Java Reference
In-Depth Information
Pushing elements onto rawTypeStack1
1.1 2.2 3.3 4.4 5.5
Popping elements from rawTypeStack1
5.5 4.4 3.3 2.2 1.1
EmptyStackException: Stack is empty, cannot pop
at Stack.pop(Stack.java:32)
at RawTypeTest.testPop(RawTypeTest.java:53)
at RawTypeTest.main(RawTypeTest.java:20)
Pushing elements onto rawTypeStack2
1.1 2.2 3.3 4.4 5.5
Popping elements from rawTypeStack2
5.5 4.4 3.3 2.2 1.1
EmptyStackException: Stack is empty, cannot pop
at Stack.pop(Stack.java:32)
at RawTypeTest.testPop(RawTypeTest.java:53)
at RawTypeTest.main(RawTypeTest.java:22)
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 RawTypeTest.testPop(RawTypeTest.java:53)
at RawTypeTest.main(RawTypeTest.java:24)
Fig. 20.11
|
Raw-type test program. (Part 3 of 3.)
Compiler Warnings
Figure 20.12 shows the warning messages generated by the compiler when the file
Raw-
TypeTest.java
(Fig. 20.11) is compiled with the
-Xlint:unchecked
option, which pro-
vides more information about potentially unsafe operations in code that uses generics. The
first warning in Fig. 20.11 is generated for line 17, which assigned a raw-type
Stack
to a
Stack<Integer>
variable—the compiler cannot ensure that all objects in the
Stack
will be
Integer
objects. The next warning occurs at line 19. The compiler determines method
testPush
's type argument from the
Double
array passed as the third argument, because
the second method argument is a raw-type
Stack
variable. In this case,
Double
is the type
argument, so the compiler expects a
Stack<Double>
as the second argument. The warning
occurs because the compiler cannot ensure that a raw-type
Stack
contains only
Double
s.
The warning at line 21 occurs for the same reason, even though the actual
Stack
that
rawTypeStack2
references is a
Stack<Double>
. The compiler cannot guarantee that the
variable will always refer to the same
Stack
object, so it must use the variable's declared
type to perform all type checking. Lines 20 and 22 each generate warnings because method
testPop
expects as an argument a
Stack
for which a type argument has been specified.
However, in each call to
testPop
, we pass a raw-type
Stack
variable. Thus, the compiler
indicates a warning because it cannot check the types used in the body of the method. In
general, you should avoid using raw types.
