CODEWITHSUNDEEP
javaarraysarraylist
Michael
Michael

Reputation: 313

How to copy elements from `2d array` to `Arraylist`?

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!
          }
      }
  }

Upvotes: 2

Views: 881

Answers (3)

Lajos Arpad
Lajos Arpad

Reputation: 76434

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
}

Upvotes: 0

afzalex
afzalex

Reputation: 8652

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

Upvotes: 1

CollinD
CollinD

Reputation: 7573

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.

Upvotes: 1

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