如何将元素从`2d array`复制到`Arraylist`？

Question

folks, what is the technique of copying elements from array to Arraylist ?

public DenseBoard(T[][] x, T fillElem){
      ArrayList<ArrayList<T>> myBoard = new ArrayList<ArrayList<T>>();
      for(int i = 0; i < x.length; i++){
          for(int j = 0; j < x[0].length; j++){
              myBoard.get(i).add(j); //<<------ getting error!
          }
      }
  }

Answer 1

You'll need to initialize each member of myBoard in the outer loop:

Untested Code Ahead

public DenseBoard(T[][] x, T fillElem){
      ArrayList<ArrayList<T>> myBoard = new ArrayList<ArrayList<T>>();
      for(int i = 0; i < x.length; i++){
          myBoard.add(new ArrayList<T>); //Gotta add something to stick stuff in
          for(int j = 0; j < x[0].length; j++){
              myBoard.get(i).add(j); //<<------ no more error?
          }
      }
  }

Since myBoard is an ArrayList of ArrayLists of Ts, we need to give it somewhere to put the T's. Initially, myBoard looks like this:

[] <-- empty ArrayList

So we give it somewhere to put data for each row, like this

myBoard.add(new ArrayList<T>);

Now it looks like

[ [] ] <--- ArrayList with an empty ArrayList in it, ready to accept T's

We add some T's, and end up with this

[ [T1, T2, T3] ].

And on the next iteration, we'll end up with something like this

[ [T1, T2, T3], [T4, T5, T6] ]

Hope that cleared things up.

Answer 2

You are getting error because your are trying to get() a value from ArrayList but not had inserted any value first. To do it your way, here is the correct code :

ArrayList<ArrayList<T>> myBoard = new ArrayList<>();
for(int i = 0; i < x.length; i++){
    ArrayList<T> values = new ArrayList<>();
    for(int j = 0; j < x[0].length; j++){
        values.add(x[i][j]);
    }
    myBoard.add(values);
}

Just use the following function :

Arrays.asList(T...a)

In your case it will be done as :

ArrayList<T> myBoard = new ArrayList<>();
for(T[] arr : x){
    myBoard.add(Arrays.asList(arr));
}

1. Try to use diamond operator <> to make code more readable
2. Don't reinvent the wheel, use utilities provided at least by core libraries
3. Make use of for-each statement where you could

Answer 3

Inspired from this post .

new ArrayList<Item>(Arrays.asList(array))

creates a new ArrayList of Item elements from an input array. For two-dimensionals. In this case, you tried this:

public DenseBoard(T[][] x, T fillElem){
      ArrayList<ArrayList<T>> myBoard = new ArrayList<ArrayList<T>>();
      for(int i = 0; i < x.length; i++){
          for(int j = 0; j < x[0].length; j++){
              myBoard.get(i).add(j); //<<------ getting error!
          }
      }
  }

The problem is that the line throwing the error assumes you have an ArrayList of ArrayList , but the inner element you try to refer to is not initialized. This should be a fix:

public DenseBoard(T[][] x, T fillElem){
      ArrayList<ArrayList<T>> myBoard = new ArrayList<ArrayList<T>>();
      for(int i = 0; i < x.length; i++){
          myBoard.add(new ArrayList<T>(Arrays.asList(x[i])));
      }
      //Do something with myBoard
}