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.

Practice

1. What is a default constructor in Java?

easy

A. A method that returns the default value of a class.

B. A constructor with no parameters that Java provides automatically if none is written.

C. A constructor that must always be written by the programmer.

D. A special method that runs only when a program ends.

Solution

1. Step 1: Understand what a constructor is

   A constructor is a special method used to create objects of a class.

2. Step 2: Identify the default constructor

   If no constructor is written, Java automatically provides a constructor with no parameters called the default constructor.

3. Final Answer:

   A constructor with no parameters that Java provides automatically if none is written. -> Option B

4. Quick Check:

   Default constructor = automatic no-parameter constructor [OK]

Hint: Default constructor has no parameters and is auto-created if missing [OK]

Common Mistakes:

• Thinking default constructor must be written manually
• Confusing default constructor with methods returning default values
• Believing default constructor runs at program end

2. Which of the following is the correct syntax for a default constructor in Java?

easy

A. public ClassName(void) { }

B. public void ClassName() { }

C. public ClassName() { }

D. void ClassName() { }

Solution

1. Step 1: Recall constructor syntax

   A constructor has the same name as the class and no return type.

2. Step 2: Check each option

   public ClassName() { } matches the class name and has no return type, so it's correct syntax.

3. Final Answer:

   public ClassName() { } -> Option C

4. Quick Check:

   Constructor syntax = class name + no return type [OK]

Hint: Constructor name = class name, no return type, parentheses empty [OK]

Common Mistakes:

• Adding void return type to constructor
• Using wrong parameter list syntax
• Using lowercase class name in constructor

3. What will be the output of this Java code?

class Car {
  String model;
}
public class Test {
  public static void main(String[] args) {
    Car c = new Car();
    System.out.println(c.model);
  }
}

medium

A. null

B. Empty string

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand default constructor usage

   No constructor is defined, so Java provides a default constructor that sets no initial values.

2. Step 2: Check default value of uninitialized String

   Instance variable 'model' is a String and defaults to null if not set.

3. Final Answer:

   null -> Option A

4. Quick Check:

   Uninitialized String = null by default [OK]

Hint: Uninitialized object fields default to null in Java [OK]

Common Mistakes:

• Expecting empty string instead of null
• Thinking default constructor sets values automatically
• Assuming compilation or runtime error

4. Identify the error in this Java class if any:

public class Book {
  String title;
  public Book() {
    title = "Java Basics";
  }
  public Book(String title) {
    title = title;
  }
}

medium

A. The second constructor does not set the instance variable correctly.

B. The default constructor is missing.

C. The class has no constructors.

D. The constructors have wrong return types.

Solution

1. Step 1: Analyze the second constructor

   It assigns parameter 'title' to itself, not to the instance variable.

2. Step 2: Understand correct assignment

   Use 'this.title = title;' to assign parameter to instance variable.

3. Final Answer:

   The second constructor does not set the instance variable correctly. -> Option A

4. Quick Check:

   Use 'this' to assign constructor parameters to fields [OK]

Hint: Use 'this.' to assign parameters to instance variables [OK]

Common Mistakes:

• Assigning parameter to itself instead of instance variable
• Missing default constructor (actually present)
• Adding return types to constructors

5. You want to create a class Person that sets the name to "Unknown" by default if no name is given. Which constructor code correctly implements this using a default constructor?

hard

A. public class Person {
String name;
public Person() {
name = name;
}
}

B. public class Person {
String name = "Unknown";
public Person(String name) {
name = name;
}
}

C. public class Person {
String name;
public Person(String name) {
this.name = name;
}
}

D. public class Person {
String name;
public Person() {
name = "Unknown";
}
public Person(String name) {
this.name = name;
}
}

Solution

1. Step 1: Check default constructor sets default value

   public class Person { String name; public Person() { name = "Unknown"; } public Person(String name) { this.name = name; } }'s default constructor sets name to "Unknown" correctly.

2. Step 2: Verify parameterized constructor sets name properly

   public class Person { String name; public Person() { name = "Unknown"; } public Person(String name) { this.name = name; } } uses 'this.name = name;' to assign parameter to instance variable.

3. Final Answer:

   public class Person { String name; public Person() { name = "Unknown"; } public Person(String name) { this.name = name; } } -> Option D

4. Quick Check:

   Default constructor sets default value, parameterized sets given value [OK]

Hint: Default constructor sets default values; use 'this' for parameters [OK]

Common Mistakes:

• Assigning parameter to itself without 'this.'
• Not setting default value in default constructor
• Missing default constructor entirely