Java Reference
In-Depth Information
Figure 8.10
All classes inherit from
Having a common superclass for all objects serves two purposes: First, we can declare polymor-
phic variables of type Object to hold any object. Having variables that can hold any object type is
not often useful, but there are some situations where this can help. Second, the Object class can
define some methods that are then automatically available for every existing object. Of particular
importance are the methods toString , equals , and hashCode which Object defines. This sec-
ond point becomes interesting a bit later, and we shall discuss this in more detail in the next chapter.
Autoboxing and wrapper classes
We have seen that, with suitable parameterization, the collection classes can store objects of
any object type. There remains one problem: Java has some types that are not object types.
As we know, the simple types—such as int , boolean , and char —are separate from object
types. Their values are not instances of classes, and they do not inherit from the Object class.
Because of this, they are not subtypes of Object , and it would not normally be possible to add
them into a collection.
This is unfortunate. There are situations in which we might want to create a list of int values or
a set of char values, for instance. What can we do?
Autoboxing is
performed auto-
matically when a
primitive-type value
is used in a context
requiring a wrapper
Java's solution to this problem is wrapper classes. Every primitive type in Java has a corre-
sponding wrapper class that represents the same type but is a real object type. The wrapper class
for int , for example, is called Integer . A complete list of simple types and their wrapper
classes is given in Appendix B.
The following statement explicitly wraps the value of the primitive int variable ix in an
Integer object:
Integer iwrap = new Integer(ix);
And now iwrap could obviously easily be stored in an ArrayList<Integer> collection, for
instance. However, storing of primitive values into an object collection is made even easier
through a compiler feature known as autoboxing.
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