What is a marker interface, and can you give examples?

A marker interface declares no methods at all — implementing it adds no new behavior directly, but signals to the JVM or a library that instances of the class should be treated specially.

Classic JDK examples:

• Serializable: marks a class as eligible for Java serialization; ObjectOutputStream checks instanceof Serializable before writing an object, throwing NotSerializableException otherwise.
• Cloneable: marks a class as allowing Object.clone() to succeed; without it, calling clone() throws CloneNotSupportedException.
• RandomAccess: marks a List implementation (like ArrayList) as supporting fast, O(1) indexed access, letting algorithms (e.g., Collections.binarySearch) choose an index-based loop instead of an iterator-based one for better performance.

class Point implements Serializable {
    int x, y; // now writable via ObjectOutputStream
}

Why they exist: before annotations (Java 5), tagging a class for special treatment required some type-level mechanism checkable via instanceof — an interface with no members was the idiom.

Modern alternative: annotations (@Deprecated, @FunctionalInterface, or custom ones processed via reflection) are now generally preferred for new APIs, since they can carry additional metadata (values, targets, retention) that a bare marker interface can't. Marker interfaces persist mainly for backward compatibility and cases (like Serializable) baked deeply into the platform.