ASA Programming Guide
  Introduction to Java in the Database
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      Understanding Java classes

Subclasses in Java

You can define classes as subclasses of other classes. A class that is a subclass of another class can use the fields and method of its parent: this is called inheritance. You can define additional methods and fields that apply only to the subclass, and redefine the meaning of inherited fields and methods.

Java is a single-hierarchy language, meaning that all classes you create or use eventually inherit from one class. This means the low-level classes (classes further up in the hierarchy) must be present before higher-level classes can be used. The base set of classes required to run Java applications is called the runtime Java classes, or the Java API.