What are some use cases for external classes in Java?

Java external classes are primarily used in the following scenarios:

1. Multiple classes need to share data: If multiple classes require access to the same data or need to share a resource, these data or resources can be defined in an external class and then instantiated in other classes to access them.
2. Code reuse: By encapsulating related functionality code in an external class, the code can be reused by instantiating an object of this external class in different classes.
3. Implementing object-oriented design patterns: The outer class can be used to implement various design patterns such as singleton pattern, factory pattern, observer pattern, etc.
4. Layered architecture: In large application programs, external classes can be used to implement a layered architecture, separating the functionality and logic of different layers to improve code maintainability and readability.
5. Plugin development: External classes can be used as plugin implementations, by loading different external classes to extend the functionality of an application.
6. Event handling: By defining an external class object as an event listener, one can achieve event processing and response.

It is important to note that Java external classes are not always necessary; in some scenarios, inner classes, anonymous inner classes, or lambda expressions can be used as more concise alternatives to achieve the same functionality.