1Z0-811 Object-Oriented Programming — 30 Questions

Question 1

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

Accepted Answer

Compilation fails. The code fails to compile because the class `Document` likely implements two interfaces, such as `Printable` and `Showable`, each declaring a default `print()` method. This creates a diamond problem where the compiler cannot determine which default method to use, resulting in a compilation error unless `Document` overrides `print()`. The error occurs at compile time, not runtime.

Answer

Printable

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Runtime exception

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Showable

Question 2

In a banking application, a class 'Account' has private field 'balance' and public methods 'getBalance()' and 'setBalance(double)'. This is an example of which OOP principle?

Accepted Answer

Encapsulation. Encapsulation hides internal data (the 'balance' field) and provides controlled access via public methods 'getBalance()' and 'setBalance(double)'. Option A is wrong because abstraction focuses on hiding implementation complexity rather than data protection; the example demonstrates data hiding which is encapsulation. Option B is wrong because inheritance involves a parent-child class relationship, which is not shown. Option C is wrong because polymorphism requires method overriding or overloading, which is not present here.

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Abstraction

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Inheritance

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Polymorphism

Question 3

A team is developing a large-scale e-commerce platform using Java. They have a class hierarchy: Product (abstract), Electronics, Clothing, Food. Each product has a discount method that applies different logic. The team notices that whenever a new product type is added, they must modify the existing discount calculation logic in multiple places, leading to high maintenance costs. They want to refactor to follow the Open/Closed Principle. Which approach should they take?

Accepted Answer

Define an interface Discountable with a method calculateDiscount() and have each product class implement it. Defining an interface Discountable with a calculateDiscount() method and having each product class implement it allows adding new product types without modifying existing code. Option B is wrong because it violates OCP by requiring changes to if-else checks. Option C is wrong because it uses reflection unnecessarily and still requires modification when new types are added. Option D is wrong because it uses a switch statement in a utility class, which also requires modification when new product types are added, violating OCP.

Answer

Create a single discount method in Product that uses if-else checks on type

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Remove the discount method and handle it externally using reflection

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Use a switch statement in a utility class to handle all product types

Question 4

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. 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 5

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. It follows encapsulation: a public getBalance() method provides read-only access to the private balance field. Subclasses cannot directly modify balance because it is private. Option D (protected) is incorrect because protected access allows subclasses to directly read and write the field, which violates the requirement that subclasses should not be able to directly modify balance.

Answer

Make balance public

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Use a static variable

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Make balance protected

Question 6

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.

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public void display() throws SQLException

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public void display() throws Exception

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public void display() throws Throwable

Question 7

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

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Abstraction

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Polymorphism

Question 8

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

Accepted Answer

Circle must override draw(). 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 9

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.

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super(); anywhere

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super(); at the end

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this.super();

Question 10

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 11

Which TWO statements are true about interfaces in Java?

Accepted Answer

Interfaces can contain private methods.. 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 12

Which THREE are benefits of using inheritance?

Accepted Answer

Code reuse. 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 13

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

Accepted Answer

Alice. 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".

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Runtime exception

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Compilation fails

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null

Question 14

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().. 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().

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

Question 15

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

Accepted Answer

Define the interface, then create a class that implements it, then implement all abstract methods, then add extra methods, then use the class. First define the interface, then create a class that implements it, implement all abstract methods, add extra methods, and then use the class.

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Create a class that implements the interface, then define the interface, then implement all abstract methods, then add extra methods, then use the class

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Implement all abstract methods, then define the interface, then create a class that implements it, then add extra methods, then use the class

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Use the class, then define the interface, then create a class that implements it, then implement all abstract methods, then add extra methods

Question 16

Match each Java exception class to its category.

Accepted Answer

IOException: Checked Exception. Checked exceptions (e.g., IOException, ClassNotFoundException) must be handled or declared; unchecked exceptions (e.g., NullPointerException, ArithmeticException) are RuntimeExceptions and do not require explicit handling.

Answer

NullPointerException: Checked Exception

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IOException: Unchecked Exception

Question 17

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

Question 18

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

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Hello World (no space)

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Compilation error

Question 19

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

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The override succeeds, final is ignored in subclasses

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Runtime error when the method is called

Question 20

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

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

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