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1. Introduction

In this quick tutorial, we’ll demonstrate how to replace a character at a specific index in a String in Java.

We’ll present four implementations of simple methods that take the original String, a character, and the index where we need to replace it.

2. Using a Character Array

Let's begin with a simple approach, using an array of char.

Here, the idea is to convert the String to char[] and then assign the new char at the given index. Finally, we construct the desired String from that array.

public String replaceCharUsingCharArray(String str, char ch, int index) {
    char[] chars = str.toCharArray();
    chars[index] = ch;
    return String.valueOf(chars);
}

This is a low-level design approach and gives us a lot of flexibility.

3. Using the substring Method

A higher-level approach is to use the substring() method of the String class.

It will create a new String by concatenating the substring of the original String before the index with the new character and substring of the original String after the index:

public String replaceChar(String str, char ch, int index) {
    return str.substring(0, index) + ch + str.substring(index+1);
}

4. Using StringBuilder

We can get the same effect by using StringBuilder. We can replace the character at a specific index using the method setCharAt():

public String replaceChar(String str, char ch, int index) {
    StringBuilder myString = new StringBuilder(str);
    myString.setCharAt(index, ch);
    return myString.toString();
}

5. Conclusion

In this article, we focused on several ways of replacing a character at a specific index in a String using Java.

String instances are immutable, so we need to create a new string or use StringBuilder to give us some mutability.

As usual, the complete source code for the above tutorial is available over on GitHub.

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Matthew Cory
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Matthew Cory

Maybe I’m missing something, but doesn’t it matter whether or not StringBuilder is thread safe in this example? It’s not shared anywhere outside the method body, and so multiple threads calling the method would be working against different instances.

Eric Martin
Member
Eric Martin

You are correct. The article has been fixed.

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