1Z0-811 Object-Oriented Programming Practice Test 3 — 15 Questions

Question 1

Refer to the exhibit. What is the potential issue with this singleton implementation in a multithreaded environment?

Accepted Answer

Not thread-safe; two threads could simultaneously create different instances. The if-check and instantiation are not atomic. Two threads could both see instance == null and create separate instances, violating the singleton guarantee.

Answer

The getInstance() method should be non-static

Answer

Compilation error because the constructor is private

Answer

Memory leak because instances are never garbage collected

Question 2

A developer wants to ensure that a class cannot be subclassed. Which keyword should be used?

Accepted Answer

final. The `final` keyword prevents a class from being subclassed. When a class is declared `final`, the compiler enforces that no other class can extend it, ensuring its implementation cannot be overridden or inherited. This is the correct mechanism in Java to prevent subclassing.

Answer

protected

Answer

private

Answer

abstract

Answer

static

Question 3

A class has a method that is marked as protected. Which statement is true about its accessibility?

Accepted Answer

Within the same package and subclasses. In Java, the protected access modifier allows access within the same package and by subclasses (even if they are in different packages). Option E correctly states this combination, which is the precise definition of protected access as specified in the Java Language Specification (JLS §6.6.2).

Answer

Only in subclasses

Answer

Only within the same package

Answer

Everywhere

Answer

Only within the same class

Question 4

Consider the following interface: public interface Drawable { void draw(); } A developer implements Drawable in class Circle. Which statement about the implementation is correct?

Accepted Answer

The draw method must be declared public. Interface methods are implicitly public and abstract unless declared as default or static. In this interface, draw() is abstract, so it cannot have a body. Therefore, the implementing class must declare draw() as public (making A correct). Option D is false because the interface method has no body. Option C is false because Circle provides an implementation, so the condition for being abstract is not met. Thus, only A is correct, making B incorrect.

Answer

Both A and D

Answer

The class must be abstract if it does not provide body for draw

Answer

The draw method can have a body in the interface

Answer

The draw method must be declared final

Question 5

Which access modifier allows a member to be accessed only within the same class?

Accepted Answer

private. Private members are accessible only within the same class.

Answer

static

Answer

public

Answer

default (no modifier)

Answer

protected

Question 6

A developer writes: Object obj = new String("Hello"); System.out.println(obj.length()); What will be the output?

Accepted Answer

Compilation error. The code fails to compile because the reference variable 'obj' is of type Object, which does not have a length() method. The length() method is defined in the String class, not in Object. Since the compiler checks the declared type (Object) for method availability, it does not find length() and produces a compilation error.

Answer

null

Answer

Runtime exception

Answer

5

Answer

0

Question 7

Which of the following best demonstrates polymorphism in Java?

Accepted Answer

Using an interface reference to call a method on an implementing object. Polymorphism in Java allows an object to take multiple forms, typically achieved through inheritance and interfaces. Option B demonstrates this by using an interface reference to invoke a method on an implementing object, where the actual method executed is determined at runtime based on the object's class, not the reference type.

Answer

Overloading a method with different parameter lists

Answer

Using static methods

Answer

Overriding a method in a subclass

Answer

Using final methods

Question 8

A class that does not define any constructor has:

Accepted Answer

A default no-argument constructor. Java automatically provides a default no-arg constructor with the same access as the class.

Answer

A private constructor

Answer

A public constructor with arguments

Answer

It cannot be instantiated

Answer

No constructor

Question 9

class Parent { void show() { System.out.print("Parent"); } }
class Child extends Parent { void show() { System.out.print("Child"); } }
public class Test { public static void main(String[] args) { Parent p = new Child(); p.show(); } }

What is the output?

Accepted Answer

Child. Although the reference is Parent, the object is Child, and show() is overridden, so Child's version is called.

Answer

Compilation error

Answer

No output

Answer

Parent

Answer

Runtime error

Question 10

Which statement about abstract classes and interfaces is true in Java?

Accepted Answer

An abstract class can have concrete methods. An abstract class in Java can contain concrete (non-abstract) methods with full implementations, which subclasses can inherit or override. This is a key distinction from interfaces (prior to Java 8), which could only declare abstract methods. Abstract classes cannot be instantiated directly, but they provide a partial implementation that concrete subclasses complete.

Answer

Both B and D

Answer

An interface can have instance variables

Answer

An abstract class can be instantiated

Answer

An interface can have constructor

Question 11

Which TWO access modifiers allow access from a subclass in a different package?

Accepted Answer

protected. (protected) is correct because the protected access modifier allows access to members of a superclass by subclasses in different packages, provided the subclass inherits from the superclass. Option E (public) is also correct because public members are accessible from any class, regardless of package or inheritance.

Answer

private

Answer

static

Answer

default (no modifier)

Question 12

Which THREE are true about Java constructors?

Accepted Answer

A constructor cannot have a return type. Constructors in Java do not have a return type, not even void. If you attempt to add a return type to a constructor, the compiler treats it as a regular method, not a constructor, which can lead to unexpected behavior. This is a fundamental rule of Java syntax.

Answer

A constructor can be marked final

Answer

A constructor can be abstract

Question 13

Which TWO statements about interfaces in Java are true?

Accepted Answer

An interface can extend another interface. Interfaces can extend other interfaces and can have default methods with a body.

Answer

An interface can have instance variables

Answer

An interface can implement another interface

Answer

An interface can have final methods

Question 14

Refer to the exhibit.

What is the output?

Accepted Answer

Vehicle. In Java, fields are not polymorphic; the reference type determines which field is accessed. v is of type Vehicle, so v.type refers to Vehicle's type field.

Answer

null

Answer

Compilation error

Answer

Car

Answer

Runtime error

Question 15

Refer to the exhibit.

What is the output?

Accepted Answer

Photo. The object is Photo, and print() is overridden, so dynamic binding calls Photo's print method.

Answer

Runtime error

Answer

No output

Answer

Compilation error

Answer

Document