# SCJP Objective 6.5 Question

karl czukoski

Greenhorn

Posts: 22

karl czukoski

Greenhorn

Posts: 22

dennis deems

Ranch Hand

Posts: 808

posted 5 years ago

Remember the contract of the Comparator interface:

compare returns a zero if the first argument is equal to the second.

compare returns a negative integer if the first argument is less than the second

compare returns a positive integer if the first argument is greater than the second.

This language could be more explicit, but "a less than b" means that

a comes before b in the sort order. And "a greater than b" means that

a comes after b in the sort order.

Comparing numerical arguments is simple; we can do arithmetic on the arguments.

For an ascending sort we can return (first argument) - (second argument).

We could implement a comparator for Integers like this:

Suppose we are sorting Integers in ascending order, and we call compare(3, 7).

Our comparator subtracts 7 from 3 and returns the result -4.

This negative value tells the sort routine calling compare that argument a comes before argument b.

The sort routine doesn't care about the actual value -- just whether it is negative, zero, or positive.

If we call compare(6, 5), the comparator returns the value 1, telling us that 6 comes after 5.

If we want to reverse the sort order, to perform a descending sort, all we need do is swap the arguments:

Now the same call compare(3, 7) will subtract 3 from 7 and return 4,

and signify that 3 comes

Likewise compare(6, 5) now returns -1, meaning that 6 comes

All the values returned from the new implementation

are the inverse of those returned by the original implementation.

Note that in either implementation, a call like compare(5, 5) will return zero.

compare returns a zero if the first argument is equal to the second.

compare returns a negative integer if the first argument is less than the second

compare returns a positive integer if the first argument is greater than the second.

This language could be more explicit, but "a less than b" means that

a comes before b in the sort order. And "a greater than b" means that

a comes after b in the sort order.

Comparing numerical arguments is simple; we can do arithmetic on the arguments.

For an ascending sort we can return (first argument) - (second argument).

We could implement a comparator for Integers like this:

Suppose we are sorting Integers in ascending order, and we call compare(3, 7).

Our comparator subtracts 7 from 3 and returns the result -4.

This negative value tells the sort routine calling compare that argument a comes before argument b.

The sort routine doesn't care about the actual value -- just whether it is negative, zero, or positive.

If we call compare(6, 5), the comparator returns the value 1, telling us that 6 comes after 5.

If we want to reverse the sort order, to perform a descending sort, all we need do is swap the arguments:

Now the same call compare(3, 7) will subtract 3 from 7 and return 4,

and signify that 3 comes

*after*7 in the sort order.Likewise compare(6, 5) now returns -1, meaning that 6 comes

*before*5.All the values returned from the new implementation

are the inverse of those returned by the original implementation.

Note that in either implementation, a call like compare(5, 5) will return zero.