2D dynamic array using ArrayList in Java

Question

I need to implement a 2D dynamic array. The number of rows is fixed, say n. But the number of columns for each row is not fixed and equivalent. For instance, the first row h

Answer 1

ArrayList<ArrayList<SomeObject>> twodlist = new ArrayList<ArrayList<SomeObject>>();
ArrayList<SomeObject> row = new ArrayList<SomeObject>();
row.add(new SomeObject(/* whatever */));
// etc
twodlist.add(row);
row = new ArrayList<SomeObject>();
// etc

Answer 2

List<ArrayList<SomeObject>> twoDList = new ArrayList<List<SomeObject>>(n);
for( int i=0; i<n; i++ )
    twoDList.add( new ArrayList<SomeObject>() );

Use as:

twoDList.get(rownumber).add(newElementInColumn);

Answer 3

You can either use a array for the rows since this dimenstion is fixed:

@SuppressWarnings("unchecked")
ArrayList<T>[] arr = new ArrayList[ fixedsize];

or use nested ArrayLists:

List<List<T>> list = new ArrayList<List<T>>( fixedsize );