org.apache.lucene.index

## Class TermsEnum

- java.lang.Object
- - org.apache.lucene.index.TermsEnum

- All Implemented Interfaces:
    BytesRefIterator
  Direct Known Subclasses:
    FilteredTermsEnum, FilterLeafReader.FilterTermsEnum, FuzzyTermsEnum, MultiTermsEnum

      public abstract class TermsEnum
      extends Object
      implements BytesRefIterator

  Iterator to seek (`seekCeil(BytesRef)`, `seekExact(BytesRef)`) or step through (`BytesRefIterator.next()` terms to obtain frequency information (`docFreq()`), `PostingsEnum` or `PostingsEnum` for the current term (`postings(org.apache.lucene.index.PostingsEnum)`.

  Term enumerations are always ordered by BytesRef.compareTo, which is Unicode sort order if the terms are UTF-8 bytes. Each term in the enumeration is greater than the one before it.

  The TermsEnum is unpositioned when you first obtain it and you must first successfully call `BytesRefIterator.next()` or one of the `seek` methods.

  WARNING: This API is experimental and might change in incompatible ways in the next release.

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `static class` | `TermsEnum.SeekStatus` Represents returned result from `seekCeil(org.apache.lucene.util.BytesRef)`.

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static TermsEnum` | `EMPTY` An empty TermsEnum for quickly returning an empty instance e.g.

  - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `TermsEnum()` Sole constructor.

  - ### Method Summary

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `AttributeSource` | `attributes()` Returns the related attributes.
    `abstract int` | `docFreq()` Returns the number of documents containing the current term.
    `abstract long` | `ord()` Returns ordinal position for current term.
    `PostingsEnum` | `postings(PostingsEnum reuse)` Get `PostingsEnum` for the current term.
    `abstract PostingsEnum` | `postings(PostingsEnum reuse, int flags)` Get `PostingsEnum` for the current term, with control over whether freqs, positions, offsets or payloads are required.
    `abstract TermsEnum.SeekStatus` | `seekCeil(BytesRef text)` Seeks to the specified term, if it exists, or to the next (ceiling) term.
    `boolean` | `seekExact(BytesRef text)` Attempts to seek to the exact term, returning true if the term is found.
    `void` | `seekExact(BytesRef term, TermState state)` Expert: Seeks a specific position by `TermState` previously obtained from `termState()`.
    `abstract void` | `seekExact(long ord)` Seeks to the specified term by ordinal (position) as previously returned by `ord()`.
    `abstract BytesRef` | `term()` Returns current term.
    `TermState` | `termState()` Expert: Returns the TermsEnums internal state to position the TermsEnum without re-seeking the term dictionary.
    `abstract long` | `totalTermFreq()` Returns the total number of occurrences of this term across all documents (the sum of the freq() for each doc that has this term).

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

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

    - ### Methods inherited from interface org.apache.lucene.util.BytesRefIterator

      `next`

- - ### Field Detail

    - #### EMPTY

          public static final TermsEnum EMPTY

      An empty TermsEnum for quickly returning an empty instance e.g. in `MultiTermQuery`

      Please note: This enum should be unmodifiable, but it is currently possible to add Attributes to it. This should not be a problem, as the enum is always empty and the existence of unused Attributes does not matter.

  - ### Constructor Detail

    - #### TermsEnum

          protected TermsEnum()

      Sole constructor. (For invocation by subclass constructors, typically implicit.)

  - ### Method Detail

    - #### attributes

          public AttributeSource attributes()

      Returns the related attributes.

