SCJP Java Generics, Enums, and Autoboxing 2

Question 1

What does a wildcard `?` represent in Java generics?

Accepted Answer

Any unknown type

Answer

The wildcard `?` in generics represents an unknown type, used when you don't know or care about the specific type argument.

Question 2

What is the difference between `List` and `List` in Java?

Accepted Answer

extends is for reading (producer); super is for writing (consumer)

Answer

The PECS principle: `` (Producer Extends) is used for reading elements; `` (Consumer Super) is used for writing elements.

Question 3

Which enum method returns the name of the enum constant as a String?

Accepted Answer

name()

Answer

The `name()` method returns the exact name of the enum constant as declared in the source code.

Question 4

Can Java enums have constructors?

Accepted Answer

Yes, but enum constructors are always private

Answer

Enum constructors are always `private` (implicitly or explicitly) because enum constants are created by the JVM, not by external code.

Question 5

What happens when you compare two Integer wrapper objects with `==` in Java?

Accepted Answer

Compares object references, not values (except for cached range -128 to 127)

Answer

`==` on Integer objects compares references; however, Integer caches values from -128 to 127, so `==` may return true for small values due to the shared cache.

SCJP - Sun Certified Java Programmer Practice Test

SCJP - Sun Certified Java Programmer Practice Test

SCJP Java Generics, Enums, and Autoboxing 2