Hardware Serial Receive (ReadString + indexOf)

void setup()
{
Serial1.begin(9600);
}

void loop()
{
checkCondition();
}


void checkCondition()
{
   String s1 = Serial1.readString();// s1 is String type variable.
//display same received Data back in serial monitor.
    if(s1.indexOf("a")>=0)
    {
            Serial1.println("a");
    }
    if(s1.indexOf("b")>=0){

        Serial1.print("b");
    }

}