germanstemfilter.java

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package org.apache.lucene.analysis.de;

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 * modification, are permitted provided that the following conditions
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import org.apache.lucene.analysis.Token;
import org.apache.lucene.analysis.TokenFilter;
import org.apache.lucene.analysis.TokenStream;
import java.io.IOException;
import java.util.Hashtable;

/**
 * A filter that stems German words. It supports a table of words that should
 * not be stemmed at all. The stemmer used can be changed at runtime after the
 * filter object is created (as long as it is a GermanStemmer).
 *
 * @author    Gerhard Schwarz
 * @version   $Id: GermanStemFilter.java,v 1.3 2002/04/19 19:08:32 otis Exp $
 */
public final class GermanStemFilter extends TokenFilter {

	/**
	 * The actual token in the input stream.
	 */
	private Token token = null;
	private GermanStemmer stemmer = null;
	private Hashtable exclusions = null;
	
	public GermanStemFilter( TokenStream in ) {
		stemmer = new GermanStemmer();
		input = in;
	}
	
	/**
	 * Builds a GermanStemFilter that uses an exclusiontable.
	 */
	public GermanStemFilter( TokenStream in, Hashtable exclusiontable ) {
		this( in );
		exclusions = exclusiontable;
	}

	/**
	 * @return  Returns the next token in the stream, or null at EOS
	 */
	public final Token next()
		throws IOException {
		if ( ( token = input.next() ) == null ) {
			return null;
		}
		// Check the exclusiontable
		else if ( exclusions != null && exclusions.contains( token.termText() ) ) {
			return token;
		}
		else {
			String s = stemmer.stem( token.termText() );
			// If not stemmed, dont waste the time creating a new token
			if ( !s.equals( token.termText() ) ) {
				return new Token( s, token.startOffset(),
				    token.endOffset(), token.type() );
			}
			return token;
		}
	}
	/**
	 * Set a alternative/custom GermanStemmer for this filter.
	 */
	public void setStemmer( GermanStemmer stemmer ) {
		if ( stemmer != null ) {
			this.stemmer = stemmer;
		}
	}
	/**
	 * Set an alternative exclusion list for this filter.
	 */
	public void setExclusionTable( Hashtable exclusiontable ) {
		exclusions = exclusiontable;
	}
}