- Direct Known Subclasses:
public class NGramTokenizer extends TokenizerTokenizes the input into n-grams of the given size(s).
On the contrary to
NGramTokenFilter, this class sets offsets so that characters between startOffset and endOffset in the original stream are the same as the term chars.
For example, "abcde" would be tokenized as (minGram=2, maxGram=3):
Term ab abc bc bcd cd cde de Position increment 1 1 1 1 1 1 1 Position length 1 1 1 1 1 1 1 Offsets [0,2[ [0,3[ [1,3[ [1,4[ [2,4[ [2,5[ [3,5[
This tokenizer changed a lot in Lucene 4.4 in order to:
- tokenize in a streaming fashion to support streams which are larger than 1024 chars (limit of the previous version),
- count grams based on unicode code points instead of java chars (and never split in the middle of surrogate pairs),
- give the ability to
pre-tokenizethe stream before computing n-grams.
Additionally, this class doesn't trim trailing whitespaces and emits tokens in a different order, tokens are now emitted by increasing start offsets while they used to be emitted by increasing lengths (which prevented from supporting large input streams).
Although highly discouraged, it is still possible to use the old behavior through
Constructors Constructor Description
NGramTokenizer(Version version, Reader input)Creates NGramTokenizer with default min and max n-grams.
NGramTokenizer(Version version, Reader input, int minGram, int maxGram)Creates NGramTokenizer with given min and max n-grams.
NGramTokenizer(Version version, AttributeSource.AttributeFactory factory, Reader input, int minGram, int maxGram)Creates NGramTokenizer with given min and max n-grams.
All Methods Instance Methods Concrete Methods Modifier and Type Method Description
end()This method is called by the consumer after the last token has been consumed, after
false(using the new
IndexWriter) use this method to advance the stream to the next token.
reset()This method is called by a consumer before it begins consumption using
Methods inherited from class org.apache.lucene.util.AttributeSource
addAttribute, addAttributeImpl, captureState, clearAttributes, cloneAttributes, copyTo, equals, getAttribute, getAttributeClassesIterator, getAttributeFactory, getAttributeImplsIterator, hasAttribute, hasAttributes, hashCode, reflectAsString, reflectWith, restoreState, toString
NGramTokenizerCreates NGramTokenizer with given min and max n-grams.
public NGramTokenizer(Version version, AttributeSource.AttributeFactory factory, Reader input, int minGram, int maxGram)Creates NGramTokenizer with given min and max n-grams.
public final boolean incrementToken() throws IOExceptionConsumers (i.e.,
IndexWriter) use this method to advance the stream to the next token. Implementing classes must implement this method and update the appropriate
AttributeImpls with the attributes of the next token.
The producer must make no assumptions about the attributes after the method has been returned: the caller may arbitrarily change it. If the producer needs to preserve the state for subsequent calls, it can use
AttributeSource.captureState()to create a copy of the current attribute state.
This method is called for every token of a document, so an efficient implementation is crucial for good performance. To avoid calls to
AttributeSource.getAttribute(Class), references to all
AttributeImpls that this stream uses should be retrieved during instantiation.
To ensure that filters and consumers know which attributes are available, the attributes must be added during instantiation. Filters and consumers are not required to check for availability of attributes in
public final void end() throws IOExceptionThis method is called by the consumer after the last token has been consumed, after
false(using the new
TokenStreamAPI). Streams implementing the old API should upgrade to use this feature. This method can be used to perform any end-of-stream operations, such as setting the final offset of a stream. The final offset of a stream might differ from the offset of the last token eg in case one or more whitespaces followed after the last token, but a WhitespaceTokenizer was used.
Additionally any skipped positions (such as those removed by a stopfilter) can be applied to the position increment, or any adjustment of other attributes where the end-of-stream value may be important.
If you override this method, always call
public final void reset() throws IOExceptionThis method is called by a consumer before it begins consumption using
Resets this stream to a clean state. Stateful implementations must implement this method so that they can be reused, just as if they had been created fresh.