Collections.sort() and reverse()

[Lucky J Verma]

Can anybody help me explaning ,
difference between answer B and C
Both give right answer according to my unserstanding.
Correct answer is C .
.
Question 189
Given:
ArrayList a = new ArrayList();
containing the values {�1�, �2�, �3�, �4�, �5�, �6�, �7�, �8�}
Which code will return 2?
A. Collections. sort(a, a.reverse());
int result = Collections.binarySearch(a, �6�);
----------------------------------------------------
B. Comparator c = Collections.reverseOrder();
Collections.sort(a, c);
int result = Collections.binarySearch(a, �6�);

C. Comparator c = Collections.reverseOrder();
Collections.sort(a, c);
int result = Collections.binarySearch(a, �6�,c);what is purpose of c here in method arguments
-------------------------------------------------
D. Comparator c = Collections.reverseOrder(a);
Collections.sort(a, c);
int result = Collections.binarySearch(a, �6�,c);
E. Comparator c = new InverseComparator(new Comparator());
Collections.sort(a);
int result = Collections.binarySearch(a, �6�,c);

[Meera Nanda]

Hi Lucky,

  Collections. sort(a, a.reverse());
int result = Collections.binarySearch(a, "6");
  

Option A is incorrect since there is no a.reverse() method.

  Comparator c = Collections.reverseOrder(a);
Collections.sort(a, c);
int result = Collections.binarySearch(a, "6",c);  

Option D also is incorrect since there is no Collections.reverseOrder(Arraylist a) method.

 Comparator c = new InverseComparator(new Comparator());
Collections.sort(a);
int result = Collections.binarySearch(a, "6",c);    

Option E is incorrect there is no class or interface called InverseComparator and also Comparator class is an abstract class which cannot be instantiated.

Can anybody help me explaning ,
difference between answer B and C

 Comparator c = Collections.reverseOrder();
Collections.sort(a, c);
int result = Collections.binarySearch(a, "6");
   

The above code compiles fine , but gives a vague result which cannot be guaranteed.In this case sometimes would be values such as -9,-10 etc.

  Comparator c = Collections.reverseOrder();
Collections.sort(a, c);
int result = Collections.binarySearch(a, "6",c);   

This code compiles fine and gives the result 2.

You could also have a look at the two binarySearch methods of Collections class inJava API which tells in detail about the differences of the 2 methods.

I hope this helps you.

Thank you
Meera



[ September 24, 2007: Message edited by: Meera Nanda ]
[ September 24, 2007: Message edited by: Meera Nanda ]

[ahmed yehia]

    
Comparator c = Collections.reverseOrder();
Collections.sort(a, c);
int result = Collections.binarySearch(a, "6",c); //what is purpose of c here in method arguments

Here you are sorting the collection using a Comparator, so you must use the same Comparator in the search method or you wont get accurate results.