String类提供了查找子串的方法，包括 indexOf(String str) ,indexOf(String str,int fromIndex),lastIndexOf(String str),lastIndexOf(String str,int fromIndex)。
我很奇怪为什么使用的是效率低下的普通算法，而没有使用高效的KMP算法，
我记得学数据结构的时候专门介绍了这个算法。下面是String的实现代码：
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 i = sourceOffset + fromIndex;
int max = sourceOffset + (sourceCount - targetCount);
startSearchForFirstChar:
while (true) {
/* Look for first character. */
while (i <= max && source[i] != first) {
i++;
}
if (i > max) {
return -1;
}
/* Found first character, now look at the rest of v2 */
int j = i + 1;
int end = j + targetCount - 1;
int k = targetOffset + 1;
while (j < end) {
if (source[j++] != target[k++]) {
i++;
/* Look for str's first char again. */
continue startSearchForFirstChar;
}
}
return i - sourceOffset; /* Found whole string. */
}
}
