Question

Say I have the string 123456789. I know that I want to replace indexes 3-5 of the string. (4, 5, and 6). Keep in mind that the replacement probably isn’t 3 characters, I might want to end up with 123foobar789.

What’s the best way to do this?

Answer 1

Firstly, you cannot do it¹, since a String is immutable in java.

However, you can create a new String object with the desired value, using the String#substring() method (for example):

String s = "123456789";
String newString = s.substring(0, 3) + "foobar" + s.substring(3+3);
System.out.println(newString);

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If you do want to achieve it efficiently, you could avoid creating some intermediate strings used by the concatinating and substring() method.

String s = "123456789";
StringBuilder sb = new StringBuilder();
char[] buff = s.toCharArray();
sb.append(buff , 0, 3).append("foobar");
sb.append(buff,3+3,buff.length -(3+3));
System.out.println(sb.toString());

However, if it is not done in a very tight loop – you should probably ignore it, and stick with the first and more readable solution.

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(1) not easily anyway, it can be done with reflection – but it should be avoided.