1Z0-811 Object-Oriented Programming Practice Test 5 — 15 Questions

Question 1

A developer is working on a Java application that processes sensor data. The code uses a class 'SensorDataProcessor' which directly instantiates specific sensor classes like 'TemperatureSensor' and 'PressureSensor' inside its methods. The team wants to make the system extensible to support new sensor types without modifying the processor class. Which design change best achieves this?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Define a Sensor interface and use dependency injection to pass sensor objects to the processor. The Open/Closed Principle says classes should be open for extension but closed for modification. By defining a Sensor interface and injecting Sensor implementations into SensorDataProcessor, the processor depends on an abstraction rather than concrete classes, so new sensor types can be added without touching the processor code. Dependency injection also improves testability because mock sensors can be substituted in unit tests. This is the canonical Strategy/DI pattern for decoupling.

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Make SensorDataProcessor abstract and subclass it for each sensor

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Use a static factory method in SensorDataProcessor to create sensors

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Create a separate configuration file listing sensors and use reflection to instantiate them

Question 2

A junior developer created a class 'BankAccount' with public fields for balance and account number. After deployment, users are able to set negative balances, causing issues. Which OOP principle should have been applied to prevent this?

Accepted Answer

Encapsulation. Encapsulation is the OOP principle that involves bundling data (fields) and methods that operate on that data within a single unit (class) and restricting direct access to some of the object's components. By making fields private and providing public getter and setter methods, the class can enforce validation rules, such as preventing negative balances. This would have prevented users from setting invalid values directly.

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Inheritance

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Abstraction

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Polymorphism

Question 3

In a Java application, a class 'OrderProcessor' contains a method that processes orders. The method currently handles multiple responsibilities: validating order data, calculating totals, updating inventory, and sending notifications. The team wants to refactor this method to follow the Single Responsibility Principle. Which action should they take?

Accepted Answer

Create separate classes or methods for each responsibility and compose them in OrderProcessor. The Single Responsibility Principle (SRP) states that a class or method should have only one reason to change. The current method mixes validation, calculation, inventory updates, and notifications — four distinct concerns. The correct refactor is to extract each responsibility into its own class or method and have OrderProcessor compose them, so each unit has a single axis of change.

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Use a single utility method that uses if-else for each case

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Override the method in subclasses for each responsibility

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Merge all responsibilities into a superclass

Question 4

Which TWO statements are true about the 'super' keyword in Java?

Accepted Answer

It can be used to call a superclass method that has been overridden.. The 'super' keyword can be used to call a superclass method that has been overridden, using 'super.methodName()'. Option D is correct because 'super()' invokes the superclass constructor. Option A is wrong because 'super' cannot access private methods of the superclass. Option B is wrong because 'super' cannot access private fields. Option E is wrong because 'super' cannot invoke static methods directly; static methods are accessed via the class name.

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It can be used to access private methods of the superclass.

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It can be used to access private fields of the superclass.

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It can be used to invoke a static method of the superclass.

Question 5

A developer defines an abstract class 'Shape' with an abstract method 'area()' and a concrete method 'describe()'. A subclass 'Rectangle' extends 'Shape' and provides an implementation of 'area()' but does not override 'describe()'. A program creates a 'Rectangle' object and calls both methods. What is the result?

Accepted Answer

The code compiles and runs; 'area()' executes the Rectangle implementation and 'describe()' executes the inherited Shape implementation.. A concrete subclass must implement all inherited abstract methods, which Rectangle does by providing 'area()'. Concrete methods such as 'describe()' are inherited and may be used without overriding. Because Rectangle is fully concrete, it can be instantiated, and calls to both methods succeed, with 'area()' using dynamic dispatch to select Rectangle's version.

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The code fails to compile because Rectangle must override every method declared in Shape, including the concrete 'describe()' method.

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The code fails to compile because Rectangle cannot be instantiated since it does not explicitly declare a constructor.

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The code compiles but throws an AbstractMethodError at runtime when 'describe()' is called on the Rectangle object.

Question 6

A developer defines a class Shape with a protected method area() and a public method describe() that calls area(). A subclass Circle overrides area() but does not override describe(). A Circle object is created and describe() is invoked. What happens?

Accepted Answer

The describe() method from Shape runs and calls Circle's overridden area().. Inheritance allows Circle to reuse the public describe() method from Shape. When describe() invokes area(), Java performs dynamic dispatch based on the runtime object, so the override in Circle executes. No compile error or exception occurs because describe() is inherited and area() is accessible from the declaring class.

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The describe() method from Shape runs and calls Shape's original area().

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A runtime exception is thrown because Circle did not override describe().

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A compile-time error occurs because describe() cannot call a protected method.

Question 7

A developer wants to model a class Employee that stores a name and a salary. The class should prevent direct modification of salary from outside the class, while allowing read access. Which design best satisfies this requirement?

Accepted Answer

Declare salary as private and provide a public getter method; do not provide a setter.. Encapsulation uses private fields with controlled access. Keeping salary private and exposing only a public getter allows read access while blocking direct writes. Not providing a setter is the simplest way to prevent modification. A public or protected field, or a public setter, would allow unwanted changes from outside the class.

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Declare salary as private and provide a public getter and a public setter.

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Declare salary as protected and provide a public getter method.

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Declare salary as public and provide a public getter method.

