1Z0-811 Object-Oriented Programming Practice Test 2 — 15 Questions

Question 1

A junior developer writes a method that attempts to modify a String: public void update() { String s = "Hello"; s.concat(" World"); System.out.println(s); } What will be printed when update() is called?

Accepted Answer

Hello. Strings are immutable in Java. The concat() method returns a new String but the original object remains unchanged because the result is not assigned.

Answer

Hello World

Answer

Hello World (no space)

Answer

Compilation error

Question 2

A banking application has a base class 'Account' with a method 'withdraw()' marked as final. A subclass 'SavingsAccount' tries to override 'withdraw()'. What is the outcome?

Accepted Answer

Compilation error because final methods cannot be overridden. A final method cannot be overridden in a subclass. Attempting to do so results in a compilation error.

Answer

The override works only if SavingsAccount is in the same package

Answer

The override succeeds, final is ignored in subclasses

Answer

Runtime error when the method is called

Question 3

A company's HR system uses an Employee class with sensitive salary information. The team wants to allow other classes to read the salary but not modify it. Which approach best preserves encapsulation?

Accepted Answer

Provide a public getSalary() getter method and keep salary private. Encapsulation is achieved by keeping fields private and providing public getters. A getter allows read-only access without exposing the internal representation.

Answer

Declare the salary field as public

Answer

Create a private method that returns salary only when called from within the same package

Answer

Provide a public setSalary() setter method and keep salary private

Question 4

A developer creates an interface 'Drawable' with a single abstract method 'draw()'. They then create a class 'Circle' that implements Drawable but forgets to provide the draw() method. Circle is not declared abstract. What will happen when compiling Circle?

Accepted Answer

Compilation fails because Circle must either implement draw() or be declared abstract. A class that implements an interface must provide implementations for all abstract methods, or it must be declared abstract. Without the implementation and without being abstract, the class does not compile.

Answer

Compilation fails because interfaces cannot be implemented without overriding all methods

Answer

Compilation succeeds, a default empty method is generated

Answer

Compilation succeeds but a warning is issued

Question 5

A security-sensitive class should not be extended by any other class. Which modifier should be applied to the class declaration?

Accepted Answer

final. The final modifier prevents a class from being subclassed, which is appropriate for security or design reasons.

Answer

abstract

Answer

static

Answer

private

Question 6

A logging utility class keeps track of the number of log entries using a static integer variable. The class is instantiated multiple times in the application. How does the count variable behave?

Accepted Answer

All instances share the same count variable. Static variables are shared among all instances of a class. There is only one copy of the variable, regardless of the number of instances.

Answer

Each instance has its own copy of count

Answer

count is reset each time a new instance is created

Answer

count cannot be accessed from static methods

Question 7

A calculator class has two overloaded methods: add(int a, int b) and add(double a, double b). A call to add(3, 4) will invoke which method?

Accepted Answer

add(int, int). Overloaded method resolution is based on compile-time types. The literal 3 and 4 are ints, so the int version is the most specific match and is called.

Answer

add(double, double)

Answer

Runtime decision based on actual parameters

Answer

Compilation error: ambiguous

Question 8

A developer writes a class 'Animal' with a method 'sound()'. The 'Cat' subclass overrides 'sound()'. If an Animal reference points to a Cat object, which method is called when sound() is invoked?

Accepted Answer

Cat's sound(). Polymorphism ensures that the overridden method in the actual object's class is called at runtime, even if the reference is of the superclass type.

Answer

Runtime error

Answer

Animal's sound()

Answer

Compilation error

Question 9

Which two of the following are fundamental principles of Object-Oriented Programming? (Choose two.)

Accepted Answer

Encapsulation. The four main OOP principles are encapsulation, inheritance, polymorphism, and abstraction.

Answer

Iteration

Answer

Compilation

Answer

Synchronization

Question 10

A developer wants to achieve loose coupling between components. Which two practices support loose coupling? (Choose two.)

Accepted Answer

Using interfaces to define contracts. Loose coupling is promoted by coding to interfaces and favoring composition over inheritance, as both reduce dependency on concrete implementations.

Answer

Using inheritance to share implementation

Answer

Using private fields with public getters

Answer

Using static methods extensively

Question 11

Which three statements about constructors in Java are true? (Choose three.)

Accepted Answer

The default constructor has no parameters. Constructors can be overloaded to provide different initialization options, they cannot return a value (not even void), and if no constructor is defined, the compiler provides a default no-arg constructor.

Answer

Constructors can be abstract

Answer

Constructors can be final

Question 12

Refer to the exhibit. What is the output when the following code is executed? Vehicle v = new Car(); v.accelerate(); System.out.println(v.speed);

Accepted Answer

20. Polymorphism applies: the accelerate() method of the actual object (Car) is called, which adds 20 to speed. The protected field speed is accessible because Car extends Vehicle.

Answer

10

Answer

0

Answer

Compilation error

Question 13

Refer to the exhibit. Why does the code fail to compile?

Accepted Answer

Cannot instantiate an abstract class. Abstract classes cannot be instantiated. The attempt to create a new Animal() causes a compile-time error because Animal is abstract.

Answer

Animal must be declared as interface

Answer

The main method is in the wrong class

Answer

The sound() method is not defined

Question 14

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 15

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