Java查找指定字符串第一次或最后一次出现的位置

思路

1. 将输入的字符串转换为char 数组

2. 从第一个字符位置开始比较，并记录匹配的次数，如果匹配的次数等于查找字符串的长度就停止检索

3. 如果找不到匹配字符串的位置返回-1

/**
* Returns the index within the given input string of the first occurrence
* of the specified substring.
*
* @param input
* @param query
*/
public static void getFirstMatchingIndex(String input, String query) {
char[] inputChars = input.toCharArray();
char[] queryChars = query.toCharArray();
int inputLength = input.length();
int queryLength = query.length();

int inputIndex = 0;
int queryIndex = 0;
while (inputIndex < inputLength && queryIndex < queryLength) {
if (inputChars[inputIndex] == queryChars[queryIndex]) {
queryIndex++;
inputIndex++;
} else {
inputIndex = inputIndex - queryIndex + 1;
queryIndex = 0;
}
}

int index = queryIndex == queryLength ? (queryLength > 1 ? inputIndex - queryLength : inputIndex - 1) : -1;
System.out.println("first matching index:" + index);
}

/**
* Returns the index within this string of the rightmost occurrence of the 
* specified substring.
*
* @param input
* @param query
*/
public static void getLastMatchingIndex(String input, String query) {
char[] inputChars = input.toCharArray();
char[] queryChars = query.toCharArray();
int inputLength = input.length();
int queryLength = query.length();
int inputIndex = inputLength - 1;
int queryIndex = queryLength - 1;
int matchingLenght = 0;
while (inputIndex >= 0 && queryIndex >= 0) {
if (inputChars[inputIndex] == queryChars[queryIndex]) {
inputIndex--;
queryIndex--;
matchingLenght++;
} else {
inputIndex = matchingLenght <= 0 ? (inputIndex - 1) : inputIndex;
queryIndex = queryLength - 1;
matchingLenght = 0;
}
System.out.println("i:" + inputIndex + ",q:" + queryIndex);
}
System.out.println("last matching index:" + (matchingLenght == queryLength ? (inputIndex + 1) : -1));
}
测试

getFirstMatchingIndex("ssssdeffsdfsdf", "sdf");

----->

first matching index:8

getFirstMatchingIndex("ssssdeffsdfsdf", "sdf");

--->