org.apache.lucene.search

## Class DocIdSet

- java.lang.Object
- - org.apache.lucene.search.DocIdSet

Direct Known Subclasses:
  DocIdBitSet, FieldCacheDocIdSet, FilteredDocIdSet, FixedBitSet, OpenBitSet

    public abstract class DocIdSet
    extends Object

A DocIdSet contains a set of doc ids. Implementing classes must only implement `iterator()` to provide access to the set.

### Field Summary

Fields
`static DocIdSet` `EMPTY_DOCIDSET` An empty `DocIdSet` instance for easy use, e.g.

### Constructor Summary

Constructors
Constructor and Description
`DocIdSet()`

### Method Summary

Methods
`Bits` `bits()` Optionally provides a `Bits` interface for random access to matching documents.
`boolean` `isCacheable()` This method is a hint for `CachingWrapperFilter`, if this `DocIdSet` should be cached without copying it into a BitSet.
`abstract DocIdSetIterator` `iterator()` Provides a `DocIdSetIterator` to access the set.

### Methods inherited from class java.lang.Object

`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

### Field Detail

#### EMPTY_DOCIDSET

    public static final DocIdSet EMPTY_DOCIDSET

An empty `DocIdSet` instance for easy use, e.g. in Filters that hit no documents.

### Constructor Detail

#### DocIdSet

    public DocIdSet()

### Method Detail

#### iterator

    public abstract DocIdSetIterator iterator()
                                       throws IOException

Provides a `DocIdSetIterator` to access the set. This implementation can return `null` or `EMPTY_DOCIDSET.iterator()` if there are no docs that match.

Throws:
  `IOException`

#### bits

    public Bits bits()
              throws IOException

Optionally provides a `Bits` interface for random access to matching documents.

Returns:
  `null`, if this `DocIdSet` does not support random access. In contrast to `iterator()`, a return value of `null` does not imply that no documents match the filter! The default implementation does not provide random access, so you only need to implement this method if your DocIdSet can guarantee random access to every docid in O(1) time without external disk access (as `Bits` interface cannot throw `IOException`). This is generally true for bit sets like `FixedBitSet`, which return itself if they are used as `DocIdSet`.
Throws:
  `IOException`

#### isCacheable

    public boolean isCacheable()

This method is a hint for `CachingWrapperFilter`, if this `DocIdSet` should be cached without copying it into a BitSet. The default is to return `false`. If you have an own `DocIdSet` implementation that does its iteration very effective and fast without doing disk I/O, override this method and return `true`.