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## org.apache.lucene.analysis.synonym
Class SynonymFilter

    java.lang.Object
      org.apache.lucene.util.AttributeSource
          org.apache.lucene.analysis.TokenStream
              org.apache.lucene.analysis.TokenFilter
                  org.apache.lucene.analysis.synonym.SynonymFilter

All Implemented Interfaces:
  Closeable

    public final class SynonymFilter

extends TokenFilter

Matches single or multi word synonyms in a token stream. This token stream cannot properly handle position increments != 1, ie, you should place this filter before filtering out stop words.

Note that with the current implementation, parsing is greedy, so whenever multiple parses would apply, the rule starting the earliest and parsing the most tokens wins. For example if you have these rules:

       a -> x
       a b -> y
       b c d -> z

Then input `a b c d e` parses to `y b c d`, ie the 2nd rule "wins" because it started earliest and matched the most input tokens of other rules starting at that point.

A future improvement to this filter could allow non-greedy parsing, such that the 3rd rule would win, and also separately allow multiple parses, such that all 3 rules would match, perhaps even on a rule by rule basis.

NOTE: when a match occurs, the output tokens associated with the matching rule are "stacked" on top of the input stream (if the rule had `keepOrig=true`) and also on top of another matched rule's output tokens. This is not a correct solution, as really the output should be an arbitrary graph/lattice. For example, with the above match, you would expect an exact `PhraseQuery` `"y b c"` to match the parsed tokens, but it will fail to do so. This limitation is necessary because Lucene's TokenStream (and index) cannot yet represent an arbitrary graph.

NOTE: If multiple incoming tokens arrive on the same position, only the first token at that position is used for parsing. Subsequent tokens simply pass through and are not parsed. A future improvement would be to allow these tokens to also be matched.

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.lucene.util.AttributeSource
`AttributeSource.AttributeFactory, AttributeSource.State`

Field Summary
`static String` | `TYPE_SYNONYM`

Fields inherited from class org.apache.lucene.analysis.TokenFilter
`input`

Constructor Summary
`SynonymFilter(TokenStream input, SynonymMap synonyms, boolean ignoreCase)`

Method Summary
`boolean` | `incrementToken()`
`void` | `reset()`

Methods inherited from class org.apache.lucene.analysis.TokenFilter
`close, end`

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`

Methods inherited from class java.lang.Object
`clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### TYPE_SYNONYM

    public static final String TYPE_SYNONYM

See Also:
  Constant Field Values

Constructor Detail

### SynonymFilter

    public SynonymFilter(TokenStream input,
                         SynonymMap synonyms,
                         boolean ignoreCase)

Parameters:
  `input` - input tokenstream
  `synonyms` - synonym map
  `ignoreCase` - case-folds input for matching with `Character.toLowerCase(int)`. Note, if you set this to true, its your responsibility to lowercase the input entries when you create the `SynonymMap`

Method Detail

### incrementToken

    public boolean incrementToken()
                           throws IOException

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

### reset

    public void reset()
               throws IOException

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

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