关于String内的indexOf方法的一些疑问

今天浏览了一下java里的String类，发现一个静态方法有点意思，就是我们常用的indexOf(String str)的底层实现，先看下代码调用链。public int indexOf(String str) { return indexOf(str, 0);} public int indexOf(String str, int fromIndex) { return indexOf(value, 0, value.length, str.value, 0, str.value.length, fromIndex);}static int indexOf(char[] source, int sourceOffset, int sourceCount, String target, int fromIndex) { return indexOf(source, sourceOffset, sourceCount, target.value, 0, target.value.length, fromIndex);}/** * Code shared by String and StringBuffer to do searches. The * source is the character array being searched, and the target * is the string being searched for. * * @param source the characters being searched. * @param sourceOffset offset of the source string. * @param sourceCount count of the source string. * @param target the characters being searched for. * @param targetOffset offset of the target string. * @param targetCount count of the target string. * @param fromIndex the index to begin searching from. /static int indexOf(char[] source, int sourceOffset, int sourceCount, char[] target, int targetOffset, int targetCount, int fromIndex) { if (fromIndex >= sourceCount) { return (targetCount == 0 ? sourceCount : -1); } if (fromIndex < 0) { fromIndex = 0; } if (targetCount == 0) { return fromIndex; } char first = target[targetOffset]; int max = sourceOffset + (sourceCount - targetCount); for (int i = sourceOffset + fromIndex; i <= max; i++) { / Look for first character. / if (source[i] != first) { while (++i <= max && source[i] != first); } / Found first character, now look at the rest of v2 / if (i <= max) { int j = i + 1; int end = j + targetCount - 1; for (int k = targetOffset + 1; j < end && source[j] == target[k]; j++, k++); if (j == end) { / Found whole string. / return i - sourceOffset; } } } return -1;}底层的字符串匹配的逻辑比较简单，就是普通的匹配模式：查找首字符，匹配target的第一个字符在source内的位置，若查找到max位置还找到，则返回-1；若在source匹配到了target的第一个字符，那么在依次比较srouce和target后面的字符，一直到target的末尾；如果target后面的字符与source都已经匹配，则返回在source上匹配到的第一个字符的相对下标，否则返回-1。但是仔细读代码会发现一个问题，就是这里int max = sourceOffset + (sourceCount - targetCount);max的计算方式，max的作用是计算出最大的首字符匹配次数，取值范围应该是"max <= sourceCount"。所以target字符串的长度是可以不用匹配的，故“sourceCount - targetCount”是没问题的。关键的地方是这里加上了sourceOffset，sourceOffset是source字符串的起始匹配偏移量，即从source的哪个字符开始匹配。所以，根据代码里的max计算方式，最终计算出来的max值是会有可能大于sourceCount。看下测试代码：package string;/* * string test /public class StringTest { static int indexOf(char[] source, int sourceOffset, int sourceCount, char[] target, int targetOffset, int targetCount, int fromIndex) { if (fromIndex >= sourceCount) { return (targetCount == 0 ? sourceCount : -1); } if (fromIndex < 0) { fromIndex = 0; } if (targetCount == 0) { return fromIndex; } char first = target[targetOffset]; int max = sourceOffset + (sourceCount - targetCount); for (int i = sourceOffset + fromIndex; i <= max; i++) { / Look for first character. / if (source[i] != first) { while (++i <= max && source[i] != first); } / Found first character, now look at the rest of v2 / if (i <= max) { int j = i + 1; int end = j + targetCount - 1; for (int k = targetOffset + 1; j < end && source[j] == target[k]; j++, k++); if (j == end) { / Found whole string. */ return i - sourceOffset; } } } return -1; } public static void main(String[] args) { String source = “abcdefghigklmn”; String target = “n”; int sourceOffset = 5; int targetOffset = 0; int index = indexOf(source.toCharArray(), sourceOffset, source.length(), target.toCharArray(), targetOffset, target.length(), 0); System.out.println(index); }}如果target在source内可以匹配到返回正确结果8（结果8是相对于sourceOffset的结果，如果转换成source内的位置则是13）。但是如果target在source内匹配不到，则会抛出java.lang.ArrayIndexOutOfBoundsException异常，如下：Exception in thread “main” java.lang.ArrayIndexOutOfBoundsException: 14 at string.StringTest.indexOf(StringTest.java:27) at string.StringTest.main(StringTest.java:52)可见报出越界的下标是14,这就是由于max = sourceOffset + (sourceCount - targetCount)引起，计算出的max值为：17。所以，个人认为max计算这里是个潜在的BUG，应该改为 int max = sourceCount - targetCount;不过这个方法是一个非public方法，只在String内部调用，同时也跟踪了所有对该方法的调用链，都是传入的默认0,在使用时不会出现数组越界问题。不知这是开发者故意为之，还是其它我未知用意，欢迎大家交流讨论！！！