Question 8

A developer is designing a Shape class hierarchy for a graphics editor. The Shape class declares a method public double area() that returns 0.0. The Circle subclass provides its own area() implementation. When the developer writes Shape s = new Circle(2.0); System.out.println(s.area()); the program prints the circle's area. Which OOP concept does this behavior demonstrate?

Accepted Answer

Dynamic method dispatch. Because Circle overrides Shape's area() and the call is made through a Shape reference holding a Circle object, the JVM defers the method selection until runtime and executes Circle's implementation. This runtime selection based on the actual object type is dynamic method dispatch, the cornerstone of polymorphic behavior in Java.

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Static binding of the area method

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Method overloading

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Encapsulation

Question 9

A class Vehicle has a constructor that takes an int parameter. A subclass Car has a constructor that takes an int parameter and needs to initialize the inherited part using the Vehicle constructor. Which statement about constructor chaining is correct?

Accepted Answer

The Car constructor must call super(value) as its first statement to invoke the Vehicle constructor.. Constructor chaining requires that a subclass constructor invoke an accessible superclass constructor. When the superclass has no no-arg constructor, the subclass must explicitly call super(...) with matching arguments as the first statement. Relying on the implicit super() call fails, and this(...) only delegates within the same class.

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The Car constructor can call this(value) instead of super(value) to initialize the Vehicle part.

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The Car constructor automatically calls the Vehicle(int) constructor without any explicit code.

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The Car constructor can call super(value) anywhere in its body as long as it is before field assignments.

Question 10

A developer writes a class named Employee that declares a public method named calculatePay. A second class named Manager extends Employee and declares its own public method named calculatePay with the exact same name, parameter list, and return type. At runtime, a Manager object assigned to an Employee reference is used to invoke calculatePay. Which concept does this scenario describe?

Accepted Answer

Method overriding. Because Manager supplies calculatePay with the same signature as the inherited method, it overrides the superclass implementation. A call through an Employee reference that points to a Manager object is resolved at runtime to Manager's version, which is runtime polymorphism. Overloading, hiding, and encapsulation describe different mechanisms and do not explain this behavior.

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Encapsulation

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Method hiding

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Method overloading

Question 11

A developer writes a class Employee with fields name and salary. The developer wants other classes to read and modify these fields only through methods that enforce validation rules, and wants to prevent direct field access from outside the class. Which pair of keywords should be applied to the fields and the accessor methods respectively to achieve this?

Accepted Answer

private on the fields and public on the accessor methods. Encapsulation requires hiding internal state and exposing only controlled operations. Private fields stop outside code from writing values that skip validation, while public accessor methods provide the single sanctioned path where rules such as non-negative salary can be enforced before assignment.

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public on the fields and public on the accessor methods

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final on the fields and static on the accessor methods

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protected on the fields and private on the accessor methods

Question 12

A developer defines an abstract class named Vehicle with an abstract method move and a concrete method describe. A class named Bicycle extends Vehicle but does not implement move. What is the result when the developer attempts to compile Bicycle?

Accepted Answer

Compilation fails because Bicycle must implement move or be declared abstract.. An abstract class can declare abstract methods that subclasses must implement. Any concrete subclass must provide bodies for all inherited abstract methods. Bicycle neither implements move nor is declared abstract, so compilation fails. Abstract classes may also contain concrete methods, so the presence of describe is not the issue.

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Compilation fails because abstract classes cannot contain concrete methods.

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Compilation succeeds, but a runtime exception is thrown when Bicycle is instantiated.

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Compilation succeeds because Vehicle already provides a default implementation of move.

Question 13

A development team is refactoring a Java application and must decide where to use abstract classes versus interfaces. Which two statements correctly describe legal and meaningful usage of an abstract class in Java? (Choose two.)

Accepted Answer

An abstract class may declare both abstract methods, which have no body, and concrete methods with full implementations.. Abstract classes uniquely combine unimplemented abstract methods with concrete methods and allow instance fields that hold state, making them ideal for partially implemented base types. They still obey single inheritance and still declare constructors that subclasses invoke, and they can never be instantiated directly with new.

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A class may extend multiple abstract classes to combine their implementations.

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An abstract class can be instantiated directly as long as all of its methods are concrete.

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An abstract class cannot define a constructor because it can never be instantiated.

Question 14

A developer designs a class named BankAccount that stores a private field accountNumber. The field must never be modified after the constructor runs, and other classes must be able to read its value. Which declaration best satisfies these requirements?

Accepted Answer

private final String accountNumber;. Marking the field private final keeps it hidden from other classes and allows a single assignment, typically inside the constructor. A public getter can then return the value for reading without exposing a setter. Public or protected fields allow later modification, and a static field would incorrectly share one value across all instances.

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protected String accountNumber;

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private static String accountNumber;

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public String accountNumber;

Question 15

A developer writes a class Logger with a private static Logger instance and a public static Logger getInstance() method. The method checks if instance is null and creates it. Two threads call getInstance() simultaneously on first use. What is the most likely problem?

Accepted Answer

Two Logger objects may be created because the null check and assignment are not atomic.. Lazy initialization without synchronization allows a race condition. Two threads can both observe instance as null, both create a Logger, and both assign it, producing multiple instances. Synchronizing getInstance(), using a static holder class, or using an enum-based singleton prevents the race and preserves the single-instance guarantee.

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A compile-time error occurs because static methods cannot access private static fields.

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The JVM throws an IllegalStateException because static initialization is not thread-safe.

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A NullPointerException is thrown because instance is not initialized before getInstance() is called.