SCJP Java Generics, Enums, and Autoboxing 5

Question 1

An enum constant can have a body with an abstract method override. What must the enum declaration itself contain for this to compile?

Accepted Answer

The abstract method must be declared abstract in the enum body

Answer

When a constant overrides a method with its own body, the enum must declare that method as abstract so each constant provides an implementation.

Question 2

What does `Collections.unmodifiableList(list)` return when passed a `List<String>`, in terms of generics?

Accepted Answer

List<String>

Answer

`unmodifiableList` is generic and preserves the type parameter, returning `List` when given a `List`.

Question 3

What is the result of: `Integer x = 200; Integer y = 200; System.out.println(x == y);`?

Accepted Answer

false

Answer

Values outside the -128 to 127 cache range create new Integer objects, so == compares references and returns false.

Question 4

Which of the following is a valid way to restrict a generic method to only accept subtypes of Number?

Accepted Answer

<T extends Number> void method(T t)

Answer

`<T extends Number>` is a bounded type parameter that restricts T to Number and its subclasses.

Question 5

Which enum method returns the zero-based position of a constant in its declaration order?

Accepted Answer

ordinal()

Answer

`ordinal()` returns the ordinal (zero-based declaration position) of the enum constant.

SCJP - Sun Certified Java Programmer Practice Test