Java Reference
In-Depth Information
public class
SomeThing {
private int
t1;
private static
public class
SomeThing {
private int
t1;
private static
int
t2;
int
t2;
public
SomeThing(
int
t1,
int
t2) {
...
public
SomeThing(
int
t1) {
...
}
}
}
public static void
setT2(
int
t2) {
SomeThing.t2 = t2;
}
}
(a)
(b)
Instance and static are integral parts of object-oriented programming. A data field or
method is either instance or static. Do not mistakenly overlook static data fields or methods.
It is a common design error to define an instance method that should have been static. For
example, the
factorial(int n)
method for computing the factorial of
n
should be
defined static, because it is independent of any specific instance.
A constructor is always instance, because it is used to create a specific instance. A static
variable or method can be invoked from an instance method, but an instance variable or
method cannot be invoked from a static method.
common design error
10.13
Describe class design guidelines.
✓
✓
Check
Point
A primitive type value is not an object, but it can be wrapped in an object using a
wrapper class in the Java API.
Key
Point
Owing to performance considerations, primitive data type values are not objects in Java.
Because of the overhead of processing objects, the language's performance would be
adversely affected if primitive data type values were treated as objects. However, many Java
methods require the use of objects as arguments. Java offers a convenient way to incorporate,
or wrap, a primitive data type into an object (e.g., wrapping
int
into the
Integer
class, and
wrapping
double
into the
Double
class). Recall that a
char
value can be wrapped into a
Character
object in Section 9.5. By using a wrapper class, you can process primitive data
type values as objects. Java provides
Boolean
,
Character
,
Double
,
Float
,
Byte
,
Short
,
Integer
, and
Long
wrapper classes in the
java.lang
package for primitive data types. The
Boolean
class wraps a Boolean value
true
or
false
. This section uses
Integer
and
Dou-
ble
as examples to introduce the numeric wrapper classes.
why wrapper class?
Note
Most wrapper class names for a primitive type are the same as the primitive data type
name with the first letter capitalized. The exceptions are
Integer
and
Character
.
naming convention
Numeric wrapper classes are very similar to each other. Each contains the methods
doubleValue()
,
floatValue()
,
intValue()
,
longValue()
,
shortValue()
, and
byteValue()
. These methods “convert” objects into primitive type values. The key features
of
Integer
and
Double
are shown in Figure 10.14.
You can construct a wrapper object either from a primitive data type value or from a string
representing the numeric value—for example,
new Double(5.0)
,
new Double("5.0")
,
new Integer(5)
, and
new Integer("5")
.
constructors
