The JDK comes with pre-written code for many objects. The most common example would be the standard command button. You do not need to code buttons from scratch each time you want to use one in your applet or application. Instead, the creators of Java have included a whole section of pre-written code that you can access and use. This code is logically organized into a directory structure.
Some components are sub-components of others.
This is known as the Application Programmer Interface or API and you can go here to see a table of these components. This source code is provided for informational purposes only, to help developers learn and use the Java programming language. These files do not include platform-specific implementation code and cannot be used to rebuild the class libraries. Do not modify core API source files. There is an assignment due in the last week.