Arrays.asList() of an array

Question

What is wrong with this conversion?

public int getTheNumber(int[] factors) {
    ArrayList f = new ArrayList(Arrays.asList(factors));  
    Co         


        

Answer 1

Arrays.asList(factors) returns a List<int[]>, not a List<Integer>. Since you're doing new ArrayList instead of new ArrayList<Integer> you don't get a compile error for that, but create an ArrayList<Object> which contains an int[] and you then implicitly cast that arraylist to ArrayList<Integer>. Of course the first time you try to use one of those "Integers" you get an exception.

Answer 2

Let's consider the following simplified example:

public class Example {
    public static void main(String[] args) {
        int[] factors = {1, 2, 3};
        ArrayList<Integer> f = new ArrayList(Arrays.asList(factors));
        System.out.println(f);
    }
}

At the println line this prints something like "[[I@190d11]" which means that you have actually constructed an ArrayList that contains int arrays.

Your IDE and compiler should warn about unchecked assignments in that code. You should always use new ArrayList<Integer>() or new ArrayList<>() instead of new ArrayList(). If you had used it, there would have been a compile error because of trying to pass List<int[]> to the constructor.

There is no autoboxing from int[] to Integer[], and anyways autoboxing is only syntactic sugar in the compiler, so in this case you need to do the array copy manually:

public static int getTheNumber(int[] factors) {
    List<Integer> f = new ArrayList<Integer>();
    for (int factor : factors) {
        f.add(factor); // after autoboxing the same as: f.add(Integer.valueOf(factor));
    }
    Collections.sort(f);
    return f.get(0) * f.get(f.size() - 1);
}

Answer 3

As far as I understand it, the sort function in the collection class can only be used to sort collections implementing the comparable interface.

You are supplying it a array of integers. You should probably wrap this around one of the know Wrapper classes such as Integer. Integer implements comparable.

Its been a long time since I have worked on some serious Java, however reading some matter on the sort function will help.

Answer 4

there are two cause of this exception:

1

Arrays.asList(factors) returns a List<int[]> where factors is an int array

2

you forgot to add the type parameter to:

ArrayList<Integer> f = new ArrayList(Arrays.asList(factors));

with:

ArrayList<Integer> f = new ArrayList<Integer>(Arrays.asList(factors));  

resulting in a compile-time error:

found   : java.util.List<int[]>
required: java.util.List<java.lang.Integer>

Answer 5

This works from Java 5 to 7:

public int getTheNumber(Integer... factors) {
    ArrayList<Integer> f = new ArrayList<Integer>(Arrays.asList(factors));
    Collections.sort(f);
    return f.get(0)*f.get(f.size()-1);
}

In Java 4 there is no vararg... :-)

Answer 6

You are trying to cast int[] to Integer[], this is not possible.

You can use commons-lang's ArrayUtils to convert the ints to Integers before getting the List from the array:

public int getTheNumber(int[] factors) {
    Integer[] integers = ArrayUtils.toObject(factors);
    ArrayList<Integer> f = new ArrayList<Integer>(Arrays.asList(integers));
    Collections.sort(f);
    return f.get(0)*f.get(f.size()-1);
}