Varargs to ArrayList problem in Java

Question

I don't understand why the following does not work:

public void doSomething(int... args){
  List<Integer> broken = new ArrayList<Integer>(Arrays.asList(args))
}

Its my understanding that the compiler converts the "int... args" to an array, so the above code should work.

Instead of working I get:

cannot find symbol symbol: constructor ArrayList(java.util.List<int[]>) location: class java.util.ArrayList<java.lang.Integer>

Thats bizarre. I'm not adding an array to array list, I'm adding each element from the list to the arraylist. Whats going on?

Answer 1

Java cannot autobox an array, only individual values. I would suggest changing your method signature to

public void doSomething(Integer... args)

Then the autoboxing will take place when calling doSomething, rather than trying (and failing) when calling Arrays.asList.

What is happening is Java is now autoboxing each individual value as it is passed to your function. What you were trying to do before was, by passing an int[] to Arrays.asList(), you were asking that function to do the autoboxing.

But autoboxing is implemented by the compiler -- it sees that you needed an object but were passing a primitive, so it automatically inserted the necessary code to turn it into an appropriate object. The Arrays.asList() function has already been compiled and expects objects, and the compiler cannot turn an int[] into an Integer[].

By moving the autoboxing to the callers of your function, you've solved that problem.

Answer 2

You can do

public void doSomething(int... args){
    List<Integer> ints = new ArrayList<Integer>(args.length);
    for(int i: args) ints.add(i);
}

or

public void doSomething(Integer... args){
    List<Integer> ints = Arrays.asList(args);
}

Answer 3

In this case, autoboxing (automatic conversion from int to Integer) doesn't work. You have to add each int manually to the list.

If you need code like that often, consider using commons lang which has org.apache.commons.lang.ArrayUtils.toObject(int[])

Answer 4

You can solve this using Guava:

List<Integer> broken = new ArrayList<>(Ints.asList(args))

Or with streams:

List<Integer> broken = Arrays
    .stream(array)
    .boxed()
    .collect(Collectors.toCollection(ArrayList::new));