Decouple Filter from BitSet

• Key: LUCENE-584
• Type: Improvement
• Status: Closed
• Resolution: Fixed
• Priority: Minor
• Assignee: Mark Miller
• Reporter: Peter Schäfer
• Component/s: Search
• Affects Version/s: 2.1
• Fix Version/s: 2.9

Description

package org.apache.lucene.search;

public abstract class Filter implements java.io.Serializable 
{
  public abstract AbstractBitSet bits(IndexReader reader) throws IOException;
}

public interface AbstractBitSet 
{
  public boolean get(int index);
}

It would be useful if the method =Filter.bits()= returned an abstract interface, instead of =java.util.BitSet=.

Use case: there is a very large index, and, depending on the user's privileges, only a small portion of the index is actually visible.
Sparsely populated =java.util.BitSet=s are not efficient and waste lots of memory. It would be desirable to have an alternative BitSet implementation with smaller memory footprint.

Though it is possibly to derive classes from =java.util.BitSet=, it was obviously not designed for that purpose.
That's why I propose to use an interface instead. The default implementation could still delegate to =java.util.BitSet=.