly found in modern operating systems.
The Java class libraries serve three purposes within the Java platform. Like other standard code libraries, they provide the programmer a well-known set of functions to perform common tasks, such as maintaining lists of items or performing complex string parsing. In addition, the class libraries provide an abstract interface to tasks that would normally depend heavily on the hardware and operating system. Tasks such as network access and file access are often heavily dependant on the native capabilities of the platform. The Java java.net and java.io libraries implement the required native code internally, then provide a standard interface for the Java applications to perform those tasks. Finally, some underlying platforms may not support all of the features a Java application expects. In these cases, the class libraries can either emulate those features using whatever is available, or provide a consistent way to check for the presence of a specific feature.
