OCP learning

Question 1

may an abstract class have methods or fields declared static?

Answer 1

Yes

an abstract class may have methods or fields declared static

Question 2

can the abstract keyword be applied to a field?

Answer 2

the abstract keyword can be applied to a class or a method but not to a field

–> no abstract variables

Question 3

an abstract class can be derived from a concrete class?

Answer 3

an abstract class can be derived from a concrete class! Although the language allows it, it is not a good idea to do so.

Question 4

wrapper classes –> final or not?

Answer 4

wrapper classes –> final class

Question 5

• which kind of class cannot be extended?

- which kind of method cannot be overridden?

Answer 5

final and static class / methods

Question 6

method of Final class –> final or not?

Answer 6

all methods of a final class are implicitly FINAL

Question 7

what is blank final?

Answer 7

If a variable declaration defines a variable as final but did not initialize it, then it is referred to as blank final.

You need to initialize a blank final all the constructors you have defined in the class; otherwise the compiler will complain.

Question 8

final parameters –> can be?

Answer 8

the keyword final can even be applied to parameters. The value of a final parameter cannot be changed once assigned.

Question 9

meaning of final keyword:

1. primitive
2. object

Answer 9

1. primitive –> value
2. object –> refers to the object reference, not its state.

–> can change the internal state of the passed final object, but you cannot change the reference itself.