Program:

class Main {
    public static void main(String[] args) {
            String str = "newBay", replace="new";
            String replaceTo = "programmer";

        int position =str.indexOf(replace);
        int len = replace.length();
        String new_string = str.substring(0, position) + replaceTo +
                str.substring(position+len);
        System.out.println(new_string);
    }
}

Output:

ProgrammerBay

Explanation

• Have a string str containing ‘newBay’
• In the above code, we need to replace  substring ‘new’ present in ‘newBay’ with ‘programmer’
• Storing the occurring position of ‘replace’ string and its length too
• Since, the position of ‘new’, we got is 0 , therefore, str.substring(0,position) would extract nothing
• replaceTo concatenated with ‘ ‘ subtring extracted by  str.substring(0,position)
• str.substring(position+len) extracted ‘Bay’ from ‘newBay’. Now after concatenating, we got ‘ProgrammerBay’