Why does the following code sort the List of objects?

Question

Could you please explain why the following code compiles and prints [1, 2, 3, 4], as expected. I'm using Java 8.

List nums = Arrays.asList(4, 3, 2, 1);
Collections.sort(nums);
System.out.println(nums);

As I understand, four Integer instances is created here. Each list entry contains an Object reference to an Integer instance. Since the Object class doesn't implement the Comparable interface, then Collections.sort should throw ClassCastException or something like this because it cannot cast Object references to Comparable references.

Could you please point out what I'm missing?

Answer 1

With 1,2,3,4 you are creating int literals. While passing them to asList(T... a) they get boxed into Integer objects which implements Comparable (public final class Integer extends Number implements Comparable<Integer>), so you can sort them.

Update

Comment: Yes, but the List is declared as List, so it's the synonym to List<Object>, not List<Integer>, and Object doesn't implement Comparable.

Answer: You do not specify a generic type for the list and the Collections.sort() method only checks if the object's class extends Comparable. If the list has no type, your compiler should only give you a warning and everything should work fine since Integer's are comparable.

The source code of the sort method

public static <T extends Comparable<? super T>> void sort(List<T> list) {
    Object[] a = list.toArray();
    Arrays.sort(a);
    ListIterator<T> i = list.listIterator();
    for (int j=0; j<a.length; j++) {
        i.next();
        i.set((T)a[j]);
    }
}

Update

Execute this piece of code to see what happens if the class does not implements Comparable.

public class Test
{
    public static void main(String[] args)
    {
        List objs = new ArrayList<>();
        objs.add(new Test());
        objs.add(new Test());
        Collections.sort(objs);
    }
}

The cast to Comparable which is done in Line 290 of ComparableTimSort.class will fail!

Exception in thread "main" java.lang.ClassCastException: src.Test cannot be cast to java.lang.Comparable
at java.util.ComparableTimSort.countRunAndMakeAscending(Unknown Source)
at java.util.ComparableTimSort.sort(Unknown Source)
at java.util.ComparableTimSort.sort(Unknown Source)
at java.util.Arrays.sort(Unknown Source)
at java.util.Collections.sort(Unknown Source)
at src.Test.main(Test.java:14)