GermanStemFilter.java

package org.apache.lucene.analysis.de;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.util.Set;

import org.apache.lucene.analysis.TokenFilter;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.tokenattributes.TermAttribute;

/**
 * A {@link TokenFilter} that stems German words. 
 * <p>
 * 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 {@link GermanStemmer}).
 * </p>
 */
public final class GermanStemFilter extends TokenFilter
{
    /**
     * The actual token in the input stream.
     */
    private GermanStemmer stemmer = null;
    private Set exclusionSet = null;

    private TermAttribute termAtt;

    public GermanStemFilter( TokenStream in )
    {
      super(in);
      stemmer = new GermanStemmer();
      termAtt = addAttribute(TermAttribute.class);
    }

    /**
     * Builds a GermanStemFilter that uses an exclusion table.
     */
    public GermanStemFilter( TokenStream in, Set exclusionSet )
    {
      this( in );
      this.exclusionSet = exclusionSet;
    }

    /**
     * @return  Returns true for next token in the stream, or false at EOS
     */
    @Override
    public boolean incrementToken() throws IOException {
      if (input.incrementToken()) {
        String term = termAtt.term();
        // Check the exclusion table.
        if (exclusionSet == null || !exclusionSet.contains(term)) {
          String s = stemmer.stem(term);
          // If not stemmed, don't waste the time adjusting the token.
          if ((s != null) && !s.equals(term))
            termAtt.setTermBuffer(s);
        }
        return true;
      } else {
        return false;
      }
    }

    /**
     * Set a alternative/custom {@link 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 setExclusionSet( Set exclusionSet )
    {
      this.exclusionSet = exclusionSet;
    }
}