What Are Annotations?

Metadata, not behaviour

An annotation (@Something) attaches metadata to code — a class, method, field, or parameter — that the compiler, a tool, or a framework (like Spring) can read and act on. The annotation itself does nothing at runtime unless something else is written to look for it.

The essential built-ins

@Override asks the compiler to verify a method really does override a supertype method — catches typos immediately. @Deprecated marks an API as discouraged, producing a compiler warning at call sites. @FunctionalInterface asks the compiler to verify an interface has exactly one abstract method, so it can be used as a lambda target.

class Old {
    @Deprecated
    void legacyMethod() { /* callers now get a compiler warning */ }
}

class Dog extends Animal {
    @Override
    String sound() { return "woof"; }   // typo in the name -> compile error, not a silent new method
}

@FunctionalInterface
interface Transformer<T, R> {
    R apply(T input);   // exactly one abstract method
}

Where you'll see more

Frameworks lean heavily on custom annotations — @Test (JUnit), @Entity (JPA), @RestController (Spring). Those live in Advanced Java; for Core Java, knowing what an annotation is and reading the built-ins comfortably is enough.