How many public classes can a Java file contain?

In Java, a source file can contain at most one public class. This rule helps maintain a clear structure and organization in the code. When a class is declared as public, it means that it can be accessed from other classes and packages, thereby establishing it as the primary interface for interaction outside its own package.

The single public class must have a filename that matches the name of the class itself, with a .java extension. For example, if you have a public class named "MyClass," the file must be named "MyClass.java." This convention ensures that the Java compiler can accurately find the corresponding source file for the public class.

While a Java file can contain multiple classes, only one of those can be public. The other classes can have package-private, protected, or private access modifiers, allowing them to exist within the same file without exposing them to the entire application.

This concept is essential for maintaining encapsulation and preventing unintended access to classes that are meant to be used only within certain contexts.