Java Reference
In-Depth Information
53
catch
(EmptyStackException emptyStackException)
54
{
55
System.out.println();
56
emptyStackException.printStackTrace();
57
}
58
}
59
}
// end class StackTest2
Pushing elements onto doubleStack
1.1 2.2 3.3 4.4 5.5
Popping elements from doubleStack
5.5 4.4 3.3 2.2 1.1
EmptyStackException: Stack is empty, cannot pop
at Stack.pop(Stack.java:32)
at StackTest2.testPop(StackTest2.java:50)
at StackTest2.main(StackTest2.java:17)
Pushing elements onto integerStack
1 2 3 4 5 6 7 8 9 10
Popping elements from integerStack
10 9 8 7 6 5 4 3 2 1
EmptyStackException: Stack is empty, cannot pop
at Stack.pop(Stack.java:32)
at StackTest2.testPop(StackTest2.java:50)
at StackTest2.main(StackTest2.java:21
Fig. 20.10
|
Passing generic
Stack
objects to generic methods. (Part 2 of 2.)
Lines 11-12 create the
Stack<Double>
and
Stack<Integer>
objects, respectively.
Lines 15-16 and 19-20 invoke generic methods
testPush
and
testPop
to test the
Stack
objects. Because type parameters can represent only reference types, to be able to pass arrays
doubleElements
and
integerElements
to generic method
testPush
, the arrays declared in
lines 7-8 must be declared with the wrapper types
Double
and
Integer
. When these arrays
are initialized with primitive values, the compiler
autoboxes
each primitive value.
Generic method
testPush
(lines 24-35) uses type parameter
T
(specified at line 24)
to represent the data type stored in the
Stack<T>
. The generic method takes three argu-
ments—a
String
that represents the name of the
Stack<T>
object for output purposes, a
reference to an object of type
Stack<T>
and an array of type
T
—the type of elements that
will be
push
ed onto
Stack<T>
. The compiler enforces
consistency
between the type of the
Stack
and the elements that will be pushed onto the
Stack
when
push
is invoked, which
is the real value of the generic method call. Generic method
testPop
(lines 38-58) takes
two arguments—a
String
that represents the name of the
Stack<T>
object for output pur-
poses and a reference to an object of type
Stack<T>
.
The test programs for generic class
Stack
in Section 20.6 instantiate
Stack
s with type ar-
guments
Double
and
Integer
. It's also possible to instantiate generic class
Stack
without
specifying a type argument, as follows:
Stack objectStack =
new
Stack(
5
);
// no type argument specified
