lucene3.5之StringHelper

这个类主要是完成2个功能：
1、版本号比较
2、生成字符串实例
package org.apache.lucene.util;

import java.util.Comparator;
import java.util.StringTokenizer;

/**
* 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.
*/


/**
* Methods for manipulating strings.
*
* @lucene.internal
*/
public abstract class StringHelper {
  /**
   * Expert:
   * The StringInterner implementation used by Lucene.
   * This shouldn't be changed to an incompatible implementation after other Lucene APIs have been used.
   */
  public static StringInterner interner = new SimpleStringInterner(1024,8);

  /** Return the same string object for all equal strings */
  public static String intern(String s) {
    return interner.intern(s);
  }

  /**
   * Compares two byte[] arrays, element by element, and returns the
   * number of elements common to both arrays.
   *
   * @param bytes1 The first byte[] to compare
   * @param bytes2 The second byte[] to compare
   * @return The number of common elements.
   */
  public static final int bytesDifference(byte[] bytes1, int len1, byte[] bytes2, int len2) {
    int len = len1 < len2 ? len1 : len2;
    for (int i = 0; i < len; i++)
      if (bytes1[i] != bytes2[i])
        return i;
    return len;
  }
 
  private StringHelper() {
  }
 
  /**
   * @return a Comparator over versioned strings such as X.YY.Z
   * @lucene.internal
   */
  public static Comparator<String> getVersionComparator() {
    return versionComparator;
  }
 
  private static Comparator<String> versionComparator = new Comparator<String>() {
    public int compare(String a, String b) {
      StringTokenizer aTokens = new StringTokenizer(a, ".");
      StringTokenizer bTokens = new StringTokenizer(b, ".");
     
      while (aTokens.hasMoreTokens()) {
        int aToken = Integer.parseInt(aTokens.nextToken());
        if (bTokens.hasMoreTokens()) {
          int bToken = Integer.parseInt(bTokens.nextToken());
          if (aToken != bToken) {
            return aToken < bToken ? -1 : 1;
          }
        } else {
          // a has some extra trailing tokens. if these are all zeroes, thats ok.
          if (aToken != 0) {
            return 1;
          }
        }
      }
     
      // b has some extra trailing tokens. if these are all zeroes, thats ok.
      while (bTokens.hasMoreTokens()) {
        if (Integer.parseInt(bTokens.nextToken()) != 0)
          return -1;
      }
     
      return 0;
    }
  };
}


上述代码中
public static StringInterner interner = new SimpleStringInterner(1024,8);

  /** Return the same string object for all equal strings */
  public static String intern(String s) {
    return interner.intern(s);
  }

intern()方法返回一个字符串的引用，
  /**
   * Compares two byte[] arrays, element by element, and returns the
   * number of elements common to both arrays.
   *
   * @param bytes1 The first byte[] to compare
   * @param bytes2 The second byte[] to compare
   * @return The number of common elements.
   */
  public static final int bytesDifference(byte[] bytes1, int len1, byte[] bytes2, int len2) {
    int len = len1 < len2 ? len1 : len2;
    for (int i = 0; i < len; i++)
      if (bytes1[i] != bytes2[i])
        return i;
    return len;
  }
bytesDifference（）方法的作用是比较2个字符串从第1位开始共有多少个字符相同，返回相同的字符串的长度


private static Comparator<String> versionComparator = new Comparator<String>() {
    public int compare(String a, String b) {
      StringTokenizer aTokens = new StringTokenizer(a, ".");
      StringTokenizer bTokens = new StringTokenizer(b, ".");
     
      while (aTokens.hasMoreTokens()) {
        int aToken = Integer.parseInt(aTokens.nextToken());
        if (bTokens.hasMoreTokens()) {
          int bToken = Integer.parseInt(bTokens.nextToken());
          if (aToken != bToken) {
            return aToken < bToken ? -1 : 1;
          }
        } else {
          // a has some extra trailing tokens. if these are all zeroes, thats ok.
          if (aToken != 0) {
            return 1;
          }
        }
类中有一个静态私有的versionComparator对象用于版本(形如 X.YY.Z)比较，StringTokenizer aTokens = new StringTokenizer将需要比较的版本字符串按“.”分割，分割后的字符通过Integer.parseInt转换为整数后进行比较，如果第一参数的版本比第二个参数的版本小就返回-1，否则返回1，如果相等，也返回1
该对象通过getVersionComparator()方法返回，getVersionComparator相当于提供版本比较对象的只读访问接口
  public static Comparator<String> getVersionComparator() {
    return versionComparator;
  }