How is it posible to add a double into an ArrayList of Integer? (Java)
I try to understand how is it possible to have a Double value into an ArrayList of Integer. The numList is an ArrayList of Integer, and the value from it is a Double.
This is the code:
package bounded.wildcards;
import java.util.ArrayList;
import java.util.List;
public class GenericsDemo {
public static void main(String args) {
// Invariance Workaround
List<Integer> numList = new ArrayList<>();
GenericsDemo.invarianceWorkaround(numList);
System.out.println(numList);
}
static <T extends Number> void invarianceWorkaround(List<T> list) {
T element = (T) new Double(23.3);
list.add(element);
}
}
This will compile and run without an error.
java generics arraylist
edited 29 mins ago
Jiri Tousek
10.4k52239
asked 56 mins ago
gabygaby
46610
add a comment |
I try to understand how is it possible to have a Double value into an ArrayList of Integer. The numList is an ArrayList of Integer, and the value from it is a Double.
This is the code:
package bounded.wildcards;
import java.util.ArrayList;
import java.util.List;
public class GenericsDemo {
public static void main(String args) {
// Invariance Workaround
List<Integer> numList = new ArrayList<>();
GenericsDemo.invarianceWorkaround(numList);
System.out.println(numList);
}
static <T extends Number> void invarianceWorkaround(List<T> list) {
T element = (T) new Double(23.3);
list.add(element);
}
}
This will compile and run without an error.
java generics arraylist
edited 29 mins ago
Jiri Tousek
10.4k52239
asked 56 mins ago
gabygaby
46610
add a comment |
I try to understand how is it possible to have a Double value into an ArrayList of Integer. The numList is an ArrayList of Integer, and the value from it is a Double.
This is the code:
package bounded.wildcards;
import java.util.ArrayList;
import java.util.List;
public class GenericsDemo {
public static void main(String args) {
// Invariance Workaround
List<Integer> numList = new ArrayList<>();
GenericsDemo.invarianceWorkaround(numList);
System.out.println(numList);
}
static <T extends Number> void invarianceWorkaround(List<T> list) {
T element = (T) new Double(23.3);
list.add(element);
}
}
This will compile and run without an error.
java generics arraylist
edited 29 mins ago
Jiri Tousek
10.4k52239
asked 56 mins ago
gabygaby
46610
I try to understand how is it possible to have a Double value into an ArrayList of Integer. The numList is an ArrayList of Integer, and the value from it is a Double.
This is the code:
package bounded.wildcards;
import java.util.ArrayList;
import java.util.List;
public class GenericsDemo {
public static void main(String args) {
// Invariance Workaround
List<Integer> numList = new ArrayList<>();
GenericsDemo.invarianceWorkaround(numList);
System.out.println(numList);
}
static <T extends Number> void invarianceWorkaround(List<T> list) {
T element = (T) new Double(23.3);
list.add(element);
}
}
This will compile and run without an error.
java generics arraylist
java generics arraylist
edited 29 mins ago
Jiri Tousek
10.4k52239
asked 56 mins ago
gabygaby
46610
edited 29 mins ago
Jiri Tousek
10.4k52239
asked 56 mins ago
gabygaby
46610
edited 29 mins ago
Jiri Tousek
10.4k52239
edited 29 mins ago
Jiri Tousek
10.4k52239
edited 29 mins ago
Jiri Tousek
10.4k52239
10.4k52239
asked 56 mins ago
gabygaby
46610
asked 56 mins ago
gabygaby
46610
asked 56 mins ago
gabygaby
46610
46610
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
This is because of type erasure used with Java generics - the type checks are only performed at compile time for generic types, and the type info for generics is then erased, effectively turning List<Integer> into List<Object>.
My IDE warns you of an "Unchecked cast from Double to T". But the compiler couldn't be sure that your code is wrong, so it does not emit an error, just a warning.
Then at runtime, the type check is no longer present due to type erasure, so the code will run without error unless you perform some operation that fails due to incompatible runtime type. System.out.println() is not such operation.
If you change the print code to
Integer num = numList.get(0);
System.out.println(num);
this will now involve runtime type check and will therefore fail:
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer
answered 45 mins ago
Jiri TousekJiri Tousek
10.4k52239
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
43 mins ago
@MCEmperor Thanks, added. I couldn't find a simple example to force type incompatibility - integer seems to basically have no meaningful methods that wouldn't either be static or already inNumber.
– Jiri Tousek
34 mins ago
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
This is because of type erasure used with Java generics - the type checks are only performed at compile time for generic types, and the type info for generics is then erased, effectively turning List<Integer> into List<Object>.
