Using if statements in public String toString - java

I am very new to java, and I am trying to get my method to return "yes" or "no" depending on whether another method returns True or False. My code below:

public String toString () {
    return "The number of cars currently in the queue is : " + numberOfCars ;
    if(isFull()) {
        return "yes";
    } else {
        return"no";
    }

When I run this, I get an error: unreachable statement. What is cauing this?