SCJP5 Study Guide Q2. Page 611/622 -- Generics and Collections

[Ravinder Singh]

Dear ranchers,

I have a query in the following question that I have customized a bit for testing purpose:

Given:-

import java.util.*;
 
class ArrayListTest {
public static void main(String[] args) {
// INSERT DECLARATION HERE
for (int i = 0; i <= 10; i++) {
List<Integer> row = new ArrayList<Integer>();
for (int j = 0; j <= 10; j++)
row.add(i * j);
table.add(row);
}
for (List<Integer> row : table)
System.out.println(row);
}
}

Which statements could be inserted at // INSERT DECLARATION HERE to allow this code to compile and run? (Choose all that apply.)

A. List<List<Integer>> table = new List<List<Integer>>();
B. List<List<Integer>> table = new ArrayList<List<Integer>>();
C. List<List<Integer>> table = new ArrayList<ArrayList<Integer>>();
D. List<List, Integer> table = new List<List, Integer>();
E. List<List, Integer> table = new ArrayList<List, Integer>();
F. List<List, Integer> table = new ArrayList<ArrayList, Integer>();
G. None of the above.

Correct Answer is B.

What if I modify option C a bit to make it look like the following ..and try to recompile and run. But it doesn't compile, why? What is wrong with the type declaration here?

List<ArrayList<Integer>> table = new ArrayList<ArrayList<Integer>>();

-------------
Ravinder

[Vijay Raj]

By saying, table.add(row), what you are actually inserting is a List of Integers. Here you cannot add an ArrayList of Integers because you cannot say that a super-class is same as a subclass.

So,

List<ArrayList<Integer>> table = new ArrayList<ArrayList<Integer>>();

will not work. It'll work if you explicitely cast it to an ArrayList.

  table.add((ArrayList) row);  

regards,
vijay.