Default constructor in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a default constructor in Java?

A default constructor is a constructor that Java provides automatically if no other constructors are defined. It has no parameters and initializes objects with default values.

intermediate

What happens if you define a constructor with parameters but no default constructor in your class?

Java will NOT provide a default constructor automatically. You must define the default constructor yourself if you want one.

beginner

How does Java initialize fields when using the default constructor?

Java sets numeric fields to 0, boolean fields to false, and object references to null by default.

beginner

Code snippet: <br>public class Car {<br>  String model;<br>}<br>What constructor does this class have by default?

This class has a default constructor with no parameters that Java provides automatically. It sets model to null by default.

intermediate

Why might you want to explicitly write a default constructor?

To add custom initialization code or to ensure the constructor exists when other constructors are defined.

What does Java provide if you do NOT write any constructor in your class?

A An error during compilation

B No constructor at all

C A constructor with parameters

D A default constructor with no parameters

Java automatically provides a default constructor with no parameters if you don't write any constructor.

If you write a constructor with parameters, does Java still provide a default constructor?

A Yes, always

B No, you must write it yourself

C Only if you use the 'default' keyword

D Only if the class is abstract

Java does not provide a default constructor if you define any constructor yourself.

What value does a boolean field have when initialized by the default constructor?

A false

B 1

C null

D true

Boolean fields are initialized to false by the default constructor.

Which of these is true about a default constructor?

A It is provided only if no constructors exist

B It must have parameters

C It throws an exception by default

D It cannot be overridden

Java provides a default constructor only if no other constructors are defined.

Why might you write your own default constructor?

A To prevent object creation

B To make the class abstract

C To add custom setup code during object creation

D To delete all fields

Writing your own default constructor lets you add custom code when creating objects.

Explain what a default constructor is and when Java provides it.

Think about what happens if you don't write any constructor.

Describe why you might need to write your own default constructor in a Java class.

Consider what happens if you add a constructor with parameters.