ABSTRACT CLASSES AND INTERFACES - Introduction to Java Programming: Comprehensive Version - page 513

13.20

You can define the compareTo method in a class without implementing the
Comparable interface. What are the benefits of implementing the Comparable
interface?

13.21

What is wrong in the following code?

public class Test {
public static void main(String[] args) {
Person[] persons = { new Person( 3 ), new Person( 4 ), new Person( 1 )};
java.util.Arrays.sort(persons);
}
}

class Person {
private int id;

Person( int id) {
this .id = id;

}
}

13.7 The Cloneable Interface
The Cloneable interface specifies that an object can be cloned.

Key
Point

Often it is desirable to create a copy of an object. To do this, you need to use the clone
method and understand the Cloneable interface.
An interface contains constants and abstract methods, but the Cloneable interface is a
special case. The Cloneable interface in the java.lang package is defined as follows:

package java.lang;

java.lang.Cloneable

public interface Cloneable {
}

This interface is empty. An interface with an empty body is referred to as a marker interface .
A marker interface does not contain constants or methods. It is used to denote that a class
possesses certain desirable properties. A class that implements the Cloneable interface is
marked cloneable, and its objects can be cloned using the clone() method defined in the
Object class.
Many classes in the Java library (e.g., Date , Calendar , and ArrayList ) implement
Cloneable . Thus, the instances of these classes can be cloned. For example, the following code

marker interface

1 Calendar calendar = new GregorianCalendar( 2013 , 2 , 1 );
2 Calendar calendar1 = calendar;
3 Calendar calendar2 = (Calendar)calendar.clone();
4 System.out.println( "calendar == calendar1 is " +
5 (calendar == calendar1));
6 System.out.println( "calendar == calendar2 is " +
7 (calendar == calendar2));
8 System.out.println( "calendar.equals(calendar2) is " +
9 calendar.equals(calendar2));

displays

calendar == calendar1 is true
calendar == calendar2 is false
calendar.equals(calendar2) is true