1Z0-819 Generics & Type Inference 2

Question 1

What is the PECS principle in Java generics?

Accepted Answer

Producer Extends, Consumer Super

Answer

PECS stands for Producer Extends, Consumer Super: use when reading (producing) from a structure, and when writing (consuming) to a structure.

Question 2

Which operation is valid on a variable declared as List<? extends Number>?

Accepted Answer

Reading elements and treating them as Number type

Answer

With an upper-bounded wildcard <? extends Number>, you can read elements as Number or its supertype, but cannot add any elements because the exact subtype is unknown to the compiler.

Question 3

Which operation is valid on a variable declared as List<? super Integer>?

Accepted Answer

Adding an Integer or any of its subtypes to the list

Answer

With a lower-bounded wildcard <? super Integer>, you can safely add Integer values (or subtypes) to the list, but reading only returns Object.

Question 4

Which of the following correctly declares a type parameter with multiple bounds?

Accepted Answer

public class MyClass>

Answer

Multiple bounds use the & separator between bounds, and all bounds must use the extends keyword — even for interfaces — as in >.

Question 5

Why does the following code NOT compile? List nums = new ArrayList();

Accepted Answer

Generic types in Java are invariant, so List is not a subtype of List

Answer

Java generics are invariant, meaning List is not a subtype of List even though Integer is a subtype of Number; this prevents heap pollution.

(1Z0-819) Oracle Java SE 11 Developer Practice Test

(1Z0-819) Oracle Java SE 11 Developer Practice Test

1Z0-819 Generics & Type Inference 2