The substring() method of the StringBuilder class is used to extract a portion of characters from an existing character sequence and return it as a new String. It always returns a new String object and does not modify the original StringBuilder. Indexing is zero-based, and passing invalid index values results in runtime exceptions.

Example.

class GFG {
    public static void main(String[] args)
    {
        StringBuilder str= new StringBuilder("GeeksForGeeks");  // string
        System.out.println("Substring = "+ str.substring(5));
    }
}

Substring = ForGeeks

Explanation: Index 5 points to the character 'F'. All characters from index 5 to the end are returned as a new String.

Syntax

public String substring(int start, int end)

Parameter:

• start: starting index (inclusive)
• end (optional): ending index (exclusive)

Return Type: String

Example 1. Extracting a Substring Starting from a Specific Index

class GFG {
    public static void main(String[] args) {
        StringBuilder str = new StringBuilder("GeeksForGeeks");
        System.out.println("Substring = " + str.substring(5));
    }
}

Substring = ForGeeks

Explanation:

• In the code, substring(5) is called on the StringBuilder object str, where index 5 corresponds to the character 'F'.
• All characters from index 5 up to the end of the sequence are extracted and returned as a new String, while the original StringBuilder remains unchanged.

Example 2. Extracting a Substring Using Start and End Index

class GFG{
    public static void main(String[] args){
        StringBuilder sb = new StringBuilder("GeeksForGeeks");
        System.out.println(sb.substring(5, 8));
    }
}

For

Explanation:

• In the statement sb.substring(5, 8), index 5 is the starting position (inclusive), pointing to the character 'F'.
• Index 8 is the ending position (exclusive), so characters at indices 5, 6, and 7 are included in the result.
• The extracted substring "For" is returned as a new String, and the original StringBuilder sb remains unchanged.

Handling Invalid Index Values

Passing a negative index or an index greater than the length of the StringBuilder causes a StringIndexOutOfBoundsException at runtime.

class GFG{
    
    public static void main(String[] args){
        StringBuilder sb = new StringBuilder("JavaDeveloper");
        try {
            sb.substring(-3);
        } 
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

java.lang.StringIndexOutOfBoundsException: Range [-3, 13) out of bounds for length 13

Explanation: Negative start index is invalid. The method throws "StringIndexOutOfBoundsException" at runtime.