Assessment Test Question 9

Question List

Question: What is the result of running the following program?

  1 import java.util.*;

  2 public class Products {

  3   private class Item<T> {}

  4   Map<Map<String, String>, Map<String, Set<Item<String>>>> products = new HashMap<>();

  5   public static void main(String...args) {

  6     Products pds = new Products();

  7     pds.test();

  8   }

  9   private void test() {

 10     HashMap<String, String> a = new HashMap<>();

 11     a.put("1","2");

 12     HashMap<String, Set<Item<String>>> b = new HashMap<>();

 13     Set<Item<String>> l = new HashSet<>();

 14     l.add(new Item<String>());l.add(new Item<String>());

 15     b.put("sports",l);

 16     products.put(a,b);

 17     System.out.println(products.get(a).get("sports").size());

 18   }

 19 }

Choice:
A. compiler error at line 3
B. compiler error at line 4
C. compiler error at line 12
D. compiler error at line 13
E. compiler error at line 15
F. compiler error at line 17
G. code compiles but throw runtime exception
H. 2



Next Question

Answer: H

Explain: This is a legitimate generic code, no compiler error and runtime exceptions. the outermost Map, variable products has key and value. The key type is Map<String, String>, the value type is Map<String, Set<Item<String>>>.  Both the key and value themselves are Map. For the value Map, Map<String, Set<Item<String>>>, the key has type String, and the value has type Set<Item<String>>. The Set contains Item objects. The Item class takes type String as class variable.
The output is 2. The reasoning is as follows, at line 16, one entry was put into the products HashMap, the key is a and the value is b. At line 17, products.get(a) retries the value of that entry, which is HashMap b. At Line 15 one entry was put into b, the key is String "sports", the value is HashSet l. products.get(a).get("sports") retrieves the value HashSet l. At line 14, 2 entries were added into HashSet l. So, products.get(a).get("sports").size() returns 2.

Study:  Create and use a generic class