How does new Comparator() work?

Question

I have this code segment in my program. It works fine, but I don't know how exactly it works.
Collections.binarySearch(vertexList, new Integer(vertexID),
new Comparator() {
public int compare(Object obj, Object key) {
PDGVertex vertex = (PDGVertex)obj;
Integer vertexID = (Integer)key;
if (vertex.id < vertexID.intValue())
return -1;
else if (vertex.id > vertexID.intValue())
return 1;
else
return 0;
I know that Comparator() has two methods, compare() and equals(). But in the code segment above, I only implemented one, compare(), in the new Comparator() body. Don't I have to implement both of the two methods?
To test it, I wrote the following test program.
interface TestInter {
void tt();
void jj();
class Test {
static TestInter testInter = new TestInter() {public void tt() {System.out.println("In tt");}};
public static void main(String args[]) {
testInter.tt();
When I compiled it, I saw the following error message:
Test.java:7: <anonymous Test$1> is not abstract and does not override abstract method jj() in TestInter
static TestInter testInter = new TestInter() {public void tt() {System.out.println("In tt");}};
^
1 error
Any thoughts? Thanks.

Answer 1

The compare method is for comparing two objects of a particular type--two Employees, two Orders, whatever. That's the Comparator's reason to live.
The equals method is just like it is on any other object---it's to compare this Comparator to another Comparator. I believe the docs say that it's always safe to leave it as the default Object implementation which is true only if the two references point to the same Comparator object.
If you do decide to override it, it's so that you can determine that two Comparators will return the same results. I suppose you might want to do this if you know that two different implementations will return the same results (maybe the implement the same rules, but one is optimized for certain kinds of data and the other for other kinds) and you want to avoid repeating a sort that's already been done. For what it's worth, I've never needed to override equals on a Comparator.