Wrapper classes

Java Primer WrapperClasses-1 © Scott MacKenzie
Wrapper Classes
Each of Java's eight primitive data types has a class dedicated to it. These are known as wrapper
classes, because they "wrap" the primitive data type into an object of that class. So, there is an
Integer class that holds an int variable, there is a Double class that holds a double
variable, and so on. The wrapper classes are part of the java.lang package, which is imported
by default into all Java programs. The following discussion focuses on the Integer wrapper
class, but applies in a general sense to all eight wrapper classes. Consult the Java API
documentation for more details.
The following two statements illustrate the difference between a primitive data type and an object
of a wrapper class:
int x = 25;
Integer y = new Integer(33);
The first statement declares an int variable named x and initializes it with the value 25. The
second statement instantiates an Integer object. The object is initialized with the value 33 and
a reference to the object is assigned to the object variable y. The memory assignments from
these two statements are visualized in Figure 1.
(a) (b)
25
x y
33
Figure 1. Variables vs. objects (a) declaration and initialization
of an int variable (b) instantiation of an Integer object
Clearly x and y differ by more than their values: x is a variable that holds a value; y is an object
variable that holds a reference to an object. As noted earlier, data fields in objects are not, in
general, directly accessible. So, the following statement using x and y as declared above is not
allowed:
int z = x + y; // wrong!
The data field in an Integer object is only accessible using the methods of the Integer class.
One such method — the intValue() method — returns an int equal to the value of the
object, effectively "unwrapping" the Integer object:
int z = x + y.intValue(); // OK!
Note the format of the method call. The intValue() method is an instance method, because it
is called through an instance of the class — an Integer object. The syntax uses dot notation,
where the name of the object variable precedes the dot.
Some of Java's classes include only instance methods, some include only class methods, some
include both. The Integer class includes both instance methods and class methods. The class
methods provide useful services; however, they are not called through instances of the class.
But, this is not new. We met a class method of the Integer class earlier. The statement
int x = Integer.parseInt("1234");
converts the string "1234" into the integer 1,234 and assigns the result to the int variable x.
The method parseInt() is a class method. It is not called through an instance of the class.

Although dot notation is used above, the term "Integer." identifies the class where the method
is defined, rather than name of an Integer object. This is one of the trickier aspects of
distinguishing instance methods from class methods. For an instance method, the dot is preceded
by the name of an object variable. For a class method, the dot is preceded by the name of a class.
It is interesting that the parseInt() method accepts a String argument and returns an int.
Neither the argument nor the returned value is an Integer object. So, the earlier comment that
class methods "provide useful services" is quite true. They do not operate on instance variables;
they provide services that are of general use. The parseInt() method converts a String to
an int, and this is a useful service — one we have called upon in previous programs.
Another useful Integer class method is the toString() method. The following statement
String s = Integer.toString(1234);
converts the int constant 1,234 to a String object and returns a reference to the object. We
have already met this method through the following shorthand notation:
String s = "" + 1234;
In general, the data fields in objects are private and are only accessible through methods of the
class. However, class definitions sometimes include constants that are "public", and, therefore,
accessible anywhere the class is accessible. Java's wrapper classes include publicly defined
constants identifying the range for each type. For example, the Integer class has constants
called MIN_VALUE and MAX_VALUE equal to 0x80000000 and 0x7fffffff respectively. These
values are the hexadecimal equivalent of the most-negative and most-positive values represented
by 32-bit integers. The table of ranges shown earlier was obtained from a simple program called
FindRanges that prints these constants. The listing is given here in Figure 2.
1 public class FindRanges
2 {
3 public static void main(String[] args)
4 {
5 System.out.println("Integer range:");
6 System.out.println(" min: " + Integer.MIN_VALUE);
7 System.out.println(" max: " + Integer.MAX_VALUE);
8 System.out.println("Double range:");
9 System.out.println(" min: " + Double.MIN_VALUE);
10 System.out.println(" max: " + Double.MAX_VALUE);
11 System.out.println("Long range:");
12 System.out.println(" min: " + Long.MIN_VALUE);
13 System.out.println(" max: " + Long.MAX_VALUE);
14 System.out.println("Short range:");
15 System.out.println(" min: " + Short.MIN_VALUE);
16 System.out.println(" max: " + Short.MAX_VALUE);
17 System.out.println("Byte range:");
18 System.out.println(" min: " + Byte.MIN_VALUE);
19 System.out.println(" max: " + Byte.MAX_VALUE);
20 System.out.println("Float range:");
21 System.out.println(" min: " + Float.MIN_VALUE);
22 System.out.println(" max: " + Float.MAX_VALUE);
23 }
24 }
Figure 2. FindRanges.java
This program generates the following output:

Integer range:
min: -2147483648
max: 2147483647
Double range:
min: 4.9E-324
max: 1.7976931348623157E308
Long range:
min: -9223372036854775808
max: 9223372036854775807
Short range:
min: -32768
max: 32767
Byte range:
min: -128
max: 127
Float range:
min: 1.4E-45
max: 3.4028235E38
In the case of float and double, the minimum and maximum values are those closest to plus
or minus zero or plus or minus infinity, respectively.
The most common methods of the Integer wrapper class are summarized in Table 1. Similar
methods for the other wrapper classes are found in the Java API documentation.
Table 1. Methods of the Integer Wrapper Class
Method Purpose
Constructors
Integer(i) constructs an Integer object equivalent to the integer i
Integer(s) constructs an Integer object equivalent to the string s
Class Methods
parseInt(s) returns a signed decimal integer value equivalent to string s
toString(i) returns a new String object representing the integer i
Instance Methods
byteValue() returns the value of this Integer as a byte
doubleValue() returns the value of this Integer as an double
floatValue() returns the value of this Integer as a float
intValue() returns the value of this Integer as an int
longValue() returns the value of this Integer as a long
shortValue() returns the value of this Integer as a short
toString() returns a String object representing the value of this Integer
In the descriptions of the instance methods, the phrase "this Integer" refers to the instance
variable on which the method is acting. For example, if y is an Integer object variable, the
expression y.doubleValue() returns the value of "this Integer", or y, as a double.

Wrapper classes

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