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

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Runtime exception

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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

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Use a static variable

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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

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public void display() throws Exception

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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.

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Inheritance

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Abstraction

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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

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Shape s = new Shape(); is valid

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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

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super(); at the end

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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.

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private

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abstract

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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.

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Interfaces can contain constructors.

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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

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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

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Compilation fails

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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

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Runtime exception

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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.

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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.

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Prototype pattern with cloning

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Builder pattern with a static inner class

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Factory pattern with a public constructor