Reflection in Java is a mechanism that allows a program to examine and manipulate the structure and behavior of objects at runtime. With reflection, a program can inspect and modify fields, methods, and constructors of a class, as well as access information about the class itself, such as its name, superclass, and interfaces.
Reflection is implemented through the java.lang.reflect package, which provides classes and interfaces for accessing and manipulating the runtime information of classes and objects. The most commonly used classes in this package include Class, Method, Field, Constructor, and Parameter.
Some common use cases for reflection include:

1. Dynamic loading of classes at runtime
2. Creation of new objects of a class at runtime
3. Examination of the fields and methods of an object to determine its type and behavior
4. Invocation of methods on an object at runtime
5. Retrieval of annotations and metadata associated with a class or method
   While reflection can be a powerful tool, it also has some drawbacks, such as performance overhead and security risks. Therefore, it should be used with care and only when necessary.

The problems are:

1. Security: Reflection can allow a program to access private fields, methods, and constructors, bypassing normal access control checks. This can be a security risk if the program is not designed to handle such access, potentially allowing malicious code to execute or sensitive data to be accessed.
2. Performance: They are slower than direct method calls or field accesses, as it involves additional overhead in looking up and invoking the appropriate method or field.
3. Code readability: It become difficult to understand and debug, as it allows a program to call methods and access fields that are not explicitly defined in the code.
4. Maintainability: Reflection can make it harder to determine how a particular method or field is being used or where it is being called from.
5. Type safety: It also lead to type safety issues, as it allows a program to cast objects to different types without proper type checking, potentially resulting in runtime errors.