1前言

不多说，直接上源码

2源码

我自己的理解，可能表述不清，多看几遍，不行就debug跟一遍代码自然就懂了。
/**
* 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. 要搜索的源字符串
* @param sourceOffset offset of the source string. 源字符串偏移量即起始位置
* @param sourceCount count of the source string. 源字符串长度
* @param target the characters being searched for. 要搜索的目标字符串
* @param targetOffset offset of the target string. 目标字符串偏移量即起始位置
* @param targetCount count of the target string. 目标字符串长度
* @param fromIndex the index to begin searching from. 开始搜索位置
*/
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;
        }

        //开始位置
        char first = target[targetOffset];
        //要循环的最大次数
        int max = sourceOffset + (sourceCount - targetCount);

        for (int i = sourceOffset + fromIndex; i <= max; i++) {
            /* Look for first character. */
        	//找到第一个字符的位置，for循环嵌套while循环直到找到
            if (source[i] != first) {
                while (++i <= max && source[i] != first);
            }

            /* Found first character, now look at the rest of v2 */
            if (i <= max) {
            	//第二个字符
                int j = i + 1;
                int end = j + targetCount - 1;
                //第一个字符之后的字符是否也一一相等
                for (int k = targetOffset + 1; j < end && source[j]
                        == target[k]; j++, k++);

                //如果j=end则说明第一个字符之后的字符也一一相等
                
                //否则继续循环找下一个字符
                if (j == end) {
                    /* Found whole string. */
                    return i - sourceOffset;
                }
            }
        }
        return -1;
    }

3后续

亮点：
1、匹配首个目标字符。for循环中，嵌套while循环，从源字符串中，找到匹配目标字符的位置。
2、小范围匹配目标字符串。for循环中，嵌套for循环，在[目标字符串]长度范围内，遍历匹配，如果全部匹配，则return。