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Java Frequently Asked Questions:
    1. What is Inheritance?
    2. What is Polymorphism?
    3. What is Encapsulation?
    4. Avoiding the Divide by Zero Error
    5. Public Classes in Java Source Files
    6. The Import Statement in Java
    7. Classes with Default Access
    8. What are Local Variables?
    9. What is an Abstract Method?
    10. Data Structures
    11. What is Autoboxing and Auto-Unboxing?
    12. What is multithreading?
    13. What are Threads?
    14. Thread States
    15. What is the Purpose of Synchronization?
    16. Thread Interaction
    17. Can You Start a Thread Twice?

Java FAQ 5 - Public Classes in Java Source Files

You cannot have two public classes in the same java file. The reason for this is fairly simple. The public class in a java file must have the same name as the java file itself.

Example: public class HelloWorld – must be in file HelloWorld.java

You cannot have two classes with the same name in the same java file, so it is impossible to have two public classes in the same java file.