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These three concepts build on each other more than they seem to at first glance: annotations give the compiler metadata to act on, one specific annotation (@FunctionalInterface) marks a particular kind of interface, and that kind of interface is exactly what makes lambda expressions possible. This post walks through all three, in that order. ## Annotations Metadata(s) for the compiler. An annotation in Java is metadata attached to code — a class, method, field, or interface — that doesn't change what the code does, but gives instructions to the compiler, a tool, or the JVM about how to treat it. You've probably already used a few without thinking about it: _@override_ , _@deprecated_ , _@SuppressWarnings_. @Override public String toString() { return "example"; } @override doesn't add behavior — it just tells the compiler "check that this method actually overrides something in the superclass, and fail if it doesn't." That's the general pattern: annotations are instructions about the code, not part of its logic. One of the most useful annotations for modern Java is _@FunctionalInterface_ , which brings us to the next concept. ## Functional interfaces A functional interface is an interface with exactly one abstract method. This single-method shape is also called a SAM interface — Single Abstract Method. interface A { void show(); } To make Java enforce that an interface stays this way, you annotate it: @FunctionalInterface interface A { void show(); } Now, if a second abstract method sneaks in, it's going to give a compile-time error instead of a bug which you will see later: @FunctionalInterface interface A { void show(); void play(); // compile error: not a functional interface anymore } Note that this restriction only applies to abstract methods. A functional interface can still have any number of default or static methods, since those already have a body and don't count toward the "single abstract method" rule: @FunctionalInterface interface A { void show(); default void log() { System.out.println("Logging from A"); } } ### Implementing a functional interface with an anonymous class Before lambdas, this is what implementing a small interface inline looked like: public class StoreApplication { public static void main(String[] args) { A obj = new A() { @Override public void show() { System.out.println("implementing interface directly...."); } }; obj.show(); } } It works, but it's verbose for something this simple — which is exactly the problem lambda expressions were introduced to solve. ## Lambda expressions Functional interfaces are what make lambda expressions possible in Java 8+: since a functional interface guarantees there's exactly one method to implement, Java can let you skip the class name, the method name, and the boilerplate, and just write the method body. ### Implementing a functional interface with a lambda @FunctionalInterface interface A { void show(); } public class StoreApplication { public static void main(String[] args) { A obj = () -> System.out.println("implementing interface using lambda...."); obj.show(); } } ### Passing an argument via lambda @FunctionalInterface interface A { void show(int i); } public class StoreApplication { public static void main(String[] args) { A obj = (i) -> System.out.println("implementing interface using lambda...." + i); obj.show(45); } } A functional interface that returns a value @FunctionalInterface interface A { int add(int a, int b); } public class StoreApplication { public static void main(String[] args) { A calculator = (i, j) -> { System.out.println("Performing addition of numbers " + i + " and " + j); return i + j; }; System.out.println("The sum is equal to " + calculator.add(45, 30)); } } Same contract, same enforcement from @FunctionalInterface — just far less ceremony to implement it. ## **Types of interfaces** , at a glance Now that we've covered the functional/SAM interface in depth, it's worth placing it alongside the other interface types you'll run into: **Normal interface** — two or more methods, no restrictions on count. **Functional / SAM interface** — exactly one abstract method; can be implemented with a lambda. **Marker interface** — has no methods at all. It exists purely to "tag" a class with metadata that other code can check via instanceof, so the JVM or a framework can treat tagged classes differently. _Serializable_ and _Cloneable_ are the classic examples — implementing Serializable doesn't add any methods, but it tells the JVM "this object is allowed to be serialized." That last one brings things full circle: marker interfaces were Java's original way of attaching metadata to a class before annotations existed. Today, annotations have largely taken over that role — which is exactly where this post started. ## Wrapping up Annotations give the compiler instructions, @FunctionalInterface uses that mechanism to enforce a single-method contract, and lambdas exist to make implementing that contract painless. Once you see the chain connecting them, a lot of modern Java code (streams, Comparator, Runnable, custom callbacks) starts making a lot more sense. Got a favorite use of lambdas or functional interfaces in your own code? Drop it in the comments.

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1. Introduction to Doctest

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Deserialization of untrusted data vulnerability that may allow class-registration checks to be bypassed during Java lambda deserialization. Only lambda capture class is affected This issue affects Apache Fory: from before 1.4.0. Users are recommended to upgrade to version 1.4.0, which fixes the i

Posted by Chaokun Yang on Jul 21 Severity: important Affected versions: - Apache Fory (org.apache.fory:fory-core) 0.5.0 before 1.4.0 Description: Deserialization of untrusted data vulnerability that may allow class-registration checks to be bypassed during Java lambda deserialization. Only lambda capture class is affected This issue affects Apache Fory: from before 1.4.0. Users are recommended to upgrade to version 1.4.0, which fixes the issue. Credit: Charles Vosburgh...

Deserialization of untrusted data vulnerability that may allow class-registration checks to be bypassed during Java lambda deserialization. Only lambda capture class is affected This issue affects Apache Fory: from before 1.4.0. Users are recommended to upgrade to version 1.4.0, which fixes the issue.