Read up on UML and class diagrams in Java.
Assuming you know Java, or some basics of Java, you can decipher the Class Diagram if you learn how to understand UML basics.
For example, this article gives a primer on UML and Java:
UML Class Diagrams for Java Programmers
If you understand how to real a UML class diagram, then all the information on how to design your Java class, the basic structure, is already there.