Reflection in Java allows programmers to examine or modify the behavior of an application at runtime. With reflection, you can inspect classes, interfaces, fields, methods, and constructors at runtime. You can also construct new objects, invoke methods, and access or modify fields on existing objects at runtime.

Reflection provides a lot of flexibility and power to your application by allowing it to adapt to different scenarios dynamically. Some common use cases of reflection include building application frameworks, creating testing tools, and creating dynamically configurable applications.

To use reflection in Java, you typically use the java.lang.reflect package, which provides a set of classes to work with reflections. Using reflection, you can get information about the class, such as its constructors, methods, and fields. You can also create new instances of the class, invoke methods, and access or modify fields.

However, reflection also comes with some downsides. Reflection can make your code slower and less type-safe, as you're operating on objects and methods based on their names in strings rather than through their defined types. It can also make your code harder to understand and maintain, as the dynamic nature of reflection can make it difficult to follow the program's control flow.

Overall, reflection is a powerful tool for Java programmers, but it should be used with caution and only when necessary.