Now you saw what a class must do to avail itself of the sorting service—it must implement a compareTo method. That’s eminently reasonable. There needs to be some way for the sort method to compare objects. But why can’t the Employee class simply provide a compareTo method without implementing the Comparable interface?

The reason for interfaces is that the Java programming language is strongly typed. When making a method call, the compiler needs to be able to check that the method actually exists. Somewhere in the sort method will be statements like this:

[CODE]if(a[i].compareTo(a[j]) > 0)
{
// rearrange a[i] and a[j]
. . .
}[/CODE]

The compiler must know that a[i] actually has a compareTo method. If a is an array of Comparable objects, then the existence of the method is assured because every class that implements the Comparable interface must supply the method.

NOTE:
You would expect that the sort method in the Arrays class is defined to accept a Comparable[] array so that the compiler can complain if anyone ever calls sort with an array whose element type doesn’t implement the Comparable interface. Sadly, that is not the case. Instead, the sort method accepts an Object[] array and uses a clumsy cast:

[CODE]//Approach used in the standard library--not recommended
if (((Comparable) a[i]).compareTo(a[j]) > 0)
{
// rearrange a[i] and a[j]
. . .
}[/CODE]

If a[i] does not belong to a class that implements the Comparable interface, the virtual machine throws an exception.