LazyArrayOfLists.java

package btools.util;

import java.util.List;
import java.util.ArrayList;

/**
 * Behaves like an Array of list
 * with lazy list-allocation at getList
 *
 * @author ab
 */
public class LazyArrayOfLists<E>
{
  private ArrayList<ArrayList<E>> lists;

  public LazyArrayOfLists( int size )
  {
    lists = new ArrayList<ArrayList<E>>( size );
    for ( int i = 0; i< size; i++ )
    {
      lists.add( null );
    }
  }

  public List<E> getList( int idx )
  {
    ArrayList<E> list = lists.get( idx );
    if ( list == null )
    {
      list = new ArrayList<E>();
      lists.set( idx, list );
    }
    return list;
  }

  public int getSize( int idx )
  {
    List<E> list = lists.get( idx );
    return list == null ? 0 : list.size();
  }
    
  public void trimAll()
  {
    for ( int idx = 0; idx< lists.size(); idx++ )
    {
      ArrayList<E> list = lists.get( idx );
      if ( list != null )
      {
        list.trimToSize();
      }
    }
  }
}