Java Tag Interfaces

In Java a tag interface (also called a marker interface) is one that has no methods. Thus a tag interface cannot be used to define behavior (in the usual sense) but it can nevertheless contain information about the given type. For example, it can

• emphasize important, high-level aspects of a class, not otherwise expressed in code
• allow quick identification of classes having specific properties, for both human readers and tools
• contain, in its javadoc documentation, all the characteristics of items that implement the given interface

Examples of Tag Interfaces (from the Java API)

java.io.Serializable
Allows the state of an object to be converted to a byte stream in such a way that the byte stream can be reverted back into a copy of the object. A Java object is serializable if its class or any of its superclasses implements the java.io.Serializable interface.

java.lang.Cloneable [Note: Does not contain the clone() method!]
Indicates to Object.clone() that it is legal for that method to make a field-for-field copy of instances of that class. By convention, classes that implement this interface should override the protected method Object.clone() with a public method.