Java：将项目添加到ArrayList时遇到麻烦 <ArrayList<Integer> &gt;

Question

I'm trying to write a program in Java that will calculate all combinations of elements in an integer array (with 5 elements) and output these combinations to an ArrayList. I've included my code below.

I use bitwise operations to find the combinations. Each combination is constructed as an ArrayList(Integer), called "writeitem". I then want to store these in another ArrayList, called "master", which has to have the form ArrayList(ArrayList(Integer)). [for formatting reasons <> have to be replaced with (); they don't show up otherwise...]

The problem arises when attempting to save each combination to the "master" ArrayList. If you run the code below, the printf function will show that the combination is constructed correctly. However, once I ask for it to be "added" to "master", it doesn't seem to be appended to the end of "master". Rather, all of "master" is overwritten with i copies of the combination just constructed.

So, for example, if I call the function on [1,2,3,4,5], my "master" array ends up being 31 copies of [1,2,3,4,5] (the 31st combination to be found).

I imagine this has something to do with using nested array lists, and there's a better way to achieve what I want. But it's equally possible I am committing some other novice error.

static ArrayList<ArrayList<Integer>> master = new ArrayList<ArrayList<Integer>>();
public static void generatecombs(int[] x){

    ArrayList<Integer> writeitem = new ArrayList<Integer>(); //empty list to construct each comb

    for(int i=1;i<32;i++){

        writeitem.clear(); //clear before constructing next combination

        if((i & 1)>0){          //check if each element is present in combination
            writeitem.add(x[0]);
        }
        if((i & 2)>0){
            writeitem.add(x[1]);
        }
        if((i & 4)>0){
            writeitem.add(x[2]);
        }
        if((i & 8)>0){
            writeitem.add(x[3]);
        }
        if((i & 16)>0){
            writeitem.add(x[4]);
        }

        System.out.printf("The %dth combination is %s\n", i,writeitem);
        master.add(writeitem); //output constructed element
        System.out.printf("The collection so far is: %s\n", master);
    }
}

Answer 1

Move the new inside the loop

static ArrayList<ArrayList<Integer>> master = new ArrayList<ArrayList<Integer>>();

public static void generatecombs(int[] x){

    for(int i=1;i<32;i++){

        ArrayList<Integer> writeitem = new ArrayList<Integer>(); // new list to construct each comb
        if((i & 1)>0){          //check if each element is present in combination
            writeitem.add(x[0]);
        }
        if((i & 2)>0){
            writeitem.add(x[1]);
        }
        if((i & 4)>0){
            writeitem.add(x[2]);
        }
        if((i & 8)>0){
            writeitem.add(x[3]);
        }
        if((i & 16)>0){
            writeitem.add(x[4]);
        }

        System.out.printf("The %dth combination is %s\n", i,writeitem);
        master.add(writeitem); //output constructed element
        System.out.printf("The collection so far is: %s\n", master);
    }
}

Answer 2

每次迭代后，clear（）从arraylist中删除值，然后在其中进行arraylist创建。

Answer 3

Move the construction of writeitem inside the for loop. You don't want to re-use the same array.

Answer 4

另一种解决方案是在清除writeItem之前将其添加到父列表时进行克隆。

master.add(writeitem.clone()); 

Answer 5

The reason you're getting 31 copies is because you're running through the for loop, wiping the writeitem array clean each time, adding to it, and printing it out while still in the for loop, which then repeats 30 more times, until i hits 32.

Remove writeitem.clear(); and see how you get on with that