Java autoboxing and unboxing

Question

I am trying to convert and array ints to a List however every time I try I get a compilation error.

   List<Integer> integers = toList(draw.getToto6From49().getFirstRound());

// getFirstRound() returns int[] ;

and here is my to toList method

public class ArraysUtil {


 public static <T> List<T> toList(T[] ts) {
  List<T> ts1 = new ArrayList<>();
  Collections.addAll(ts1, ts);
  return ts1;
 }
}

java: method toList in class com.totoplay.util.ArraysUtil cannot be applied to given types;

  required: T[]
  found: int[]
  reason: inferred type does not conform to declared bound(s)
    inferred: int
    bound(s): java.lang.Object

Some scientists believe it's helpful to specify your errors in your question. — Jeroen Vannevel, Aug 10 '14 at 19:53
`int` is not of type `T`, because `T` is a subclass of `Object` and `int` is not a subclass of `Object`. — christopher, Aug 10 '14 at 19:56
@Adio what is this returning: draw.getToto6From49().getFirstRound() ?? int[] ? if so, look at christopher comment above. — LeTex, Aug 10 '14 at 20:00

Jon Skeet · Accepted Answer · 2014-08-10T20:16:56.713

5

Yes, you can't do that, basically. You're expecting to be able to call toList with a type argument of int, and Java doesn't allow primitives to be used as type arguments. You basically need a different method for each primitive you want to handle like this, e.g.

public static List<Integer> toList(int[] ts) {
  List<Integer> list = new ArrayList<>(ts.length);
  for (int i in ts) {
    list.add(i);
  }
}

At least, that's the case in Java 7 and before. In Java 8 it's slightly simpler:

int[] array = { 1, 2, 3 };
IntStream stream = Arrays.stream(array);
List<Integer> list = stream.boxed().collect(Collectors.toList());

edited Aug 10 '14 at 20:16

answered Aug 10 '14 at 19:57

Jon Skeet

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I would recommend `new ArrayList<>(ts.length);` to avoid duplicating the size of the internal array – Balázs Édes Aug 10 '14 at 20:01

score 0 · Answer 2 · edited May 23 '17 at 12:28

0

It's because primitive types don't play nicely with generics. All generics need to be treatable as objects, and primitives can't be. More Detail

edited May 23 '17 at 12:28

Community

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answered Aug 10 '14 at 19:58

Mshnik

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score 0 · Answer 3 · edited Jun 20 '20 at 09:12

In Java 8 you can do this:

List<Integer> list = new ArrayList<>();
list.add(101); // autoboxing will work
int aValue = list.get(0); // autounboxing will work, too

Specifying the Integer type in angle brackets (< Integer >), while creating the List object, tells the compiler that the List will hold object of only Integer type. This gives the compiler freedom to wrap and unwrap your primitive int values while you work with the List object

from the book: Beginning Java 8 Fundamentals

Java autoboxing and unboxing

3 Answers3