What are Marker Interfaces and why are they required?

Definition:

An interface with no methods or fields.

Examples:

• Serializable
• Cloneable
• Remote

Purpose:

• They act as tags or "flags" for the JVM or a library.
• For example, if a class implements Cloneable, it tells Object.clone() that it is safe to make a field-for-field copy. Without it, CloneNotSupportedException is thrown.
• In modern Java, these are often replaced by Annotations, which are more flexible.