Why should a Java class implement comparable?

Question

Why is Java Comparable used? Why would someone implement Comparable in a class? What is a real life example where you need to implement comparable

Answer 1

OK, but why not just define a compareTo() method without implementing comparable interface. For example a class City defined by its name and temperature and

public int compareTo(City theOther)
{
    if (this.temperature < theOther.temperature)
        return -1;
    else if (this.temperature > theOther.temperature)
        return 1;
    else
        return 0;
}