# Interfaces Functional interfaces only have a single method. They can be annotated with @FunctionalInterface to ensure, during compile, that they fit this requirement. If the marked interface gets more than one method then it won't compile. ```java @FunctionalInterface public interface Function ``` These interfaces are very useful when working with the functional paradigm, as with them it is easy making sure that operations have a single concern. By default Java comes with several functional patterns. ## Default Methods These interfaces mix well with default methods, and the functional interfaces included in the JDK make use of them. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://bernardo.gitbook.io/development-docs-java/functional/interfaces.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.