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Lucene example source code file (SimpleStringInterner.java)

This example Lucene source code file (SimpleStringInterner.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lucene tags/keywords

entry, entry, override, simplestringinterner, simplestringinterner, string, string, stringinterner, stringinterner

The Lucene SimpleStringInterner.java source code

package org.apache.lucene.util;
/**
 * 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.
 */


/**
 * Simple lockless and memory barrier free String intern cache that is guaranteed
 * to return the same String instance as String.intern()
 * does.
 *
 * @lucene.internal
 */
public class SimpleStringInterner extends StringInterner {

  private static class Entry {
    final private String str;
    final private int hash;
    private Entry next;
    private Entry(String str, int hash, Entry next) {
      this.str = str;
      this.hash = hash;
      this.next = next;
    }
  }

  private final Entry[] cache;
  private final int maxChainLength;

  /**
   * @param tableSize  Size of the hash table, should be a power of two.
   * @param maxChainLength  Maximum length of each bucket, after which the oldest item inserted is dropped.
   */
  public SimpleStringInterner(int tableSize, int maxChainLength) {
    cache = new Entry[Math.max(1,BitUtil.nextHighestPowerOfTwo(tableSize))];
    this.maxChainLength = Math.max(2,maxChainLength);
  }

  @Override
  public String intern(String s) {
    int h = s.hashCode();
    // In the future, it may be worth augmenting the string hash
    // if the lower bits need better distribution.
    int slot = h & (cache.length-1);

    Entry first = this.cache[slot];
    Entry nextToLast = null;

    int chainLength = 0;

    for(Entry e=first; e!=null; e=e.next) {
      if (e.hash == h && (e.str == s || e.str.compareTo(s)==0)) {
      // if (e.str == s || (e.hash == h && e.str.compareTo(s)==0)) {
        return e.str;
      }

      chainLength++;
      if (e.next != null) {
        nextToLast = e;
      }
    }

    // insertion-order cache: add new entry at head
    s = s.intern();
    this.cache[slot] = new Entry(s, h, first);
    if (chainLength >= maxChainLength) {
      // prune last entry
      nextToLast.next = null;
    }
    return s;
  }
}

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