In Java, reflection is the ability of a program to examine, introspect, and modify itself at runtime. This is achieved through the use of the Java Reflection API, which allows a program to access its own class, method, and field definitions, as well as to invoke methods and access fields at runtime.

Reflection in Java is useful for a number of reasons. For example, it can be used to implement features such as object serialization, where objects can be converted to and from a string representation. It can also be used for debugging and testing, since it allows a program to inspect and modify its own behavior at runtime. Additionally, reflection can be used to implement dynamic loading of classes, where classes can be loaded and instantiated at runtime, rather than at compile time. This can make a program more flexible and adaptable, since new classes can be added without the need to recompile the entire program.