Inheritance - Data Structures and Problem Solving Using Java - page 145

adapter class is typically used when the interface of a class is not exactly what
is needed, and provides a wrapping effect, while changing the interface.
In Java, we have already seen that although every reference type is com-
patible with Object , the eight primitive types are not. As a result, Java provides
a wrapper class for each of the eight primitive types. For instance, the wrapper
for the int type is Integer . Each wrapper object is immutable (meaning its
state can never change), stores one primitive value that is set when the object
is constructed, and provides a method to retrieve the value. The wrapper
classes also contain a host of static utility methods.
As an example, Figure 4.25 shows how we can use the Java 5 ArrayList to
store integers. Note carefully that we cannot use ArrayList<int> .

4.6.3 autoboxing/unboxing

The code in Figure 4.25 is annoying to write because using the wrapper
class requires creation of an Integer object prior to the call to add , and then
the extraction of the int value from the Integer , using the intValue method.
Prior to Java 1.4, this is required because if an int is passed in a place
where an Integer object is required, the compiler will generate an error
message, and if the result of an Integer object is assigned to an int , the
compiler will generate an error message. This resulting code in Figure 4.25
accurately reflects the distinction between primitive types and reference
types, yet, it does not cleanly express the programmer's intent of storing
int s in the collection.
Java 5 rectifies this situation. If an int is passed in a place where an Inte-
ger is required, the compiler will insert a call to the Integer constructor behind
the scenes. This is known as auto-boxing. And if an Integer is passed in a

figure 4.25
An illustration of the
Integer wrapper class
using Java 5 generic
ArrayList

1 import java.util.ArrayList;
2
3 public class BoxingDemo
4 {
5 public static void main( String [ ] args )
6 {
7 ArrayList<Integer> arr = new ArrayList<Integer>( );
8
9 arr.add( new Integer( 46 ) );
10 Integer wrapperVal = arr.get( 0 );
11 int val = wrapperVal.intValue( );
12 System.out.println( "Position 0: " + val );
13 }
14 }