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problem

 
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1)Collection<?> c = new ArrayList<String>();
....//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)

but it is unkown type then how it will be add null
 
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Quoting from Gilad Bracha's article

Generics in the Java Programming Language



Collection<?> c = new ArrayList<String>();
c.add(new Object()); // compile time error
Since we don�t know what the element type of c stands for, we cannot add objects
to it. The add() method takes arguments of type E, the element type of the collection.
When the actual type parameter is ?, it stands for some unknown type. Any parameter
we pass to add would have to be a subtype of this unknown type. Since we don�t know
what type that is, we cannot pass anything in. The sole exception is null, which is a
member of every type.


[ August 27, 2007: Message edited by: Ram Chandramouli ]
 
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Ram is wright.

Give a look to Gilad Bracha's article
 
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