Replace part of a string between indexes in Java [closed]

Try below code. With help of StringBuffer you do replacement. StringBuffer replace java docs

public static void main(String[] args) {

        StringBuffer buf = new StringBuffer("123456789");

        int start = 3;
        int end = 6;
        buf.replace(start, end, "foobar"); 
        System.out.println(buf);

    }

}

After running above program below will be out put.

123foobar789

Demo