The replace(int start, int end, String str) method of StringBuilder is used to replace a specified range of characters with a given string. It modifies the existing StringBuilder object directly, making it efficient for frequent updates to strings.

class Main {
    public static void main(String[] args) {

        StringBuilder sb = new StringBuilder("Hello World");
        sb.replace(6, 11, "Java");

        System.out.println(sb);
    }
}

Hello Java

Explanation:

• A StringBuilder object is created with the initial value "Hello World".
• The replace(6, 11, "Java") method replaces the characters from index 6 to 10 ("World") with "Java".
• The modified StringBuilder is printed, showing the updated string "Hello Java".

Syntax

public StringBuilder replace(int start, int end, String str)

Parameters: This method accepts three parameters:

• start - Starting index (inclusive)
• end - Ending index (exclusive)
• str - String to replace the specified range

Return Value: This method returns StringBuilder object after successful replacement of characters. 

Note:

• The character at index start is included, while the character at index end is excluded.
• If start == end, the string is inserted at that position.

Examples of StringBuilder replace() Method

Example 1: Basic Replacement

class GFG {
    public static void main(String[] args) {

        StringBuilder sb = new StringBuilder("WelcomeGeeks");

        System.out.println("Original: " + sb);

        sb.replace(1, 7, "e are ");

        System.out.println("After: " + sb);
    }
}

Original: WelcomeGeeks
After: We are Geeks

Explanation: Characters from index 1 to 6 are replaced with "e are ".

Example 2: Replacing a Substring in the Middle

class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("I like Python");
        sb.replace(7, 13, "Java");
        System.out.println(sb);
    }
}

I like Java

Explanation: The replace(7, 13, "Java") call replaces the substring "Python" with "Java" in the middle of the StringBuilder.

Example 3: Inserting Using replace()

class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Programming");
        sb.replace(0, 0, "Java ");
        System.out.println(sb);
    }
}

Java Programming

Explanation: When start and end are the same, the string is inserted at that position.

Example 4: Replacing Entire String

class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Old Value");
        sb.replace(0, sb.length(), "New Value");
        System.out.println(sb);
    }
}

New Value

Explanation: It replaces all characters from index 0 to the length of the StringBuilder, effectively replacing the entire string with "New Value".

Example 5: Invalid Index Handling

class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Example");
        sb.replace(3, 20, "Test");
    }
}

Output:

StringIndexOutOfBoundsException

Explanation: The end index exceeds the length of the StringBuilder.