Kotlin is designed to have seamless interoperability with Java, allowing developers to leverage existing Java code and libraries within Kotlin projects and vice versa. Here are some key aspects of how Kotlin and Java interoperate :

1. Calling Java Code from Kotlin :
   * Kotlin can directly call Java code without any additional configuration or wrappers. You can use Java classes, methods, and libraries in your Kotlin code as if they were Kotlin entities.
   * Kotlin provides null safety features that help prevent null pointer exceptions when calling nullable Java code from Kotlin. Kotlin handles nullability through type inference and the use of nullable and non-nullable types.

2. Calling Kotlin Code from Java :
   * Kotlin code can be called from Java in a similar manner as calling Java code. Kotlin classes, functions, and properties are visible to Java code without any special handling.
   * Kotlin's extension functions, which allow adding functions to existing classes, can be used seamlessly from Java code as if they were regular static utility methods.

3. Interoperability of Types :
   * Kotlin maps most Java types to their corresponding Kotlin counterparts. For example, Java's `String` is mapped to Kotlin's `String`, and Java's `List` is mapped to Kotlin's `List`.
   * Kotlin has its own nullability system, distinguishing nullable types from non-nullable types. Java types are treated as platform types in Kotlin, meaning the nullability is unknown. This allows Kotlin to handle nullability more safely, but developers must use appropriate null safety operators when working with Java types.

4. Java Annotations :
   * Kotlin fully supports Java annotations. You can use Java annotations in Kotlin code, define new annotations in Kotlin, and even write Kotlin code within Java annotations.

5. Java Interop Annotations :
   * Kotlin provides a set of annotations specifically for Java interoperability. These annotations allow fine-grained control over how Kotlin code is represented and accessed from Java, such as controlling the generated Java bytecode or specifying default values for properties.

6. Gradual Adoption :
   * Kotlin is designed to facilitate gradual adoption in existing Java projects. You can introduce Kotlin code into a Java codebase seamlessly, with both languages coexisting and interoperating without issues.
   * Kotlin and Java files can be mixed within the same project, and the IDEs provide support for converting Java code to Kotlin and vice versa.