How is it possible 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
asked 2 hours ago
gabygaby
48911
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
asked 2 hours ago
gabygaby
48911
Possible duplicate of type erasure in implementation of ArrayList in Java
– vaxquis
44 mins ago
also, stackoverflow.com/questions/339699/… and many others... I think that someone can find an exact duplicate, but they are are extremely closely related.
– vaxquis
44 mins ago
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
asked 2 hours ago
gabygaby
48911
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
asked 2 hours ago
gabygaby
48911
asked 2 hours ago
gabygaby
48911
edited 46 mins ago
gaby
asked 2 hours ago
gabygaby
48911
asked 2 hours ago
gabygaby
48911
asked 2 hours ago
gabygaby
48911
48911
Possible duplicate of type erasure in implementation of ArrayList in Java
– vaxquis
44 mins ago
also, stackoverflow.com/questions/339699/… and many others... I think that someone can find an exact duplicate, but they are are extremely closely related.
– vaxquis
44 mins ago
add a comment |
Possible duplicate of type erasure in implementation of ArrayList in Java
– vaxquis
44 mins ago
also, stackoverflow.com/questions/339699/… and many others... I think that someone can find an exact duplicate, but they are are extremely closely related.
– vaxquis
44 mins ago
Possible duplicate of type erasure in implementation of ArrayList in Java
– vaxquis
44 mins ago
Possible duplicate of type erasure in implementation of ArrayList in Java
– vaxquis
44 mins ago
also, stackoverflow.com/questions/339699/… and many others... I think that someone can find an exact duplicate, but they are are extremely closely related.
– vaxquis
44 mins ago
also, stackoverflow.com/questions/339699/… and many others... I think that someone can find an exact duplicate, but they are are extremely closely related.
– vaxquis
44 mins ago
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 2 hours ago
Jiri TousekJiri Tousek
10.5k52240
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
2 hours 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
1 hour ago
@JiriTousek You may be interested in Java is Unsound. Basically all java compilers will add cast checks in places to avoid issues because they know that the type system is broken.
– Giacomo Alzetta
1 hour ago
effectively turning intoList<Number>notList<Object>because of upper bound isNumber. if its List<Object> then you can store String but you can't
– Akash Shah
1 hour ago
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
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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 2 hours ago
Jiri TousekJiri Tousek
10.5k52240
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
2 hours 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
1 hour ago
@JiriTousek You may be interested in Java is Unsound. Basically all java compilers will add cast checks in places to avoid issues because they know that the type system is broken.
– Giacomo Alzetta
1 hour ago
effectively turning intoList<Number>notList<Object>because of upper bound isNumber. if its List<Object> then you can store String but you can't
– Akash Shah
1 hour 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 2 hours ago
Jiri TousekJiri Tousek
10.5k52240
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
2 hours 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
1 hour ago
@JiriTousek You may be interested in Java is Unsound. Basically all java compilers will add cast checks in places to avoid issues because they know that the type system is broken.
– Giacomo Alzetta
1 hour ago
effectively turning intoList<Number>notList<Object>because of upper bound isNumber. if its List<Object> then you can store String but you can't
– Akash Shah
1 hour 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 2 hours ago
Jiri TousekJiri Tousek
10.5k52240
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 2 hours ago
Jiri TousekJiri Tousek
10.5k52240
edited 1 hour ago
answered 2 hours ago
Jiri TousekJiri Tousek
10.5k52240
answered 2 hours ago
Jiri TousekJiri Tousek
10.5k52240
answered 2 hours ago
Jiri TousekJiri Tousek
10.5k52240
10.5k52240
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
2 hours 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
1 hour ago
@JiriTousek You may be interested in Java is Unsound. Basically all java compilers will add cast checks in places to avoid issues because they know that the type system is broken.
– Giacomo Alzetta
1 hour ago
effectively turning intoList<Number>notList<Object>because of upper bound isNumber. if its List<Object> then you can store String but you can't
– Akash Shah
1 hour ago
add a comment |
3
Note that aClassCastExceptionis emitted when one tries to do this:Integer i = numList.get(0).
– MC Emperor
2 hours 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
1 hour ago
@JiriTousek You may be interested in Java is Unsound. Basically all java compilers will add cast checks in places to avoid issues because they know that the type system is broken.
– Giacomo Alzetta
1 hour ago
effectively turning intoList<Number>notList<Object>because of upper bound isNumber. if its List<Object> then you can store String but you can't
– Akash Shah
1 hour ago
3
3
Note that a
ClassCastException is emitted when one tries to do this: Integer i = numList.get(0).– MC Emperor
2 hours ago
Note that a
ClassCastException is emitted when one tries to do this: Integer i = numList.get(0).– MC Emperor
2 hours 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
1 hour 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
1 hour ago
@JiriTousek You may be interested in Java is Unsound. Basically all java compilers will add cast checks in places to avoid issues because they know that the type system is broken.
– Giacomo Alzetta
1 hour ago
@JiriTousek You may be interested in Java is Unsound. Basically all java compilers will add cast checks in places to avoid issues because they know that the type system is broken.
– Giacomo Alzetta
1 hour ago
effectively turning into
List<Number> not List<Object> because of upper bound is Number. if its List<Object> then you can store String but you can't– Akash Shah
1 hour ago
effectively turning into
List<Number> not List<Object> because of upper bound is Number. if its List<Object> then you can store String but you can't– Akash Shah
1 hour ago
add a comment |
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Possible duplicate of type erasure in implementation of ArrayList in Java
– vaxquis
44 mins ago
also, stackoverflow.com/questions/339699/… and many others... I think that someone can find an exact duplicate, but they are are extremely closely related.
– vaxquis
44 mins ago