1Z0-811 Object-Oriented Programming — Timed 20 Questions

Question 1

A developer writes a class 'Vehicle' with a method 'move()' that prints 'Vehicle moves'. A subclass 'Car' overrides 'move()' to print 'Car moves'. Given: Vehicle v = new Car(); v.move(); What is the output?

Accepted Answer

Car moves. Option D is correct because Java uses dynamic method dispatch (runtime polymorphism). Even though the reference variable is of type 'Vehicle', the actual object is a 'Car' instance. At runtime, the JVM calls the overridden 'move()' method of the 'Car' class, printing 'Car moves'.

Answer

Vehicle moves

Answer

Runtime exception

Answer

Compilation fails

Question 2

In a banking application, a class 'Account' has a private field 'balance'. Which is the best way to allow subclasses to read but not directly modify 'balance'?

Accepted Answer

Provide a public getBalance() method. Option B is correct because it follows the encapsulation principle of object-oriented programming. By providing a public getBalance() method, subclasses can read the balance value without having direct access to the private field, thus preventing unintended modification. This is the standard Java pattern for controlled access to private instance variables.

Answer

Make balance public

Answer

Use a static variable

Answer

Make balance protected

Question 3

A class 'Base' has a method 'public void display() throws IOException'. Subclass 'Derived' overrides display(). Which exception specifications are allowed in the overriding method?

Accepted Answer

public void display() throws FileNotFoundException. In Java, an overriding method can throw the same exception, a subclass of the exception thrown by the parent method, or no exception at all. The parent method throws IOException, so the overriding method may throw FileNotFoundException (a subclass of IOException). This follows the rule that the overriding method cannot throw a broader checked exception than the overridden method.

Answer

public void display() throws SQLException

Answer

public void display() throws Exception

Answer

public void display() throws Throwable

Question 4

Which design principle is violated by making all fields public in a class?

Accepted Answer

Encapsulation. Making all fields public violates the principle of encapsulation, which requires that an object's internal state be hidden from external access and only modifiable through controlled methods (getters/setters). In Java, public fields allow any class to directly read or modify the field, breaking data integrity and the ability to enforce invariants. Encapsulation is a core OOP concept that protects the internal representation of an object.

Answer

Inheritance

Answer

Abstraction

Answer

Polymorphism

Question 5

Given: abstract class Shape { abstract void draw(); } class Circle extends Shape { void draw() {} } Which is true?

Accepted Answer

Circle must override draw(). Option B is correct because Circle is a concrete class that extends the abstract class Shape, which declares the abstract method draw(). A concrete subclass of an abstract class must provide an implementation for all inherited abstract methods, unless the subclass is also declared abstract. Circle provides an implementation of draw() with an empty body, which satisfies the override requirement.

Answer

Shape cannot have a constructor

Answer

Shape s = new Shape(); is valid

Answer

draw() must be public in Circle

Question 6

Which is the correct way to call a superclass constructor from a subclass constructor?

Accepted Answer

super(); as first statement. In Java, a call to the superclass constructor using `super()` must be the first statement in a subclass constructor. This ensures that the superclass initialization completes before any subclass-specific code executes, maintaining the inheritance chain. Option B correctly identifies this requirement.

Answer

super(); anywhere

Answer

super(); at the end

Answer

this.super();

Question 7

A developer wants to prevent a method from being overridden. Which modifier should be used?

Accepted Answer

final. The `final` modifier prevents a method from being overridden in a subclass. When a method is declared as `final`, any attempt to override it in a subclass results in a compile-time error, ensuring the method's implementation remains unchanged.

Answer

private

Answer

abstract

Answer

static

Question 8

Which TWO statements are true about interfaces in Java?

Accepted Answer

Interfaces can contain private methods.. Option A is correct because, since Java 9, interfaces can contain private methods to share common code between default methods or static methods within the interface, without exposing that logic to implementing classes. Option D is correct because all variables declared in an interface are implicitly public, static, and final, meaning they are constants that cannot be changed once assigned.

Answer

Interfaces can be instantiated.

Answer

Interfaces can contain constructors.

Answer

Interfaces cannot have default methods.

Question 9

Which THREE are benefits of using inheritance?

Accepted Answer

Code reuse. Option A is correct because inheritance allows a subclass to reuse fields and methods from a superclass without rewriting them. This directly reduces code duplication and promotes maintainability in Java.

Answer

Improved security

Answer

Reduces memory usage

Question 10

What is the output of: System.out.println(new Manager("Alice", 5).getName());

Accepted Answer

Alice. Option C is correct because the code creates a new Manager object with the name "Alice" and a level of 5, then calls getName() on it. Assuming Manager extends a class (likely Employee) that has a constructor setting the name field via super(name), getName() returns the stored name "Alice". The code compiles and runs without exception, printing "Alice".

Answer

Runtime exception

Answer

Compilation fails

Answer

null

Question 11

What is the result of: new Document().print();

Accepted Answer

Compilation fails. The code fails to compile because the class `Document` does not have a `print()` method defined. In Java, you can only invoke methods that are declared in the class or inherited from its superclass. Since no such method exists, the compiler issues an error.

Answer

Printable

Answer

Runtime exception

Answer

Showable

Question 12

A development team is building a library management system. The system has classes 'LibraryItem', 'Book', and 'DVD'. LibraryItem has a method 'getTitle()' that returns the title. Book and DVD extend LibraryItem. The team wants to ensure that when a LibraryItem is borrowed, a message specific to its type is displayed. They have a 'Borrower' class with a method 'borrow(LibraryItem item)', which currently calls 'item.getTitle()' and prints the title. Now they need to display 'Book borrowed' or 'DVD borrowed' based on the actual item type. They want to avoid using 'instanceof' checks in the 'borrow' method to keep it open for new item types. Which design should they use?

Accepted Answer

Define an abstract method 'getBorrowMessage()' in LibraryItem, and implement it in Book and DVD. Call that method in borrow().. Option C is correct because it applies the Open/Closed Principle: LibraryItem defines an abstract getBorrowMessage() method, and each subclass provides its own implementation. The borrow() method calls this polymorphic method without needing instanceof or type checks, so adding new item types (e.g., Magazine) only requires implementing getBorrowMessage() in the new subclass without modifying existing code.

Answer

Add a String field 'type' to LibraryItem and set it in each subclass constructor. In borrow(), check the field.

Answer

Use a switch statement on the class name in borrow().

Answer

Override the 'borrow' method in each subclass and call a different method.

Question 13

Arrange the steps to implement an interface in a Java class in the correct order.

Question 14

Match each Java exception class to its category.

Question 15

A company is developing a configuration manager that must be shared across all components to ensure consistent settings. The manager should prevent direct instantiation and provide a single access point. Which design pattern and implementation should be used?

Accepted Answer

Singleton pattern with a private constructor and a static getInstance() method. The Singleton pattern ensures a class has only one instance and provides a global point of access. A private constructor prevents direct instantiation, and a static getInstance() method provides controlled access.

Answer

Prototype pattern with cloning

Answer

Builder pattern with a static inner class

Answer

Factory pattern with a public constructor

Question 16

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 17

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 18

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 19

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 20

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