Interfaces and Inner Classes - Java - page 673

Display 13.8 Implementation of the Method clone (Harder Case)

1 public class YourCloneableClass2 implements Cloneable
2{
3 private DataClass someVariable;

DataClass is a mutable class. Any other
instance variables are each of a primitive
type or of an immutable type like String .

4 .

5 .

6 .

7 public Object clone()

8 {

9 try

10 {

11 YourCloneableClass2 copy =

12 (YourCloneableClass2)super.clone();

13 copy.someVariable = (DataClass)someVariable.clone();

14 return copy;

15 }

16 catch (CloneNotSupportedException e)

17 { //This should not happen.

18 return null ; //To keep the compiler happy.

19 }

20 }

If the clone method return type is DataClass rather
than Object , then this type cast is not needed.

21 .

22 .

23 .

24 }

The class DataClass must also properly implement
the Cloneable interface including defining the clone
method as we are describing.

variable someVariable . So, the reference is replaced by a reference to a copy of the
object named by someVariable . This is done with the following line: 3

copy.someVariable = (DataClass)someVariable.clone();

The object named by copy is now safe and so can be returned by the clone method.
If there are more instance variables that have a mutable class type, then you repeat
what we did for someVariable for each of the mutable instance variables.
This requires that the class type DataClass has a correctly working clone method
that is marked public , but that will be true if the class type DataClass implements the
Cloneable interface in the way we are now describing. That is, DataClass should
implement the Cloneable interface following the model of Display 13.7 or Display 13.8,

3 Depending on how the clone method was defined in the class DataClass , the type cast may or may
not be needed, but causes no harm in any case.