Parameterized constructor in Java - Time & Space Complexity

Time Complexity: Parameterized constructor

O(n)

Understanding Time Complexity

Let's see how the time it takes to run a parameterized constructor changes as we create more objects.

We want to know how the work grows when we make many objects using this constructor.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public class Person {
    String name;
    int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

// Creating multiple Person objects
for (int i = 0; i < n; i++) {
    Person p = new Person("Name" + i, i);
}

This code defines a parameterized constructor and creates n Person objects using it.

Identify Repeating Operations

• Primary operation: Creating a Person object using the constructor.
• How many times: The constructor runs once for each of the n objects in the loop.

How Execution Grows With Input

Each new object requires the constructor to run once, so the total work grows directly with the number of objects.

Input Size (n)	Approx. Operations
10	10 constructor calls
100	100 constructor calls
1000	1000 constructor calls

Pattern observation: The work increases evenly as we create more objects.

Final Time Complexity

Time Complexity: O(n)

This means the time to create n objects grows in a straight line with n.

Common Mistake

[X] Wrong: "The constructor runs only once no matter how many objects we create."

[OK] Correct: Each object needs its own constructor call, so the constructor runs n times for n objects.

Interview Connect

Understanding how constructors work with many objects helps you explain how your code scales in real projects.

Self-Check

"What if the constructor did some extra work inside a loop? How would that affect the time complexity?"

Practice

1.

What is the main purpose of a parameterized constructor in Java?

easy

A. To initialize an object with specific values when it is created

B. To create multiple objects without any initial values

C. To delete an object from memory

D. To define a method that returns a value

Solution

1. Step 1: Understand constructor purpose

   A constructor is used to initialize objects when they are created.

2. Step 2: Identify parameterized constructor role

   A parameterized constructor takes arguments to set initial values for the object's fields.

3. Final Answer:

   To initialize an object with specific values when it is created -> Option A

4. Quick Check:

   Parameterized constructor = initialize with values [OK]

Hint: Constructors with parameters set initial object values [OK]

Common Mistakes:

• Confusing constructors with methods
• Thinking constructors delete objects
• Believing constructors don't take parameters

2.

Which of the following is the correct syntax for a parameterized constructor in Java?

public class Car {
    String model;
    int year;

    // Constructor here
}

easy

A. void Car(String model, int year) { this.model = model; this.year = year; }

B. public Car(String model, int year) { this.model = model; this.year = year; }

C. public Car() { model = ""; year = 0; }

D. public void Car(String model, int year) { model = model; year = year; }

Solution

1. Step 1: Check constructor syntax

   A constructor has no return type and matches the class name exactly.

2. Step 2: Verify parameter usage

   public Car(String model, int year) { this.model = model; this.year = year; } correctly uses parameters and assigns them to fields with this.

3. Final Answer:

   public Car(String model, int year) { this.model = model; this.year = year; } -> Option B

4. Quick Check:

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

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

Common Mistakes:

• Adding return type to constructor
• Not using 'this' to assign fields
• Using void or other return types

3.

What will be the output of the following Java code?

class Book {
    String title;
    int pages;

    Book(String title, int pages) {
        this.title = title;
        this.pages = pages;
    }

    void display() {
        System.out.println(title + ": " + pages + " pages");
    }
}

public class Main {
    public static void main(String[] args) {
        Book b = new Book("Java Basics", 250);
        b.display();
    }
}

medium

A. Java Basics: 250 pages

B. Java Basics pages

C. 250 pages

D. Compilation error

Solution

1. Step 1: Understand constructor initialization

   The constructor sets title to "Java Basics" and pages to 250.

2. Step 2: Analyze display method output

   The display() method prints the title, colon, pages, and "pages" text.

3. Final Answer:

   Java Basics: 250 pages -> Option A

4. Quick Check:

   Constructor sets fields, display prints them [OK]

Hint: Constructor sets fields, display prints combined string [OK]

Common Mistakes:

• Ignoring 'this' keyword effect
• Expecting only title or pages printed
• Assuming syntax error without reason

4.

Identify the error in this Java class with a parameterized constructor:

public class Student {
    String name;
    int age;

    public Student(String n, int a) {
        name = n;
        age = a;
    }

    public Student() {
        name = "Unknown";
        age = 0;
    }

    public void Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

medium

A. No default constructor defined

B. Missing return type in parameterized constructor

C. The method named Student with void return type is not a constructor

D. Fields name and age are not declared

Solution

1. Step 1: Check constructor definitions

   Constructors have no return type and match class name exactly.

2. Step 2: Identify method named Student with void

   The method public void Student(String name, int age) is not a constructor but a method, which is confusing.

3. Final Answer:

   The method named Student with void return type is not a constructor -> Option C

4. Quick Check:

   Constructors have no return type [OK]

Hint: Constructors never have a return type, not even void [OK]

Common Mistakes:

• Writing constructors with void return type
• Confusing methods with constructors
• Missing default constructor when needed

5.

Given the class below, what will be the output when creating two Employee objects with different parameters and printing their details?

class Employee {
    String name;
    double salary;

    public Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    public void printInfo() {
        System.out.println(name + " earns $" + salary);
    }
}

public class Test {
    public static void main(String[] args) {
        Employee e1 = new Employee("Alice", 50000);
        Employee e2 = new Employee("Bob", 60000);
        e1.printInfo();
        e2.printInfo();
    }
}

hard

A. Runtime error due to uninitialized fields

B. Alice earns $60000.0
Bob earns $50000.0

C. Compilation error due to missing default constructor

D. Alice earns $50000.0
Bob earns $60000.0

Solution

1. Step 1: Analyze object creation with parameters

   Two Employee objects are created with names and salaries passed to the parameterized constructor.

2. Step 2: Check printInfo output

   Each object's printInfo() prints the name and salary correctly.

3. Final Answer:

   Alice earns $50000.0 Bob earns $60000.0 -> Option D

4. Quick Check:

   Parameterized constructor sets fields, printInfo shows them [OK]

Hint: Each object holds its own values set by constructor [OK]

Common Mistakes:

• Mixing up values between objects
• Expecting errors without reason
• Assuming default constructor is needed here