Mock Exam on Collections & Generics - 3

1)
Collection c = new ArrayList();
....//line 1

Which line inserted independtly at line 1 will allow to compile?

a)c.add(new Object());

b)c.add("Java");

c)c.add(null);

d)All 3 lines above inserted independtely will allow to compile

e)All 3 lines above inserted independtely will not allow to compile


Answer c)

a)-->we don’t know what the element type c will be,So we cannot add Object to it.

b)-->we don’t know what the element type c will be,So we cannot add String to it.

c)-->null is allowed because it is member of every type.

d)-->Because of option a & b, d also wrong.

e)-->Because of option c, e also wrong.


2) Which line(s) is free from compile time error?

Assume that Parent is a Base class and Child is a Derived class

List alList = new ArrayList(); //line 1
List alList = new ArrayList(); //line 2
List alList = new ArrayList(); //line 3
List alList = new ArrayList(); //line 4
List alList = new ArrayList();  //line 5

a)only 1

b)only 4

c)only 5

d)2 and 3

e)1 and 4


Answer c)

a)-->Reference can hold only Parent class and its super types.

b)-->Reference can hold only Child class and its sub types.

c)-->Wild card types are applicable for references and it can accept anytype parameter object assign to it

d)-->We cannot use wildcard notation in the object creation.

e)-->line 1 failed because Reference can hold only Parent class & its super types and
   line 2 failed because Reference can hold only Child class & its sub types.


3)
public static  void addandDisp(Collection cs, T t)
{
 for (T o : cs)
 {
  s.add(o);
 }
 for (T o : cs)
 { 
  System.out.println(o);
 }
}

 //call 1
 List ls1 = new LinkedList