为什么我的递归Java方法中的字段会更改？

Question

I have been learning Java for about a month now and have learned a lot by reading others' questions (and answers) on this site. I don't think that this has been asked before (but if it has, I would appreciate a pointer ...)

In the code below, pretty much everything which is not indented are error-checking printouts, so the code is quite a bit shorter than it seems. The code is my attempt at using recursion to list all the ways that numBalls balls can be placed in numBins bins.

Main question: The method works fine for numBins < 3. As soon as numBins is set to 3, the endList field (in the recursive call) has more than 1 "row", and as soon as j hits 1 in the loop directly below, the indexList field gets changed. For example, calling testList = distributeBallsInBins(1,3,""); in the main method (as it is configured), leads to a change in the second row of indexList from {0 1 0} to {0 0 1} (as shown in the output) , but I cannot see how/why it has changed when all I have done is moved to the next j (ie from j=0 to j=1)

Secondary question: I have replaced all the Integer[] occurrences with int[] and there seems to be no difference. Should there be? I guess I need to read more on the difference between primitive and reference types, but I don't really understand the difference here.

Thank you in advance, Mike

import java.util.*;

public class testRecursion
{
     public static List<Integer[]> distributeBallsInBins(int numBalls, int numBins, String tmpTxt)
    {
        if (numBins==1)
        {
            List<Integer[]> lastList = new ArrayList<Integer[]>();
            lastList.add((new Integer[] {numBalls}));
            return lastList;
        }
        else if (numBalls==0)
        {
            List<Integer[]> lastList = new ArrayList<Integer[]>();
            Integer[] tmpNum = new Integer[numBins];
            for (int k=0; k<numBins; k++)
                tmpNum[k] = 0;
            lastList.add(tmpNum);
            return lastList;
        }
        else
        {
            List<Integer[]> indexList = new ArrayList<Integer[]>();
            for (int i=numBalls; i>=0; i--)
            {
                Integer[] newLine = new Integer[numBins];
                newLine[0] = i;
                List<Integer[]> endList = distributeBallsInBins((numBalls-i), (numBins-1), (tmpTxt + "    "));
                for (int j=0; j<endList.size(); j++)
                {
                    Integer[] newLineEnd = endList.get(j);
                    for (int k=0; k<numBins-1; k++)
                        newLine[k+1] = newLineEnd[k]; 
                    indexList.add(newLine);
                }
            }
        return indexList;
        }
    }

    public static void main(String[] args)
    {
        List<Integer[]> testList = distributeBallsInBins(1,3,"");
    }
}

Answer 1

Your problem is, that you are always modifying and inserting the same array into your result list. Since Java handles all objects per reference, you'll end up with a list containing the same array over and over again.

So you need to either clone the array before adding it to the list:

indexList.add(newLine.clone());

Or create the newLine array new every time you iterate your j -loop:

for (int j = 0; j < endList.size(); j++) {
    Integer[] newLine = new Integer[numBins];
    newLine[0] = i;
    Integer[] newLineEnd = endList.get(j);
    for (int k = 0; k < numBins - 1; k++)
        newLine[k + 1] = newLineEnd[k];
    indexList.add(newLine);
} // next j

Regarding Objects versus primitives: Changing to int[] did not help because the array itself is still an Object and thus passed by reference.

Answer 2

just replace this line

indexList.add(newLine);

with Line

indexList.add(newLine.clone());

this will work...... this will pass new array rather than pass the reference of old array.......