如何在不使用.replace（）的情况下替换字符串中的某些字符？

Question

I have to write a programme that encrypts and decrypts hidden messages. My problem is changing the alphabet. Originally I was going to use .replace() method, however, my teacher said we're not allowed to. The only methods we are allowed to do are,

.indexof();
.length();
.substring();

I have no idea how I'm supposed to do this. For example, Apple would be &**$# but how can I do this without .replace();

Answer 1

Below is the idea, can be optimised by yourself :

public String replace(String original, String toBeReplacedStr, String withStr) {
    while(true) {
      int i = original.indexOf(toBeReplacedStr);
      if (i == -1) {
         break;
      }
      original = original.substring(0, i) + withStr + original.substring(i + toBeReplacedStr.length());
    }
    return original;
}

Or can use the StringBuilder with recursion:

public String replace(String original, String toBeReplacedStr, String withStr) {
    int i = original.indexOf(toBeReplacedStr);
    if (i < 0) {
      return original;
    }
    StringBuilder sb = new StringBuilder();
    String before = original.substring(0, i);

    String rawAfter  = original.substring(i + toBeReplacedStr.length());
    String replacedAfter = replace(rawAfter, toBeReplacedStr, withStr);
    return sb.append(before).append(withStr).append(replacedAfter).toString();
}

Answer 2

I suggest you read source code of String.replace(char oldChar, char newChar) to implement your own method.

public String replace(char oldChar, char newChar) {
        if (oldChar != newChar) {
            int len = value.length;
            int i = -1;
            char[] val = value; /* avoid getfield opcode */

            while (++i < len) {
                if (val[i] == oldChar) { // comment 1, aims to find 
          // first index of oldChar. You can use indexOf() to achieve this.
                    break;
                }
            }
            if (i < len) {
                char buf[] = new char[len];
                for (int j = 0; j < i; j++) {
                    buf[j] = val[j]; // comment 2, copy prefix chars 
          // before the oldChar to buf. You can use subString() to achieve this.
                }
                while (i < len) {
                    char c = val[i];
                    buf[i] = (c == oldChar) ? newChar : c; // comment 3, 
        // replace oldChar with newChar, for other chars, just copy. 
        // You can use the thought above. I suggest you divide to 3 
        // methods to do "replace" thing.
                    i++;
                }
                return new String(buf, true);
            }
        }
        return this;
    }