Package org.apache.lucene.analysis.ngram

# Class NGramTokenizer

java.lang.Object
org.apache.lucene.util.AttributeSource
org.apache.lucene.analysis.TokenStream
org.apache.lucene.analysis.Tokenizer
org.apache.lucene.analysis.ngram.NGramTokenizer

All Implemented Interfaces:
  `Closeable`, `AutoCloseable`

Direct Known Subclasses:
  `EdgeNGramTokenizer`

public class NGramTokenizer extends Tokenizer
Tokenizes 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):

ngram tokens example
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-tokenize` the 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).

## Nested Class Summary

## Nested classes/interfaces inherited from class org.apache.lucene.util.AttributeSource

`AttributeSource.State`

## Field Summary

Fields
Modifier and Type
Field
Description
`static final int`
`DEFAULT_MAX_NGRAM_SIZE`
`static final int`
`DEFAULT_MIN_NGRAM_SIZE`

### Fields inherited from class org.apache.lucene.analysis.Tokenizer

`input`

### Fields inherited from class org.apache.lucene.analysis.TokenStream

`DEFAULT_TOKEN_ATTRIBUTE_FACTORY`

## Constructor Summary

Constructors
Constructor
Description
`NGramTokenizer()`
Creates NGramTokenizer with default min and max n-grams.
`NGramTokenizer(int minGram, int maxGram)`
Creates NGramTokenizer with given min and max n-grams.
`NGramTokenizer(AttributeFactory factory, int minGram, int maxGram)`
Creates NGramTokenizer with given min and max n-grams.

## Method Summary

Modifier and Type
Method
Description
`final void`
`end()`
`final boolean`
`incrementToken()`
`protected boolean`
`isTokenChar(int chr)`
Only collect characters which satisfy this condition.
`final void`
`reset()`

### Methods inherited from class org.apache.lucene.analysis.Tokenizer

`close, correctOffset, setReader, setReaderTestPoint`

### Methods inherited from class org.apache.lucene.util.AttributeSource

`addAttribute, addAttributeImpl, captureState, clearAttributes, cloneAttributes, copyTo, endAttributes, equals, getAttribute, getAttributeClassesIterator, getAttributeFactory, getAttributeImplsIterator, hasAttribute, hasAttributes, hashCode, reflectAsString, reflectWith, removeAllAttributes, restoreState, toString`

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

`clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

## Field Details

### DEFAULT_MIN_NGRAM_SIZE

public static final int DEFAULT_MIN_NGRAM_SIZE

See Also:
  - Constant Field Values

### DEFAULT_MAX_NGRAM_SIZE

public static final int DEFAULT_MAX_NGRAM_SIZE

See Also:
  - Constant Field Values

## Constructor Details

### NGramTokenizer

public NGramTokenizer(int minGram, int maxGram)
Creates NGramTokenizer with given min and max n-grams.

Parameters:
  `minGram` - the smallest n-gram to generate
  `maxGram` - the largest n-gram to generate

### NGramTokenizer

public NGramTokenizer(AttributeFactory factory, int minGram, int maxGram)
Creates NGramTokenizer with given min and max n-grams.

Parameters:
  `factory` - `AttributeFactory` to use
  `minGram` - the smallest n-gram to generate
  `maxGram` - the largest n-gram to generate

### NGramTokenizer

public NGramTokenizer()
Creates NGramTokenizer with default min and max n-grams.

## Method Details

### incrementToken

public final boolean incrementToken() throws IOException

Specified by:
  `incrementToken` in class `TokenStream`
Throws:
  `IOException`

### isTokenChar

protected boolean isTokenChar(int chr)
Only collect characters which satisfy this condition.

### end

public final void end() throws IOException

Overrides:
  `end` in class `TokenStream`
Throws:
  `IOException`

### reset

public final void reset() throws IOException

Overrides:
  `reset` in class `Tokenizer`
Throws:
  `IOException`