My IDE warns you of an "Unchecked cast from Double to T". But the compiler couldn't be sure that your code is wrong, so it does not emit an error, just a warning.
Then at runtime, the type check is no longer present due to type erasure, so the code will run without error unless you perform some operation that fails due to incompatible runtime type. System.out.println() is not such operation.
If you change the print code to
Integer num = numList.get(0);
System.out.println(num);
this will now involve runtime type check and will therefore fail:
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer
answered 45 mins ago
Jiri TousekJiri Tousek
10.4k52239
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
43 mins ago
@MCEmperor Thanks, added. I couldn't find a simple example to force type incompatibility - integer seems to basically have no meaningful methods that wouldn't either be static or already inNumber.
– Jiri Tousek
34 mins ago
add a comment |
This is because of type erasure used with Java generics - the type checks are only performed at compile time for generic types, and the type info for generics is then erased, effectively turning List<Integer> into List<Object>.
My IDE warns you of an "Unchecked cast from Double to T". But the compiler couldn't be sure that your code is wrong, so it does not emit an error, just a warning.
Then at runtime, the type check is no longer present due to type erasure, so the code will run without error unless you perform some operation that fails due to incompatible runtime type. System.out.println() is not such operation.
If you change the print code to
Integer num = numList.get(0);
System.out.println(num);
this will now involve runtime type check and will therefore fail:
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer
answered 45 mins ago
Jiri TousekJiri Tousek
10.4k52239
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
43 mins ago
@MCEmperor Thanks, added. I couldn't find a simple example to force type incompatibility - integer seems to basically have no meaningful methods that wouldn't either be static or already inNumber.
– Jiri Tousek
34 mins ago
add a comment |
This is because of type erasure used with Java generics - the type checks are only performed at compile time for generic types, and the type info for generics is then erased, effectively turning List<Integer> into List<Object>.
My IDE warns you of an "Unchecked cast from Double to T". But the compiler couldn't be sure that your code is wrong, so it does not emit an error, just a warning.
Then at runtime, the type check is no longer present due to type erasure, so the code will run without error unless you perform some operation that fails due to incompatible runtime type. System.out.println() is not such operation.
If you change the print code to
Integer num = numList.get(0);
System.out.println(num);
this will now involve runtime type check and will therefore fail:
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer
answered 45 mins ago
Jiri TousekJiri Tousek
10.4k52239
This is because of type erasure used with Java generics - the type checks are only performed at compile time for generic types, and the type info for generics is then erased, effectively turning List<Integer> into List<Object>.
My IDE warns you of an "Unchecked cast from Double to T". But the compiler couldn't be sure that your code is wrong, so it does not emit an error, just a warning.
Then at runtime, the type check is no longer present due to type erasure, so the code will run without error unless you perform some operation that fails due to incompatible runtime type. System.out.println() is not such operation.
If you change the print code to
Integer num = numList.get(0);
System.out.println(num);
this will now involve runtime type check and will therefore fail:
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer
answered 45 mins ago
Jiri TousekJiri Tousek
10.4k52239
edited 36 mins ago
answered 45 mins ago
Jiri TousekJiri Tousek
10.4k52239
answered 45 mins ago
Jiri TousekJiri Tousek
10.4k52239
answered 45 mins ago
Jiri TousekJiri Tousek
10.4k52239
10.4k52239
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
43 mins ago
@MCEmperor Thanks, added. I couldn't find a simple example to force type incompatibility - integer seems to basically have no meaningful methods that wouldn't either be static or already inNumber.
– Jiri Tousek
34 mins ago
add a comment |
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
43 mins ago
@MCEmperor Thanks, added. I couldn't find a simple example to force type incompatibility - integer seems to basically have no meaningful methods that wouldn't either be static or already inNumber.
– Jiri Tousek
34 mins ago
3
3
Note that a
ClassCastException is emitted when one tries to do this: Integer i = numList.get(0).– MC Emperor
43 mins ago
Note that a
ClassCastException is emitted when one tries to do this: Integer i = numList.get(0).– MC Emperor
43 mins ago
@MCEmperor Thanks, added. I couldn't find a simple example to force type incompatibility - integer seems to basically have no meaningful methods that wouldn't either be static or already in
Number.– Jiri Tousek
34 mins ago
@MCEmperor Thanks, added. I couldn't find a simple example to force type incompatibility - integer seems to basically have no meaningful methods that wouldn't either be static or already in
Number.– Jiri Tousek
34 mins ago
add a comment |
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