About Question enthuware.ocpjp.v7.2.1307 :

[icepeanuts]

According to API (http://docs.oracle.com/javase/7/docs/ap ... rator.html), Comparator has 2 methods. But this one only has the implementation of compare(). I understand the equals() of this interface is equal to Object's equals(), but not 100% sure if this class should implement its own equals() as it is defined in the interface.

[admin]

All classes get equals and hashCode methods from Object class. So it is not necessary to define them in every class.
But yes, it is a good idea to implement both to ensure consistency of equals as well as compare().
HTH,
Paul.

[profnotime]

Note that this guarantees that the return value will be >= 0 if and only if the key is found.

Why then does

Code: Select all

Arrays.binarySearch(sa, "c", new MyStringComparator()); 

return 0 when "c" is not in the array of items?

Answer:
This is because the comparator compares length, not actual values.
Therefore, the position for an item with length one is zero.
Replacing "c" with any one length string will yield the same result e.g. "q", "=", "+"

I hope this saves someone a few minutes

[Venceslas]

Hello,

I was in doubt about this because sort has not been called before binary search. Maybe it will worth the effort to add this call in the example?

[admin]

It is not necessary to call sort if you know that the list is already sorted.

HTH,
Paul.

[Venceslas]

Humm, actually I'm not sure the list is sorted.

Indeed:

String[] sa = { "d", "bbb", "aaaa" };
Arrays.sort(sa);
System.out.format("%s %s %s%n", sa[0],sa[1],sa[2] ); => aaaa bbb d

Did I miss something please?

[admin]

For the binarySearch method to work, the list has to be sorted as per the Comparator that is being used to search.

[Venceslas]

Oups, thank you.

[Sergiy Romankov]

Could you please explain?

I believed that if there no such element in array the return value must be less than 0, but
in this case :

Arrays.binarySearch(sa, "c", new MyStringComparator());

return value is 0.

Yes Comparator compares length of Strings, but shouldn`t it be negative ?

[admin]

You are right. If there is no such element in the array the return value must be less than 0.
But "c" is there in the array sa, as per MyStringComparator.