What is the difference between inner classes and outer classes in Java?

An inner class in Java is a class defined within another class, while an outer class is a class that exists independently. The main differences between them are as follows:

1. Access permissions: Inner classes have access to all members of the outer class, including private members; while the outer class can only access the public members of the inner class.
2. Static nature: Inner classes can be static or non-static; whereas outer classes can only be non-static.
3. Instantiation: The instantiation of an inner class must rely on an instance of the outer class, meaning that the creation of an inner class must come after the outer class object; whereas the outer class can be instantiated directly.
4. Scope: The scope of inner classes is limited to the inner portion of the outer class and cannot be accessed directly by any other classes outside of the outer class, while the outer class can be accessed by other classes.
5. Name restriction: Inner classes can have the same name as outer classes; while the name of the outer class must be the same as the file name.

In general, inner classes are primarily used to establish logical connections, allowing easy access to the members and methods of the outer class, while also enabling multiple inheritance. On the other hand, outer classes are standalone classes designed to implement independent functional modules.