如何使用Java接口比较器进行通用合并排序？

Question

I'm trying to merge sort lists but get ArrayOutOfBoundsError and don't manage to fix it.

This is for generic Lists using the Comparator interface. For now I'm running test with Integer.

This is my merge sort class:

public class ListMS<T> {
    private List<T> wholeList;
    private Comparator<T> comparator;

    public ListMS(List<T> list, Comparator<T> C){
        wholeList=new ArrayList<>();
        wholeList.addAll(list);
        comparator=C;
        mergeSort(wholeList, comparator);

    }

    public static <T> void merge(List<T> L1, List<T> L2,List<T> L, Comparator<T> C){
        int i=0;
        int j=0;
        while(i+j<L.size()){
            if(j==L2.size() || (i<L.size() && C.compare(L1.get(i),L2.get(j))<0)){
                L.set(i+j,L1.get(i++));
            }
            else{
                L.set(i+j,L2.get(j++));
            }
        }
    }

    public static <T> void mergeSort(List<T> L, Comparator<T> C){
        int size=L.size();
        if(size<2){
            return;
        }
        int half=size/2;
        List<T> L1=L.subList(0,half);
        List<T> L2=L.subList(half,size);

        mergeSort(L1,C);
        mergeSort(L1,C);

        merge(L1,L2,L,C);
    }

    public List<T> getWholeList(){
        return wholeList;
    }
}

What I'm testing it with:

public static void main(String[] args) {

        Comparator<Integer> C=new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o1.compareTo(o2);
            }
        };

        List<Integer> list1K=generateList(1000);
        ListMS<Integer> list1KMerged=new ListMS<>(list1K, C);

        printList(list1K);
        printList(list1KMerged.getWholeList());


    }


    public static List<Integer> generateList(int size){
        List<Integer> listNumber=new ArrayList<>();
        for(int i=0;i<size;i++){
            int min=0;
            int max=size-1;
            int num=(int)(Math.random() * ((max - min) + 1)) + min;
            listNumber.add(num);
        }
        return listNumber;
    }

    public static void printList(List<Integer> L){
        for(int i=0;i<L.size();i++){
            System.out.print(L.get(i));
        }
    }

But I get multiple ArrayOutOfBounds:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 1, Size: 1 at java.util.ArrayList$SubList.rangeCheck(ArrayList.java:1225) at java.util.ArrayList$SubList.get(ArrayList.java:1042) at ListMS.merge(ListMS.java:19) at ListMS.mergeSort(ListMS.java:40) at ListMS.mergeSort(ListMS.java:37) at ListMS.mergeSort(ListMS.java:37) at ListMS.mergeSort(ListMS.java:37) at ListMS.mergeSort(ListMS.java:37) at ListMS.mergeSort(ListMS.java:37) at ListMS.mergeSort(ListMS.java:37) at ListMS.mergeSort(ListMS.java:37) at ListMS.mergeSort(ListMS.java:37) at ListMS.(ListMS.java:11) at TestListMS.main(TestListMS.java:16)

Answer 1

Basically, there is an error in your loop counters in merge() methid at this line:

if(j == L2.size() || (i < L.size() && C.compare(L1.get(i), L2.get(j)) < 0))

Modify the merge function as so, it will work:

public static <T> void merge(List<T> L1, List<T> L2,List<T> L, Comparator<T> C){
        int i=0;
        int j=0;
        int k=0;
        while(i < L1.size() && j < L2.size()) {
            if(C.compare(L1.get(i), L2.get(j)) < 0) {
                L.set(k++, L1.get(i++));
            }else {
                L.set(k++, L2.get(j++));
            }   
        }
        while(i < L1.size()) {
            L.set(k++, L1.get(i++));
        }
        while(j < L2.size()) {
            L.set(k++, L2.get(j++));
        }
    }

UPDATE:

There are multiple errors in mergeSort function as well. Change it as so:

public static <T> void mergeSort(List<T> L, Comparator<T> C){
    int size=L.size();
    if(size<2){
        return;
    }
    int half=size/2;
    List<T> L1=new ArrayList<T>(L.subList(0,half));
    List<T> L2=new ArrayList<T>(L.subList(half,size));

    mergeSort(L1,C);
    mergeSort(L2,C);

    merge(L1,L2,L,C);
    printList(L);
}

L1 and L2 were not new array-list in old code. They were just two pointers pointing to same memory locations as in the List L. So modifying L in merge function also modified L1 and L2. In order to solve this, you need to create two new subarrays with separate memory allocations.

Also you were calling mergeSort(L1,C); twice instead of calling it on L1 and L2 each.