Question

3.       What Collection Framework classes can and can't do. Which of the following things can be done by collection classes (List, Set, or Map)? If you can do it, outline how, mentioning relevant methods, else say that it is impossible and why.

1. A program is designed to have a TreeMap<Customer, String>, but Customer has no compareTo method. Is it possible to get the TreeMap to work without changing Customer.java?
2. You have a Map of Strings to Integers. You want to make a List of the key strings.

3. You have a Map of Strings to Integers. You now want to use the same (String,Integer) pairs to make a Map of Integers to Strings, the inverse map. Is this always possible?

Answer 1

Ans:

a) Yes , It is possible to get Treemap<Customer,String> to work by implementing compareTo() method inside an anonymous inner class .

eg: TreeMap<Customer,String> map = new TreeMap<>(new Comparator<Customer>() {

          public int compare(Customer C1 , Customer C2)

          {

               // Write comparison code here

           }

       });

b) Yes , It is possible to make a list of key strings from a map of string to integers by:

     List<String> list = map.keySet().stream().collect(Collectors.toList());

    It will make a list of Strings . For this we need to import java.util.stream.*;

c) No , It is not always possible because for a map the key should be unique but values may repeat . So , while reversing the map the repeated values will act as key which will produce error .