Java is an object-oriented language. Its instructions (source code) come in the form of classes. You write and compile the Java instructions outside the database into compiled classes (byte code), which are binary files holding Java instructions.
You then install the compiled classes into the database, where they can be executed in the database server.
Adaptive Server is a runtime environment for Java classes. You need a Java development environment, such as Sybase PowerJ™ or Sun Microsystems Java Development Kit (JDK), to write and compile Java.