How can it be possible Creating instance of interface in Java? [duplicate]

I'm trying to understand the following code and a little bit confused. After some Googling, I found Anonymus Classes In OOP. But can't understand why in this code they declared Sorting object and calling an instance of MergeSort with sorting = new MergeSort(); ? Can someone explain to me?

interface Sorting {
   List sort(List list);
}
class MergeSort implements Sorting {
    public List sort(List list) {
// sort implementation
        return list;
    }
}
class QuickSort implements Sorting {
    public List sort(List list) {
// sort implementation
        return list;
    }
}
class DynamicDataSet {
    Sorting sorting;
    public DynamicDataSet() {
        sorting = new MergeSort();
    }
// DynamicDataSet implementation
}
class SnapshotDataSet {
    Sorting sorting;
    public SnapshotDataSet() {
        sorting = new QuickSort();
    }
// SnapshotDataSet implementation
}