What is a marker interface? Why is it needed?

Marker interface is an interface without methods that serves to mark or identify a class as possessing a certain property.

An example in Java is the Serializable interface. A class implementing this interface signals to the JVM and other tools that its objects can be serialized.

Why is a marker interface needed:

• It allows using instanceof to check if an object belongs to a certain category.
• It is used in APIs and frameworks to enable or disable certain behaviors without adding methods.
• It provides type safety and code clarity.

Example:

public interface MyMarker {}

public class MyClass implements MyMarker {
    // the class does not need to implement anything
}

// somewhere in the code
if (obj instanceof MyMarker) {
    // perform special logic
}

Thus, a marker interface is a simple way to mark a class for subsequent processing.