Autoboxing
Up through version 4 of Java, an assignment like
Integer b;
b= 25;
int i;
i= b;
were illegal, because the types of the variable and expression did not match —one is an int and the other is
class Integer.
In version 5 of Java, autoboxing was introduced, which makes such assignments legal. In the first
assignment, b= 25; the integer 25 is automatically wrapped in a new instance of class Integer; it is as if the
programmer had written
b= new Integer(25);
In the second assignment, i= b; since an int value is needed, the int value is automatically extracted from
Integer b; it is as if the programmer had written
i= b.intValue();
Autoboxing is a convenience for the programmer, allowing them to shorten their code.
Autoboxing is done with all primitive types and their corresponding wrapper classes. For example, here is
autoboxing of a character:
Character c= 'g';
Moreover, the autoboxing can be carried out in places other than assignments.
We give the rule for autoboxing for type int and leave to you to write the rule for other primitive types.
Autoboxing rule for int: If an expression e (say) of type int appears in a position in which an object
of class Integer is expected (or possible), the expression new Integer(e) is used in place of e.
Here is an example of the use of this rule. The parameter of function isFive,
/** = "obj is 5". */
public static boolean isFive(Integer obj) {
return obj.equals(new Integer(5));
}
is of type Integer. Yet, in a call of it, we can put a value of type int.
AutoboxDemo.isFive(5)
There is an unboxing rule also —a rule for extracting a value from a wrapper class object. Here is the rule for
class Integer; you can generalize it to the other wrapper classes yourself.
Autounboxing rule for Integer: If an expression e (say) of type Integer appears in a position in
which an expression of type int is expected, then the expression e.intValue() is used in place of e.
Here is an example of the use of this rule. The parameter of function isFour,
/** = "c is 4". */
public static boolean isFour(int c) {
return c == 4;
}
We can call isFour using the call AutoboxDemo.isFour(new Integer(4)), and the int value is
extracted from the argument expression.
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