在java中复制多维数组

Question

In Java, how do we make a copy of a 3-D array? Thing is, when we use new_array.clone() or something of that sort, we are putting the address of the entry into the another_array, and not the actual value. Hence when i clear() old_array , new_array is empty too

private List<List<List<String>>> moves = new ArrayList<List<List<String>>>();
private List<List<List<String>>> moves1 = new ArrayList<List<List<String>>>();

.blah
.blah 
.blah

mid_array = new ArrayList<List<String>>();//will use this array to insert inot moves1

for(int f = 0; f < moves.size(); f++)//make a copy of original array.
{

    List<String> row_st = moves.get(f).get(0);
    List<String> deck_st = moves.get(f).get(1);

    mid_array.add(row_st);//current array of the Rows
    mid_array.add(deck_st);//current array of the Deck

    moves1.add(mid_array);

    System.out.println("Moves1 "+moves1);//displays the new array correctly

    mid_array.clear();//clear mid_array, NOT moves1 or moves arrays

    System.out.println("Moves1 "+moves1);//new array is now empty

}

Answer 1

In Java, Objects are always referenced so when you do:

   moves1.add(mid_array);

It means mid_array is added to moves1 but still referenced with mid_array . This way, when you call mid_array.clear() , its clear from both the places.

If you want to maintain the list inside moves1 , then better to create a new instance of mid_array inside the for loop eg below:

 for(int f = 0; f < moves.size(); f++)//make a copy of original array.
 {
    List<List<String>> mid_array = new ArrayList<List<String>>();
    List<String> row_st = moves.get(f).get(0);
    List<String> deck_st = moves.get(f).get(1);

    mid_array.add(row_st);//current array of the Rows
    mid_array.add(deck_st);//current array of the Deck

    moves1.add(mid_array);

    System.out.println("Moves1 "+moves1);//displays the new array correctly

    //No need to clear as in each iteration, it will instantiate a new mid_array
 }

Answer 2

This produces a real copy, inclusive copying all Elements.

public static <T> List<List<List<T>>> list3DCopy(List<List<List<T>>> source) {
    List<List<List<T>>> result = new ArrayList<>(source.size());
    Cloner cloner = new Cloner();   
    for(List<List<T>> innerList : source) {
        List<List<T>> copy = new ArrayList<>(innerList.size());
        for (List<T> innerInnerList : innerList) {
            List<T> innerCopy = new ArrayList<>(innerInnerList.size());
            for (T item : innerInnerList) {
                T clone = cloner.deepClone(item);
                innerCopy.add(clone);
            }
            copy.add(innerCopy);
        }
        result.add(copy);
    }
    return result;
}

It uses this cloning library to copy the elements.

Use it as follows:

List<List<List<YourClass>>> original = ...
List<List<List<YourClass>>> copy = list3DCopy(original);

Answer 3

Generally you have to do deep copies manually (unless the object supports it for you, the documentation would tell you). In this case you would need to use two nested loops and clone the bottommost ArrayList .

Answer 4

You can create subclass of ArrayList and override the clone() method to perform deep copy instead of shallow copy. Eg:

public class DeepCopyArrayList extends ArrayList
{
  public Object clone()
  {
     // implement deep copy clone here
  }
}

Once you have this you only need to do

myArray.clone()

Answer 5

Well you can do it the ugly way if you want. I forget if there is a function to do it but if you want to manually do it here is the way.

 int array[10][10][10] = ...// just assume they are all filled.
    int newArray[10][10][10];
        for(int i = 0; i< 10; i++)
        {
           for(int j = 0; j< 10; j++)
           {
               for(int k = 0; k< 10; k++)
               {
                  newArray[i][j][k] = array[i][j][k];
               }
           }
        }

    array = NULL;

This is just an example and probably isn't correct syntax but this way should work. The i< 10 or any of the others should actually be i< length of the array dimension.