Constructor execution flow in Java - Step-by-Step Execution

Concept Flow - Constructor execution flow

Create Object

↓

Call Constructor

↓

Initialize Instance Variables

↓

Execute Constructor Body

↓

Return Object Reference

↓

Object Ready

When creating an object, Java calls the constructor which runs its code to initialize the object before returning it.

Execution Sample

Java

class Box {
  int width;
  Box(int w) {
    width = w;
  }
}
Box b = new Box(5);

This code creates a Box object with width set to 5 by running the constructor.

Execution Table

Step	Action	Variable/Field	Value	Notes
1	Call constructor Box(5)	w	5	Parameter w receives 5
2	Assign width = w	width	5	Instance variable width set to 5
3	Constructor ends	-	-	Object initialization complete
4	Object reference assigned	b	Box@<address>	Variable b holds new object reference

💡 Constructor finishes, object is fully initialized and ready to use

Variable Tracker

Variable/Field	Start	After Step 1	After Step 2	Final
w	-	5	5	5
width	0 (default)	0 (default)	5	5
b	null	null	null	Box@<address>

Key Moments - 3 Insights

Why does the instance variable 'width' have value 0 before assignment?

Java initializes instance variables to default values (0 for int) before the constructor runs, as shown in variable_tracker after start.

What happens if we do not assign 'width = w' inside the constructor?

The instance variable 'width' remains at its default value 0, so the object won't have the intended width, as seen in step 2 of execution_table.

When is the object reference 'b' assigned the new object?

After the constructor finishes (step 3), the new object reference is assigned to 'b' (step 4), so 'b' points to the fully initialized object.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'width' after step 2?

A null

B 0

C 5

D undefined

💡 Hint

Check the 'width' column in execution_table row for step 2

At which step is the constructor body executed?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column describing assignment inside constructor

If we remove 'width = w;' from the constructor, what changes in variable_tracker?

A 'w' becomes 0

B 'width' stays 0 after all steps

C 'b' becomes null

D No change

💡 Hint

Refer to variable_tracker row for 'width' and how assignment affects it

Concept Snapshot

Constructor execution flow in Java:
- Object creation calls constructor
- Constructor parameters receive arguments
- Instance variables initialized to defaults before constructor
- Constructor body runs, assigns values
- Object reference assigned after constructor ends

Full Transcript

When you create an object in Java, the constructor runs first. The constructor receives any parameters you pass. Before the constructor runs, Java sets instance variables to default values like 0 for numbers. Then the constructor code runs and assigns values to these variables. After the constructor finishes, the new object reference is assigned to your variable. This process ensures your object is ready to use with all fields properly set.