Object creation in Java - Time & Space Complexity

Time Complexity: Object creation

O(n)

Understanding Time Complexity

When we create objects in Java, it takes some time for the computer to set up each new object.

We want to understand how the time needed grows as we create more objects.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public class Example {
    public static void createObjects(int n) {
        for (int i = 0; i < n; i++) {
            Object obj = new Object();
        }
    }
}
    

This code creates n new objects one after another in a loop.

Identify Repeating Operations

• Primary operation: Creating a new object inside the loop.
• How many times: The loop runs n times, so n objects are created.

How Execution Grows With Input

Each time we increase n, we create more objects, so the time grows directly with n.

Input Size (n)	Approx. Operations
10	10 object creations
100	100 object creations
1000	1000 object creations

Pattern observation: If you double the number of objects to create, the time roughly doubles too.

Final Time Complexity

Time Complexity: O(n)

This means the time to create objects grows in a straight line with the number of objects you want to make.

Common Mistake

[X] Wrong: "Creating objects inside a loop is instant and does not affect performance."

[OK] Correct: Each object creation takes time, so doing it many times adds up and affects how long the program runs.

Interview Connect

Understanding how object creation time grows helps you write code that runs smoothly and shows you can think about efficiency clearly.

Self-Check

"What if we created objects only when a condition inside the loop is true? How would the time complexity change?"

Practice

1. Which of the following is the correct way to create an object of class Car in Java?

easy

A. Car myCar = new Car();

B. Car myCar = Car();

C. new Car myCar();

D. Car myCar = new car();

Solution

1. Step 1: Understand object creation syntax

   In Java, objects are created using the new keyword followed by the class name and parentheses.

2. Step 2: Check each option

   Car myCar = new Car(); uses new Car() correctly with proper capitalization and assignment. Others have syntax errors or wrong capitalization.

3. Final Answer:

   Car myCar = new Car(); -> Option A

4. Quick Check:

   Use new ClassName() to create objects [OK]

Hint: Remember: new + ClassName() creates an object [OK]

Common Mistakes:

• Omitting the new keyword
• Using wrong capitalization for class name
• Missing parentheses after class name

2. Which of the following lines will cause a syntax error when creating an object of class Book?

easy

A. Book b=new Book();

B. Book b = new Book();

C. Book b = new Book;

D. Book b = new Book( );

Solution

1. Step 1: Recall syntax for object creation

   In Java, when creating an object, parentheses must follow the class name even if the constructor has no parameters.

2. Step 2: Identify the incorrect option

   Book b = new Book; misses the parentheses after new Book, causing a syntax error. Others are correct.

3. Final Answer:

   Book b = new Book; -> Option C

4. Quick Check:

   Always use parentheses after class name in new [OK]

Hint: Always include () after class name when using new [OK]

Common Mistakes:

• Forgetting parentheses after class name
• Confusing object creation with method calls
• Using semicolon inside parentheses

3. What will be the output of the following code?

class Dog {
    String name;
    Dog(String n) {
        name = n;
    }
}

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog("Buddy");
        System.out.println(d.name);
    }
}

medium

A. Buddy

B. null

C. Dog@someHashCode

D. Compilation error

Solution

1. Step 1: Understand constructor assignment

   The constructor sets the name field to the string passed, which is "Buddy".

2. Step 2: Check output of print statement

   Printing d.name outputs the string "Buddy" stored in the object.

3. Final Answer:

   Buddy -> Option A

4. Quick Check:

   Constructor sets field, printing field shows assigned value [OK]

Hint: Constructor sets values; print field to see stored data [OK]

Common Mistakes:

• Expecting default null instead of assigned value
• Confusing object reference print with field print
• Missing constructor parameters

4. Identify the error in the following code snippet:

class Person {
    String name;
    Person(String n) {
        name = n;
    }
}

public class Test {
    public static void main(String[] args) {
        Person p = Person("Alice");
        System.out.println(p.name);
    }
}

medium

A. Missing semicolon after System.out.println

B. Constructor name does not match class name

C. Variable p is not declared

D. Missing new keyword when creating object

Solution

1. Step 1: Check object creation syntax

   The code tries to create an object with Person("Alice") but misses the new keyword.

2. Step 2: Confirm other parts are correct

   Constructor name matches class name, variable is declared, and semicolon is present.

3. Final Answer:

   Missing new keyword when creating object -> Option D

4. Quick Check:

   Use new keyword to create objects [OK]

Hint: Always use new before class name to create objects [OK]

Common Mistakes:

• Omitting new keyword
• Confusing method call with object creation
• Incorrect constructor naming

5. Given the class below, which code correctly creates two Student objects with names "John" and "Jane" and prints their names?

class Student {
    String name;
    Student(String n) {
        name = n;
    }
}

hard

A. Student s1 = new Student("John");
Student s2 = new Student();
System.out.println(s1.name + ", " + s2.name);

B. Student s1 = new Student("John");
Student s2 = new Student("Jane");
System.out.println(s1.name + ", " + s2.name);

C. Student s1 = new Student();
Student s2 = new Student();
System.out.println(s1.name + ", " + s2.name);

D. Student s1 = Student("John");
Student s2 = Student("Jane");
System.out.println(s1.name + ", " + s2.name);

Solution

1. Step 1: Check constructor usage

   The constructor requires a String parameter. Student s1 = new Student("John"); Student s2 = new Student("Jane"); System.out.println(s1.name + ", " + s2.name); correctly passes names "John" and "Jane".

2. Step 2: Verify object creation and printing

   Student s1 = new Student("John"); Student s2 = new Student("Jane"); System.out.println(s1.name + ", " + s2.name); creates both objects properly and prints their names separated by a comma.

3. Final Answer:

   Student s1 = new Student("John"); Student s2 = new Student("Jane"); System.out.println(s1.name + ", " + s2.name); -> Option B

4. Quick Check:

   Use new with constructor parameters to create objects [OK]

Hint: Pass required parameters in new ClassName(params) [OK]

Common Mistakes:

• Calling constructor without new keyword
• Using default constructor when none exists
• Not passing required parameters