Using Classes and Objects - Java Software Solutions: Foundations of Program Design - page 143

Autoboxing
Autoboxing is the automatic conversion between a primitive value and a cor-
responding wrapper object. For example, in the following code, an int value is
assigned to an Integer object reference variable:

Integer obj1;
int num1 = 69;
obj1 = num1; // automatically creates an Integer object

The reverse conversion, called unboxing, also occurs automatically when needed.
For example:

Integer obj2 = new Integer(69);
int num2;
num2 = obj2; // automatically extracts the int value

Assignments between primitive types and object types
are generally incompatible. The ability to autobox occurs
only between primitive types and corresponding wrapper
classes. In any other case, attempting to assign a primitive
value to an object reference variable, or vice versa, will
cause a compile-time error.

KEY CONCEPT
Autoboxing provides automatic con-
versions between primitive values
and corresponding wrapper objects.

SELF-REVIEW QUESTIONS (see answers in Appendix N)
SR 3.32 How can we represent a primitive value as an object?
SR 3.33 What wrapper classes correspond to each of the following primitive
types: byte, int, double, char , and boolean ?
SR 3.34 Suppose that an int variable named number has been declared and ini-
tialized and an Integer variable named holdNumber has been declared.
Show two approaches in Java for having holdNumber represent the
value stored in number .
SR 3.35 Write a statement that prints out the largest possible int value.

3.9 Components and Containers

In the Graphics Track sections of Chapter 2 we introduced the Java capabilities
to draw shapes using the Graphics and Color classes from the Java standard
class library. We also defined the concept of an applet, a Java program that is
intended to be embedded in a Web page and executed through a browser. Recall
that, in contrast to applets, Java applications are stand-alone programs that are
not executed through the Web.