1Z0-819 Generics & Type Inference 3

Question 1

What is the var keyword available in Java 10 and Java 11?

Accepted Answer

A keyword for local variable type inference that lets the compiler determine the type

Answer

var is a reserved type name for local variable type inference; the compiler determines the variable's type from the right-hand side initializer expression at compile time.

Question 2

Which of the following var declarations is INVALID in Java 11?

Accepted Answer

var x;

Answer

var requires an initializer expression so the compiler can infer the type; declaring var x; without an initializer is not allowed because the type cannot be determined.

Question 3

Where can the var keyword NOT be used in Java 11?

Accepted Answer

As the declared type of an instance field in a class

Answer

var can only be used for local variable type inference; it cannot be used for instance fields, static fields, method parameters, or method return types.

Question 4

Why can you NOT instantiate a generic type parameter with new T() in Java?

Accepted Answer

Due to type erasure, T is not available at runtime so the JVM cannot know which class to instantiate

Answer

Because of type erasure, T is erased to Object (or its upper bound) in bytecode, so the JVM has no information about the actual class T represents when new T() would be executed.

Question 5

Which of the following correctly relies on type inference for a generic method call?

Accepted Answer

List<String> list = Collections.emptyList();

Answer

The compiler infers the type argument for Collections.emptyList() from the target type List<String> on the left-hand side, so no explicit type witness is needed.

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

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

1Z0-819 Generics & Type Inference 3