Parameterized constructor in Java - Step-by-Step Execution

Concept Flow - Parameterized constructor

Start Object Creation

↓

Call Constructor with Parameters

↓

Assign Parameters to Fields

↓

Object Initialized with Given Values

↓

End

When creating an object, the constructor receives values (parameters) and assigns them to the object's fields to initialize it.

Execution Sample

Java

class Car {
  String model;
  int year;
  Car(String m, int y) {
    model = m;
    year = y;
  }
}

Defines a Car class with a parameterized constructor that sets model and year when a new Car is created.

Execution Table

Step	Action	Parameter Values	Field Assignments	Resulting Object State
1	Create new Car object	model = "Tesla", year = 2023	model = "Tesla", year = 2023	Car{model='Tesla', year=2023}
2	Create new Car object	model = "Ford", year = 2018	model = "Ford", year = 2018	Car{model='Ford', year=2018}
3	Create new Car object	model = "BMW", year = 2020	model = "BMW", year = 2020	Car{model='BMW', year=2020}
4	No more objects created	-	-	Execution ends

💡 No more objects created, program ends

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
car1.model	null	"Tesla"	"Tesla"	"Tesla"	"Tesla"
car1.year	0	2023	2023	2023	2023
car2.model	null	null	"Ford"	"Ford"	"Ford"
car2.year	0	0	2018	2018	2018
car3.model	null	null	null	"BMW"	"BMW"
car3.year	0	0	0	2020	2020

Key Moments - 3 Insights

Why do we pass values inside parentheses when creating a new object?

Because the parameterized constructor needs those values to set the object's fields, as shown in execution_table rows 1-3.

What happens if we don't provide the right number or type of parameters?

The code will not compile because the constructor expects specific parameters, as seen in the constructor signature in execution_sample.

Are the object's fields set before or after the constructor finishes?

They are set during the constructor execution, so after the constructor finishes, the object fields hold the passed values (execution_table rows 1-3).

Visual Quiz - 3 Questions

Look at the execution_table, what is the model of car2 after step 2?

A "Tesla"

B "Ford"

C null

D "BMW"

💡 Hint

Check the 'Parameter Values' and 'Field Assignments' columns for step 2 in execution_table.

At which step does the year field of car3 get assigned?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Field Assignments' column for car3 in execution_table.

If we create a new Car with model "Audi" and year 2022, what will be the model of the new object?

A "Tesla"

B "BMW"

C "Audi"

D null

💡 Hint

The constructor assigns the passed model to the object's model field as shown in variable_tracker.

Concept Snapshot

Parameterized constructor syntax:
class ClassName {
  Type field;
  ClassName(Type param) {
    field = param;
  }
}

Used to initialize objects with specific values when created.

Full Transcript

A parameterized constructor is a special method in a class that takes inputs (parameters) to set initial values for the object's fields. When you create a new object and pass values inside parentheses, the constructor uses those values to assign to the object's variables. This way, each object can start with different data. The execution table shows step-by-step how each object is created with its own model and year. The variable tracker follows how each field changes from null or zero to the assigned values. Beginners often wonder why parameters are needed or when fields get set; these happen during the constructor call. If parameters are missing or wrong, the program won't compile. This concept helps create flexible and meaningful objects easily.