SCJP Java Generics, Enums, and Autoboxing 4

Question 1

Which of the following correctly declares a bounded wildcard that accepts a List of any type that is a supertype of Integer?

Accepted Answer

List<? super Integer>

Answer

The lower-bounded wildcard `? super Integer` accepts Integer and any of its supertypes (Number, Object).

Question 2

What is the result of unboxing a null Integer reference?

Accepted Answer

Throws NullPointerException

Answer

Unboxing a null wrapper object throws a NullPointerException at runtime.

Question 3

Given `enum Planet { MERCURY, VENUS, EARTH; }`, what does `Planet.values()` return?

Accepted Answer

An array Planet[] of all constants

Answer

`values()` is a compiler-generated static method that returns a Planet[] array containing all enum constants.

Question 4

Which statement about generic type erasure is TRUE?

Accepted Answer

Type parameters are replaced by Object or their bound at compile time

Answer

Type erasure replaces unbounded type parameters with Object and bounded ones with their upper bound in bytecode.

Question 5

What is printed by: `Integer a = 127; Integer b = 127; System.out.println(a == b);`?

Accepted Answer

true

Answer

Integer values between -128 and 127 are cached, so autoboxed instances with the same value share the same reference.

SCJP - Sun Certified Java Programmer Practice Test