涉及 StringBuilder 的 Java replaceSubstring() 方法？

Question

A part of my assignment is to create a method that replaces all the occurences of string 2 that occur in string 1 with string 3. So if the sentence was : "the dog jumped over the fence", I'd want the method to replace all occurrences of whatever string 2 is which let's say is "the" with the content of string 3 which let's say is "that".

So I'd want it to say "that dog jumped over that fence".

This is really easy if my teacher professor allowed for a more convenient way, but this entire course is just inconvenient when it comes to learning, so I HAVE to use a StringBuilder object.

So far my code for replaceSubstring() is

   public static String replaceSubstring(String str1, String str2, String str3)
   {
      String str1Copy = str1, str2Copy = str2, str3Copy = str3;

      if (str2Copy.equals(str3Copy))
      {
         return str1Copy;
      }

      StringBuilder b = new StringBuilder(str1Copy);

      int index = b.indexOf(str2Copy);

      b.replace(index, (index + str2Copy.length()), str3Copy);

      index = b.indexOf(str3Copy);


      return b.toString();
   }

However I'm running into an issue, because when I run this code in an application class that prints out the return statement of this method, I get

After replacing "the" with "that", the string: that dog jumped over the fence

In my console. The original string is "the dog jumped over the fence", and my code should change it to "that dog jumped over that fence", however it just changes the first occurrence of "the" and not the second one. I'm really scratching my head over this because I understand how I could just do something like

return string1.replaceAll(string2, string3);

And call it a day, but I'd lose points for not doing it how my professor wants it which is by using a StringBuilder object. What am I missing here? Also, I can't import any packages created by someone else. I HAVE to use the generic and basic java kit.

EDIT: New code that seems to work

   public static String replaceSubstring(String str1, String str2, String str3)
   {
      String str1Copy = new String (str1), str2Copy = new String (str2), str3Copy = new String (str3);

      if (str2Copy.equals(str3Copy))
      {
         return str1Copy;
      }

      StringBuilder b = new StringBuilder(str1Copy);

      int index = b.indexOf(str2Copy);
      while (index != -1)
      {
         b.replace(index, (index + str2Copy.length()), str3Copy);
         index = b.indexOf(str2Copy, index + 1);
      }

      return b.toString();
   }

Answer 1

You need to loop while there are no more occurrences of str2 in str1 . indexOf() returns -1 if there are no more occurrences so you can use this to solve this problem. Inside the loop you use the overload indexOf(String str, int fromIndex) . Also you do not need the copies of the String 's:

public static String replaceSubstring(String str1, String str2, String str3)
{ 
    if (str2.equals(str3))
    {
        return str1;
    }

    StringBuilder b = new StringBuilder(str1);
    int index = b.indexOf(str2); 
    //Loop while there is still an occurrence of str2       
    while(index != -1) {
        b.replace(index, (index + str2.length()), str3);
        index = b.indexOf(str2, index+str3.length());
    }

    return b.toString();
}

---

The line:

index = b.indexOf(str2, index+str3.length());

Moves the search to past where we've already found an occurrence. On the first iteration we just call indexOf() without specifying the starting index, so it will start at the beginning of the String :

the dog jumped over the fence
^index points to the first occurrence of the (0)

Once we call indexOf() specifying the starting index as index + str3.length() , the starting index will be 0 + 4 so it will move the search to:

that dog jumped over the fence
    ^Start the search here.

To see why this is important if we replace the with tthe , without specifying the starting index it will look like this:

the dog jumped over the fence
^found first occurrence. Replace with tthe

Second iteration of loop:

tthe dog jumped over the fence
 ^Found another one! replace again.

Third iteration of loop:

ttthe dog jumped over the fence
  ^Another occurrence of the. 

Etc, etc.