Java Reference
In-Depth Information
15.4
Why do the following two lines of code compile but cause a runtime error?
✓
✓
Check
Point
Number numberRef =
new
Integer(
0
);
Double doubleRef = (Double)numberRef;
15.5
Why do the following two lines of code compile but cause a runtime error?
Number[] numberArray =
new
Integer[
2
];
numberArray[
0
] =
new
Double(
1.5
);
15.6
Show the output of the following code.
public class
Test {
public static void
main(String[] args) {
Number x =
3
;
System.out.println(x.intValue());
System.out.println(x.doubleValue());
}
}
15.7
What is wrong in the following code? (Note that the
compareTo
method for the
Integer
and
Double
classes was introduced in Section 10.12.)
public class
Test {
public static void
main(String[] args) {
Number x =
new
Integer(
3
);
System.out.println(x.intValue());
System.out.println(x.compareTo(
new
Integer(
4
)));
}
}
15.8
What is wrong in the following code?
public class
Test {
public static void
main(String[] args) {
Number x =
new
Integer(
3
);
System.out.println(x.intValue());
System.out.println((Integer)x.compareTo(
new
Integer(
4
)));
}
}
GregorianCalendar
is a concrete subclass of the abstract class
Calendar
.
Key
Point
An instance of
java.util.Date
represents a specific instant in time with millisecond
precision.
java.util.Calendar
is an abstract base class for extracting detailed calendar
information, such as the year, month, date, hour, minute, and second. Subclasses of
Calendar
can implement specific calendar systems, such as the Gregorian calendar, the lunar calendar,
and the Jewish calendar. Currently,
java.util.GregorianCalendar
for the Gregorian
calendar is supported in Java, as shown in Figure 15.3. The
add
method is abstract in the
Calendar
class, because its implementation is dependent on a concrete calendar system.
You can use
new GregorianCalendar()
to construct a default
GregorianCalendar
with the current time and
new GregorianCalendar(year, month, date)
to construct
a
GregorianCalendar
with the specified
year
,
month
, and
date
. The
month
parameter
is
0
based—that is,
0
is for January.
VideoNote
Calendar
and
GregorianCalendar
classes
abstract
add
method
constructing calendar
