Using multiple interfaces to define a generic type

[L K Kakani]

Hi all,

I was solving the mock test from Khalid Mughal and came across this question

  class Interval<________________________> { // (1) INSERT TYPE CONSTRAINT HERE
private N lower, upper;
public void update(N value) {
if (lower == null || value.compareTo(lower) < 0)
lower = value;
if (upper == null || value.compareTo(upper) > 0)
upper = value;
}
}

Which type constraints, when inserted at (1), will allow the class to compile?
Select the four correct answers.
(a) N extends Object
(b) N extends Comparable<N>
(c) N extends Object & Comparable<N>
(d) N extends Number
(e) N extends Number & Comparable<N>
(f) N extends Comparable<N> & Number
(g) N extends Integer
(h) N extends Integer & Comparable<N>


Ans: (b),(c),(e) and (g).

I did not come across a combination generic type like this before and just wanted to know if there are any rules in how to define such generic types.

For example can we say <N> Number<N> & Comparable<N> instead of N extends Number & Comparable<N> ?

Please explain the specifics of this kind of implementation.

Thanks,
Kakani

[Wouter Oet]

Well it's just a case of trying. Look for what the method needs. This method needs an Object with the method compareTo thus we need an class that implements Comparable<N>. I'll try to explain by example:

A: N always extends Object but does not implement Comparable<N>
B: Correct
C: N extends Object is implicit but correct because it implements Comparable<N>
D: Incorrect because Number doesn't implement Comparable
E: Correct
F: Incorrect because in the generic definition the concrete classes must come first and then the interface. It would be correct is it was N extends Number & Comparable<N>
G: Correct because Integer implements Comparable
H: Incorrect because "java.lang.Comparable cannot be inherited with different arguments: <N> and <java.lang.Integer>". Basically it means that because Integer already defines Comparable you can't define it again.

[L K Kakani]

So if we are using more than one class/interface to describe a generic type the concrete class must come first and then the interface. I guess I need to go through generics once more. Thank you Wouter.