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9

1

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

share|improve this question

edited 20 mins ago

Jiri Tousek

10.4k52239

asked 47 mins ago

gabygaby

46610

add a comment | 

9

1

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

share|improve this question

edited 20 mins ago

Jiri Tousek

10.4k52239

asked 47 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

share|improve this question

edited 20 mins ago

Jiri Tousek

10.4k52239

asked 47 mins ago

gabygaby

46610

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);

    }



}

share|improve this question

edited 20 mins ago

Jiri Tousek

10.4k52239

asked 47 mins ago

gabygaby

46610

share|improve this question

edited 20 mins ago

Jiri Tousek

10.4k52239

asked 47 mins ago

gabygaby

46610

edited 20 mins ago

Jiri Tousek

10.4k52239

edited 20 mins ago

Jiri Tousek

10.4k52239

asked 47 mins ago

gabygaby

46610

asked 47 mins ago

gabygaby

46610

1 Answer
1

active

oldest

votes

9

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

share|improve this answer

edited 27 mins ago

answered 36 mins ago

Jiri TousekJiri Tousek

10.4k52239

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Note that a ClassCastException is emitted when one tries to do this: Integer i = numList.get(0).
  
  – MC Emperor
  35 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
  25 mins ago
  

  
  
  
  

add a comment | 

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9

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

share|improve this answer

edited 27 mins ago

answered 36 mins ago

Jiri TousekJiri Tousek

10.4k52239

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Note that a ClassCastException is emitted when one tries to do this: Integer i = numList.get(0).
  
  – MC Emperor
  35 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
  25 mins ago
  

  
  
  
  

add a comment | 

9

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

share|improve this answer

edited 27 mins ago

answered 36 mins ago

Jiri TousekJiri Tousek

10.4k52239

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Note that a ClassCastException is emitted when one tries to do this: Integer i = numList.get(0).
  
  – MC Emperor
  35 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
  25 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

share|improve this answer

edited 27 mins ago

answered 36 mins ago

Jiri TousekJiri Tousek

10.4k52239

Integer num = numList.get(0);

System.out.println(num);

share|improve this answer

edited 27 mins ago

answered 36 mins ago

Jiri TousekJiri Tousek

10.4k52239

answered 36 mins ago

Jiri TousekJiri Tousek

10.4k52239

answered 36 mins ago

Jiri TousekJiri Tousek

10.4k52239