Classes and objects in Java - Time & Space Complexity

Time Complexity: Classes and objects

O(n)

Understanding Time Complexity

We want to understand how the time to run code with classes and objects changes as we create more objects or call methods.

How does the program's work grow when we use more objects?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public class Box {
    int length, width, height;

    public Box(int l, int w, int h) {
        length = l;
        width = w;
        height = h;
    }

    public int volume() {
        return length * width * height;
    }
}

public class Main {
    public static void main(String[] args) {
        Box[] boxes = new Box[100];
        for (int i = 0; i < boxes.length; i++) {
            boxes[i] = new Box(i, i+1, i+2);
            System.out.println(boxes[i].volume());
        }
    }
}
    

This code creates 100 Box objects, each with different sizes, and prints their volumes.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop that creates and processes each Box object.
• How many times: It runs once for each box, so 100 times in this example.

How Execution Grows With Input

As the number of boxes increases, the program does more work creating and calculating volumes for each box.

Input Size (n)	Approx. Operations
10	About 10 creations and volume calculations
100	About 100 creations and volume calculations
1000	About 1000 creations and volume calculations

Pattern observation: The work grows directly with the number of boxes; doubling boxes doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line with the number of objects created and processed.

Common Mistake

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

[OK] Correct: Each object creation and method call takes time, so more objects mean more total work.

Interview Connect

Understanding how object creation and method calls scale helps you write efficient code and explain your reasoning clearly in interviews.

Self-Check

"What if we added a nested loop inside the volume method that runs n times? How would the time complexity change?"

Practice

1. What is the main purpose of a class in Java?

easy

A. To serve as a blueprint for creating objects

B. To execute the program's main method

C. To store data permanently on disk

D. To perform input and output operations

Solution

1. Step 1: Understand the role of a class

   A class defines the structure and behavior that objects created from it will have.

2. Step 2: Identify the correct purpose

   Classes are not for running programs or storing data on disk; they are blueprints for objects.

3. Final Answer:

   To serve as a blueprint for creating objects -> Option A

4. Quick Check:

   Class = blueprint for objects [OK]

Hint: Classes define objects' structure and behavior [OK]

Common Mistakes:

• Confusing classes with methods
• Thinking classes store data permanently
• Believing classes execute the program

2. 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. Car myCar = new Car();

D. new Car myCar = Car();

Solution

1. Step 1: Recall object creation syntax

   In Java, objects are created using the new keyword followed by the class constructor with parentheses.

2. Step 2: Check each option

   Car myCar = new Car(); uses correct syntax: Car myCar = new Car();. Others miss parentheses or have wrong order.

3. Final Answer:

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

4. Quick Check:

   Use new ClassName() to create objects [OK]

Hint: Use 'new ClassName()' to create objects [OK]

Common Mistakes:

• Omitting parentheses after class name
• Placing 'new' keyword incorrectly
• Missing semicolon at the end

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

class Dog {
  String name;
  void bark() {
    System.out.println(name + " says Woof!");
  }
}

public class Main {
  public static void main(String[] args) {
    Dog dog1 = new Dog();
    dog1.name = "Buddy";
    dog1.bark();
  }
}

medium

A. null says Woof!

B. Woof!

C. Compilation error

D. Buddy says Woof!

Solution

1. Step 1: Understand object property assignment

   The object dog1 has its name set to "Buddy" before calling bark().

2. Step 2: Analyze the bark method output

   The method prints the name followed by " says Woof!" so it prints "Buddy says Woof!".

3. Final Answer:

   Buddy says Woof! -> Option D

4. Quick Check:

   Object property used in method = Buddy says Woof! [OK]

Hint: Check object fields before method calls for output [OK]

Common Mistakes:

• Assuming default null value prints
• Forgetting to assign the name
• Thinking method prints only 'Woof!'

4. Identify the error in the following code snippet:

class Person {
  String name;
  void setName(String name) {
    name = name;
  }
}

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

medium

A. Missing semicolon after setName method

B. The method setName does not assign the parameter to the instance variable

C. Cannot print p.name directly

D. Class Person should be public

Solution

1. Step 1: Analyze the setName method

   The assignment name = name; assigns the parameter to itself, not to the instance variable.

2. Step 2: Understand instance variable shadowing

   To assign correctly, use this.name = name; to refer to the instance variable.

3. Final Answer:

   The method setName does not assign the parameter to the instance variable -> Option B

4. Quick Check:

   Use 'this' to assign instance variables [OK]

Hint: Use 'this.variable' to refer to instance variables [OK]

Common Mistakes:

• Confusing parameter and instance variable names
• Forgetting 'this' keyword
• Assuming assignment works without 'this'

5. Given the class below, how can you create a method that returns a new Rectangle object with double the width and height of the current one?

class Rectangle {
  int width;
  int height;

  Rectangle(int w, int h) {
    width = w;
    height = h;
  }

  // Your method here
}

hard

A. Rectangle doubleSize() { return new Rectangle(width * 2, height * 2); }

B. void doubleSize() { width *= 2; height *= 2; }

C. Rectangle doubleSize() { width *= 2; height *= 2; return this; }

D. Rectangle doubleSize() { return new Rectangle(width + 2, height + 2); }

Solution

1. Step 1: Understand the requirement

   The method should return a new Rectangle object with width and height doubled, without changing the current object.

2. Step 2: Evaluate each option

   Rectangle doubleSize() { return new Rectangle(width * 2, height * 2); } creates and returns a new Rectangle with doubled dimensions. void doubleSize() { width *= 2; height *= 2; } changes current object and returns void. Rectangle doubleSize() { width *= 2; height *= 2; return this; } changes current object and returns it. Rectangle doubleSize() { return new Rectangle(width + 2, height + 2); } adds 2 instead of doubling.

3. Final Answer:

   Rectangle doubleSize() { return new Rectangle(width * 2, height * 2); } -> Option A

4. Quick Check:

   Return new object with doubled size = Rectangle doubleSize() { return new Rectangle(width * 2, height * 2); } [OK]

Hint: Return new object; don't modify current one [OK]

Common Mistakes:

• Modifying current object instead of returning new
• Adding instead of multiplying dimensions
• Returning void instead of new object