OOP Intro

Question 1

What is the difference between a class and an object?

Answer 1

A class is the blueprint for an object. An object is an instance of a class.

Question 2

What does instantiating mean?

Answer 2

“instantiating” is creating an object instance of a class.

Question 3

What is the difference between stack and heap memory?

Answer 3

Stack is used for storing primitive types (numbers, boolean and character) and variables that store references to objects in the heap. Variables stored in the stack are immediately cleared when they go out of scope (eg when a method finishes execution). Objects stored in the heap get removed later on when they’re no longer references. This is done by Java’s garbage collector.

Question 4

What are the problems of procedural code

Answer 4

Big classes with several unrelated methods focusing on different concerns and responsibilities.

Question 5

What is encapsulation?

Answer 5

It suggests that we should bundle the data and operations on the data inside a single unit (class).

Question 6

Why should we declare fields as private

Answer 6

We want to keep implementation details secret and prevent objects from entering bad state.

Question 7

What is abstraction?

Answer 7

Suggests that we should reduce complexity by hiding the unnecessary implementation details. Encourages users to work with an interface.

Question 8

What is coupling?

Answer 8

The level of dependency between software entities (eg classes). The more our classes are dependent on each other, the harder it is to change them.

Question 9

How does the abstraction principle help reduce coupling?

Answer 9

By hiding the implementation details, we prevent other classes from getting affected when we change these details

Question 10

What are constructors?

Answer 10

Called when we instantiate our class. We use them to initialize our objects.

Question 11

What is method overloading?

Answer 11

Declaring a method with the same name but with different signatures. The number, type and order of its parameters will be different.

Question 12

What are static methods?

Answer 12

These methods are accessible via classes, not objects.

Question 13

How can we make one class inherit from another?

Answer 13

Through use of the extends keyword.

Question 14

What does the hashCode() method of an object return?

Answer 14

It returns a numeric value that is calculated based on the address of the object in memory.

Question 15

What is a default constructor?

Answer 15

A constructor without any parameters. If we don’t create it, the Java compiler will automatically add one to our classes.

Question 16

How can we add a constructor to a class?

Answer 16

Create a method with no return type.

Question 17

Why is the super keyword used?

Answer 17

a reference to the base or parent class. We can use it to access the members (fields and methods) or call the constructors of the base class.

Question 18

What is the difference between private and protected access modifiers?

Answer 18

protected or private access modifiers are only accessible inside of a class. Protected members are inherited and are accessible by child (derived) classes. Private members are not.

Question 19

How accessible is a field or method if it’s declared without an access modifier?

Answer 19

If we omit the access modifier, the member will have the default access modifier which makes that member public in package. In other words, that member will be public in the package but private outside of the package.

Question 20

What’s the difference between method overriding and method overloading?

Answer 20

Method overriding means changing the implementation of an inherited method in a subclass. Method overloading means declaring a method with different signatures (different number, type and order of parameters).

Question 21

What is the benefit of using @override notation?

Answer 21

It signals the Java compiler that we’re overriding a method in the base class and this helps the compiler check our code for correctness. It will ensure the signature of the method in the subclass matches the on declared in the base class. Also, if we remove this method from the base class, the compiler will let us know and we can remove the method in the subclass as well.

Question 22

How is the instanceOf operator used?

Answer 22

It tells us if an object is an instance of a class. We use it before casting an object to a different type to make sure we don’t get a casting exception.

Question 23

What are the 4 principles of OOP?

Answer 23

• Encapsulation: bundling the data and operations on the data inside a single unit (class).
• Abstraction: reducing complexity by hiding unnecessary details (metaphor: the implementation detail of a remote control is hidden from us. We only work with its public interface.) -Inheritance: a mechanism for reusing code. -Polymorphism: a mechanism that allows an object to take many forms and behave differently. This will help us build extensible applications.

Question 24

When do we use abstract classes?

Answer 24

An abstract class is a partially-implemented (half-cooked) class. We cannot instantiate them. But we use them to share some common code across their subclasses.

Question 25

Can we have an abstract class without abstract methods?

Answer 25

Yes! An abstract class does not need abstract methods. But if we mark a method as abstract, we should mark the class as abstract as well.

Question 26

When do we use final classes?

Answer 26

Final classes cannot be inherited. We use them when we’ve made certain assumptions about a class and we want to prevent other classes extending our class and break those assumptions.

Question 27

What is the diamond problem?

Answer 27

The diamond problem happens in languages that support multiple inheritance. Java does not support multiple inheritiance.