    - #### seekExact

          public boolean seekExact(BytesRef text)
                            throws IOException

      Attempts to seek to the exact term, returning true if the term is found. If this returns false, the enum is unpositioned. For some codecs, seekExact may be substantially faster than `seekCeil(org.apache.lucene.util.BytesRef)`.
      Throws:
        `IOException`

    - #### seekCeil

          public abstract TermsEnum.SeekStatus seekCeil(BytesRef text)
                                                 throws IOException

      Seeks to the specified term, if it exists, or to the next (ceiling) term. Returns SeekStatus to indicate whether exact term was found, a different term was found, or EOF was hit. The target term may be before or after the current term. If this returns SeekStatus.END, the enum is unpositioned.
      Throws:
        `IOException`

    - #### seekExact

          public abstract void seekExact(long ord)
                                  throws IOException

      Seeks to the specified term by ordinal (position) as previously returned by `ord()`. The target ord may be before or after the current ord, and must be within bounds.
      Throws:
        `IOException`

    - #### seekExact

          public void seekExact(BytesRef term,
                                TermState state)
                         throws IOException

      Expert: Seeks a specific position by `TermState` previously obtained from `termState()`. Callers should maintain the `TermState` to use this method. Low-level implementations may position the TermsEnum without re-seeking the term dictionary.

      Seeking by `TermState` should only be used iff the state was obtained from the same `TermsEnum` instance.

      NOTE: Using this method with an incompatible `TermState` might leave this `TermsEnum` in undefined state. On a segment level `TermState` instances are compatible only iff the source and the target `TermsEnum` operate on the same field. If operating on segment level, TermState instances must not be used across segments.

      NOTE: A seek by `TermState` might not restore the `AttributeSource`'s state. `AttributeSource` states must be maintained separately if this method is used.

      Parameters:
        `term` - the term the TermState corresponds to
        `state` - the `TermState`
      Throws:
        `IOException`

    - #### term

          public abstract BytesRef term()
                                 throws IOException

      Returns current term. Do not call this when the enum is unpositioned.
      Throws:
        `IOException`

    - #### ord

          public abstract long ord()
                            throws IOException

      Returns ordinal position for current term. This is an optional method (the codec may throw `UnsupportedOperationException`). Do not call this when the enum is unpositioned.
      Throws:
        `IOException`

    - #### docFreq

          public abstract int docFreq()
                               throws IOException

      Returns the number of documents containing the current term. Do not call this when the enum is unpositioned. `TermsEnum.SeekStatus.END`.
      Throws:
        `IOException`

    - #### totalTermFreq

          public abstract long totalTermFreq()
                                      throws IOException

      Returns the total number of occurrences of this term across all documents (the sum of the freq() for each doc that has this term). This will be -1 if the codec doesn't support this measure. Note that, like other term measures, this measure does not take deleted documents into account.
      Throws:
        `IOException`

    - #### postings

          public final PostingsEnum postings(PostingsEnum reuse)
                                      throws IOException

      Get `PostingsEnum` for the current term. Do not call this when the enum is unpositioned. This method will not return null.

      NOTE: the returned iterator may return deleted documents, so deleted documents have to be checked on top of the `PostingsEnum`.

      Use this method if you only require documents and frequencies, and do not need any proximity data. This method is equivalent to `postings(reuse, PostingsEnum.FREQS)`

      Parameters:
        `reuse` - pass a prior PostingsEnum for possible reuse
      Throws:
        `IOException`
      See Also:
        `postings(PostingsEnum, int)`

    - #### postings

          public abstract PostingsEnum postings(PostingsEnum reuse,
                                                int flags)
                                         throws IOException

      Get `PostingsEnum` for the current term, with control over whether freqs, positions, offsets or payloads are required. Do not call this when the enum is unpositioned. This method may return null if the postings information required is not available from the index

      NOTE: the returned iterator may return deleted documents, so deleted documents have to be checked on top of the `PostingsEnum`.

      Parameters:
        `reuse` - pass a prior PostingsEnum for possible reuse
        `flags` - specifies which optional per-document values you require; see `PostingsEnum.FREQS`
      Throws:
        `IOException`

    - #### termState

          public TermState termState()
                              throws IOException

      Expert: Returns the TermsEnums internal state to position the TermsEnum without re-seeking the term dictionary.

      NOTE: A seek by `TermState` might not capture the `AttributeSource`'s state. Callers must maintain the `AttributeSource` states separately

      Throws:
        `IOException`
      See Also:
        `TermState`, `seekExact(BytesRef, TermState)`

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