Mutable, Immutable, and Cloneable Objects - Data Structures and Abstractions with Java - page 784

30.9

We have one more concern. Imagine a class that has a Name object as a data field. The class has acces-
sor methods, including one that returns the Name field, but has no mutator methods. However, a client
of this class can access the Name field and then use Name 's set methods to alter the field's value. In
other words, the class is not read only. To make it read only, we can define the field as final. Note that
the fields of ImmutableName are strings, which are immutable, so we need not make them final.

Note: Design guidelines for a read-only class

•

The class should be final.

•

Data fields should be private.

•

The class should not have public set methods.

•

Data fields that are mutable objects should be final.

Question 1 Define a constructor for ImmutableName that has a Name object as a parameter.

Companion Classes
30.10

Although immutable objects are desirable for certain applications, mutable objects have their place.
Sometimes we will want to represent an object in both immutable and mutable forms. In such cases, a
pair of companion classes can be convenient. The classes ImmutableName and Name are examples of
two such companion classes. The objects in both classes represent names, but one type of object can-
not be altered, while the other can be.
To make the classes even more convenient, you could include constructors and/or methods that
convert an object from one type to the other. For example, we might add the following constructor
and method to the class ImmutableName :

// add to the class ImmutableName:

public ImmutableName(Name aName)
{

first = aName.getFirst();
last = aName.getLast();
} // end constructor
public Name getMutable()
{

return new Name(first, last);
} // end getMutable
Similarly, we could add the following constructor and method to the class Name :

// add to the class Name

public Name(ImmutableName aName)
{

first = aName.getFirst();
last = aName.getLast();
} // end constructor
public ImmutableName getImmutable()
{

return new ImmutableName(first, last);
} // end getMutable
Figure 30-3 illustrates the two classes Name and ImmutableName .