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

Dear ranchers,

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

Given:-


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>>();

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Ravinder

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.



regards,
vijay.