OCP Java Class Design and OOP Questions and Answers

Question 1

Given the following class definitions, which statement is true?

public class Animal {
    public Number getAge() throws Exception {
        return 10;
    }
}

public class Dog extends Animal {
    // INSERT METHOD HERE
}

Accepted Answer

Overriding the method with `public Integer getAge() throws Exception` is a valid covariant return.

Answer

Overriding the method with `public Number getAge()` is invalid because a checked exception is removed.

Answer

Overriding the method with `public Object getAge() throws Exception` is a valid override.

Answer

Overriding the method with `public Integer getAge() throws RuntimeException` is invalid because the return type is changed.

Question 2

Which of the following statements about Java Records is correct?

Accepted Answer

All fields in a record are implicitly final.

Answer

A record can extend another class.

Answer

A record can be declared as abstract.

Answer

A record's constructor must explicitly assign all fields.

Question 3

What is the primary purpose of a `sealed` class in Java 17?

Accepted Answer

To restrict which other classes or interfaces may extend or implement it.

Answer

To prevent a class from being instantiated directly.

Answer

To ensure that all methods of the class are final.

Answer

To create a class that can only be accessed from within the same package.

Question 4

A software developer is designing a system for a library. They have created a `Book` class with private fields for `title` and `author`, and public `getter` and `setter` methods to access these fields. Which object-oriented programming principle is best demonstrated by this design?

Accepted Answer

Encapsulation

Answer

Polymorphism

Answer

Inheritance

Answer

Abstraction

Question 5

Which of the following is a valid declaration for a class that extends a sealed class named `Vehicle`?

Accepted Answer

public non-sealed class Car extends Vehicle { } // Assumes Car is in the permits list

Answer

public class Car extends Vehicle { } // Assumes Car is in the permits list

Answer

public open class Car extends Vehicle { } // Assumes Car is in the permits list

Answer

public class Car extends Vehicle permits Truck { }

Question 6

Examine the following code:

public class Calculator {
    public int add(int a, int b) { return a + b; }
    public double add(double a, double b) { return a + b; }
    public int add(int a, int b, int c) { return a + b + c; }
}

This code is an example of which OOP concept?

Accepted Answer

Method Overloading

Answer

Method Overriding

Answer

Abstraction

Answer

Inheritance

Given the following class definitions, which statement is true?public class Animal { public Number getAge() throws Exception { return 10; }}public class Dog extends Animal { // INSERT METHOD HERE}

Given the following class definitions, which statement is true?
public class Animal {
public Number getAge() throws Exception {
return 10;
}
}
public class Dog extends Animal {
// INSERT METHOD HERE
}