 |
AppletTalk.com
Java discussions newsgroups
|
| View previous topic :: View next topic |
| Author |
Message |
Guest
|
 Posted: Mon Feb 27, 2006 2:12 pm Post subject: Problem with Widcards |
 |
|
I cannot understand the error.
Test.java:24: test(java.util.Map<java.lang.Long,java.util.List<?>>) in
Test cannot be applied to
(java.util.Map<java.lang.Long,java.util.List<java.lang.Double>>)
test(map);
^
1 error
What am I doing wrong?
As an example here is a code:
import java.util.*;
public class Test {
static void test(Map<Long, List<?>> map) {
for(Long id : map.keySet()) {
System.out.println(id);
for(Object obj : map.get(id)) {
System.out.println(obj);
}
}
}
public static void main(String[] args) {
Map<Long, List<Double>> map = new TreeMap<Long, List<Double>>();
for(long i = 1; i < 5; i++) {
List<Double> array = new ArrayList<Double>();
for(int j=3; j < 6; j++) {
array.add((double)i*j);
}
map.put(i, array);
}
test(map);
}
} |
|
| Back to top |
|
 |
IchBin
Guest
|
| Posted: Mon Feb 27, 2006 4:12 pm Post subject: Re: Problem with Widcards |
|
|
astyskin (AT) gmail (DOT) com wrote:
| Quote: | I cannot understand the error.
Test.java:24: test(java.util.Map<java.lang.Long,java.util.List<?>>) in
Test cannot be applied to
(java.util.Map<java.lang.Long,java.util.List<java.lang.Double>>)
test(map);
^
1 error
What am I doing wrong?
As an example here is a code:
import java.util.*;
public class Test {
static void test(Map<Long, List<?>> map) {
|
static void test(Map<Long, List<Double>> map) {
| Quote: |
for(Long id : map.keySet()) {
System.out.println(id);
for(Object obj : map.get(id)) {
System.out.println(obj);
}
}
}
public static void main(String[] args) {
Map<Long, List<Double>> map = new TreeMap<Long, List<Double>>();
for(long i = 1; i < 5; i++) {
List<Double> array = new ArrayList<Double>();
for(int j=3; j < 6; j++) {
array.add((double)i*j);
|
array.add( new Double( i * j));
| Quote: | }
map.put(i, array);
}
test(map);
}
}
|
--
Thanks in Advance...
IchBin, Pocono Lake, Pa, USA
http://weconsultants.servebeer.com/JHackerAppManager
__________________________________________________________________________
'If there is one, Knowledge is the "Fountain of Youth"'
-William E. Taylor, Regular Guy (1952-) |
|
| Back to top |
|
|
Hendrik Maryns
Guest
|
| Posted: Thu Mar 02, 2006 9:12 pm Post subject: Re: Problem with Widcards |
|
|
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
NotDashEscaped: You need GnuPG to verify this message
Hendrik Maryns uitte de volgende tekst op 03/02/2006 09:23 PM:
| Quote: | -----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
NotDashEscaped: You need GnuPG to verify this message
astyskin (AT) gmail (DOT) com uitte de volgende tekst op 02/27/2006 03:09 PM:
I cannot understand the error.
Test.java:24: test(java.util.Map<java.lang.Long,java.util.List<?>>) in
Test cannot be applied to
(java.util.Map<java.lang.Long,java.util.List<java.lang.Double>>)
test(map);
^
1 error
What am I doing wrong?
As an example here is a code:
import java.util.*;
public class Test {
static void test(Map<Long, List<?>> map) {
for(Long id : map.keySet()) {
System.out.println(id);
for(Object obj : map.get(id)) {
System.out.println(obj);
}
}
}
public static void main(String[] args) {
Map<Long, List<Double>> map = new TreeMap<Long, List<Double>>();
for(long i = 1; i < 5; i++) {
List<Double> array = new ArrayList<Double>();
for(int j=3; j < 6; j++) {
array.add((double)i*j);
}
map.put(i, array);
}
test(map);
}
}
Working with multilevel generics is very tricky. You want test() to
accept any Map which has Longs as keys and any subtype of List as
values. What you have above only accepts Maps that have Longs as keys
and Lists of *the specific type List<?>* as keys.
|
That?s wrong. The problem is, that Map<Long, List<?>> can map Longs on
any type of list. I.e., 3 could map to a List<String>, whereas 5 maps
to List<Number> and so on. Thus, the moment you give Java a Map<Long,
List<Double>>, it says: cannot assign, because imagine that this would
work, then the following code would be possible, and of course we don?t
want that:
Map<Long, List<?>> badMap = new HashMap<Long, List<Double>>; //will not
compile
map.put(5, new ArrayList<String>());
Because, if you look at the type of Map, it can accept any map as value.
Because the assignment of actual arguments to formal arguments in a
method invocation is the same as a normal assignment, this is rejected.
The rest of my answer is correct.
--
Hendrik Maryns
==================
www.lieverleven.be
http://aouw.org
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.2 (GNU/Linux)
iD8DBQFEB1YXiOEY3xKMFEERAnd6AKDEPyD4zIVmFknJ01sZfB/zi57wCgCfUOUl
4H7ADyuv75+AFAalDDcPPB8=
=6C/u
-----END PGP SIGNATURE----- |
|
| Back to top |
|
|
Hendrik Maryns
Guest
|
| Posted: Thu Mar 02, 2006 9:12 pm Post subject: Re: Problem with Widcards |
|
|
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
NotDashEscaped: You need GnuPG to verify this message
astyskin (AT) gmail (DOT) com uitte de volgende tekst op 02/27/2006 03:09 PM:
| Quote: | I cannot understand the error.
Test.java:24: test(java.util.Map<java.lang.Long,java.util.List<?>>) in
Test cannot be applied to
(java.util.Map<java.lang.Long,java.util.List<java.lang.Double>>)
test(map);
^
1 error
What am I doing wrong?
As an example here is a code:
import java.util.*;
public class Test {
static void test(Map<Long, List<?>> map) {
for(Long id : map.keySet()) {
System.out.println(id);
for(Object obj : map.get(id)) {
System.out.println(obj);
}
}
}
public static void main(String[] args) {
Map<Long, List<Double>> map = new TreeMap<Long, List<Double>>();
for(long i = 1; i < 5; i++) {
List<Double> array = new ArrayList<Double>();
for(int j=3; j < 6; j++) {
array.add((double)i*j);
}
map.put(i, array);
}
test(map);
}
}
|
Working with multilevel generics is very tricky. You want test() to
accept any Map which has Longs as keys and any subtype of List as
values. What you have above only accepts Maps that have Longs as keys
and Lists of *the specific type List<?>* as keys. What you want is
test(Map<Long, ? extends List<?>>).
Have a look at Angelika Langer?s excellent FAQ about Java generics,
specifically the question ?How do I implement a method that takes a
multi-level wildcard argument??:
http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#How%20do%20I%20implement%20a%20method%20that%20takes%20a%20multi-level%20wildcard%20argument?,
also under http://tinyurl.com/o4a82.
Hope this helps, H.
--
Hendrik Maryns
==================
www.lieverleven.be
http://aouw.org
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.2 (GNU/Linux)
iD8DBQFEB1QxiOEY3xKMFEERAs/JAJ41E7IvQlvG/QJiUHaGU/ZF+wzhQACeLZj3
AiboznhvN7rNzRzZ7RZ08ck=
=xKN0
-----END PGP SIGNATURE----- |
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
|
|
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
