Default constructor in Java - Step-by-Step Execution

Concept Flow - Default constructor

Class declared

↓

No constructor written?

Yes↓

Compiler adds default constructor

↓

Object created using default constructor

↓

Object initialized with default values

↓

Program continues

When no constructor is written, Java adds a default constructor automatically to create objects with default values.

Execution Sample

Java

class Car {
  int speed;

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

Creates a Car object using the default constructor and prints the default speed value.

Execution Table

Step	Action	Evaluation	Result
1	Class Car declared with int speed	No constructor defined	Compiler will add default constructor
2	Create object c with new Car()	Calls default constructor	Object c created, speed initialized to 0
3	Print c.speed	speed is 0 by default	Output: 0
4	Program ends	No more actions	Execution stops

💡 Program ends after printing default value of speed

Variable Tracker

Variable	Start	After Object Creation	Final
c.speed	undefined	0	0

Key Moments - 2 Insights

Why can we create an object without writing a constructor?

Because Java automatically adds a default constructor if none is written, as shown in execution_table step 1 and 2.

What value does the speed variable have after creating the object?

It has the default value 0 for int type, as shown in execution_table step 3 and variable_tracker.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 2?

A The program prints the speed value

B The default constructor is called and object c is created

C The speed variable is set to 10

D The class Car is deleted

💡 Hint

Check execution_table row with Step 2 describing object creation

According to variable_tracker, what is the value of c.speed after object creation?

A undefined

B null

C 0

D 1

💡 Hint

Look at variable_tracker row for c.speed after object creation

If we add a constructor with parameters, what happens to the default constructor?

A It is removed and not added automatically

B It is still added automatically

C It runs twice

D The program crashes

💡 Hint

Recall that default constructor is only added if no constructor is written (see concept_flow)

Concept Snapshot

Default constructor in Java:
- Automatically added if no constructor is defined
- Has no parameters
- Initializes object with default values
- Allows object creation without explicit constructor
- If any constructor is written, default is NOT added

Full Transcript

In Java, when you create a class without writing any constructor, the compiler automatically adds a default constructor. This default constructor has no parameters and initializes the object with default values like 0 for int. For example, if you have a class Car with an int speed variable and no constructor, creating a new Car object calls this default constructor. The speed variable is set to 0 by default. This allows you to create objects easily without writing constructors. However, if you write any constructor yourself, the default constructor is not added automatically.