Constructor execution flow in Java - Time & Space Complexity

Time Complexity: Constructor execution flow

O(n)

Understanding Time Complexity

We want to understand how the time taken by constructors grows as we create more objects or use inheritance.

How does the constructor's work change when the program runs?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Parent {
    Parent() {
        System.out.println("Parent constructor");
    }
}

class Child extends Parent {
    Child() {
        super();
        System.out.println("Child constructor");
    }
}

public class Main {
    public static void main(String[] args) {
        Child c = new Child();
    }
}
    

This code creates a Child object, which calls the Parent constructor first, then its own.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Constructor calls during object creation.
• How many times: Each constructor runs once per object creation; Parent constructor runs before Child constructor.

How Execution Grows With Input

Each time we create a new Child object, both Parent and Child constructors run once.

Input Size (n)	Approx. Operations
10	20 constructor calls (10 Parent + 10 Child)
100	200 constructor calls (100 Parent + 100 Child)
1000	2000 constructor calls (1000 Parent + 1000 Child)

Pattern observation: The total constructor calls grow linearly with the number of objects created.

Final Time Complexity

Time Complexity: O(n)

This means the time to create n objects grows directly in proportion to n.

Common Mistake

[X] Wrong: "The Parent constructor runs multiple times per object creation."

[OK] Correct: Each object creation calls the Parent constructor exactly once before the Child constructor.

Interview Connect

Understanding constructor execution flow helps you explain object creation and inheritance clearly, a useful skill in many coding discussions.

Self-Check

"What if the Child constructor called another method that creates multiple objects? How would the time complexity change?"

Practice

1. In Java, when you create an object of a child class, which constructor runs first?

easy

A. The parent class constructor

B. The child class constructor

C. Both constructors run simultaneously

D. No constructor runs automatically

Solution

1. Step 1: Understand constructor call order

   In Java, when creating an object, the parent class constructor runs before the child class constructor.

2. Step 2: Reason about object creation

   This ensures the parent part of the object is set up before the child adds its own setup.

3. Final Answer:

   The parent class constructor -> Option A

4. Quick Check:

   Parent constructor runs first [OK]

Hint: Parent constructor always runs before child constructor [OK]

Common Mistakes:

• Thinking child constructor runs first
• Believing constructors run simultaneously
• Assuming constructors don't run automatically

2. Which of the following is the correct way to call a parent class constructor from a child class in Java?

easy

A. this();

B. parent();

C. super();

D. base();

Solution

1. Step 1: Recall Java syntax for parent constructor call

   Java uses the keyword super() to call the parent class constructor explicitly.

2. Step 2: Check other options

   parent(), this(), and base() are not valid Java syntax for this purpose.

3. Final Answer:

   super(); -> Option C

4. Quick Check:

   Use super() to call parent constructor [OK]

Hint: Use super() to call parent constructor explicitly [OK]

Common Mistakes:

• Using this() instead of super()
• Trying parent() or base() which don't exist
• Omitting the call when needed

3. What is the output of the following Java code?

class Parent {
    Parent() {
        System.out.print("P");
    }
}
class Child extends Parent {
    Child() {
        System.out.print("C");
    }
}
public class Test {
    public static void main(String[] args) {
        new Child();
    }
}

medium

A. CP

B. PC

C. C

D. P

Solution

1. Step 1: Identify constructor calls

   Creating new Child() calls the Child constructor, which implicitly calls the Parent constructor first.

2. Step 2: Trace output order

   Parent constructor prints "P" first, then Child constructor prints "C".

3. Final Answer:

   PC -> Option B

4. Quick Check:

   Parent prints P, then Child prints C [OK]

Hint: Parent constructor output appears before child output [OK]

Common Mistakes:

• Assuming child prints before parent
• Ignoring implicit super() call
• Expecting only one letter output

4. Consider this Java code snippet:

class A {
    A() {
        System.out.print("A");
    }
}
class B extends A {
    B() {
        System.out.print("B");
    }
}
class C extends B {
    C() {
        System.out.print("C");
    }
}
public class Main {
    public static void main(String[] args) {
        new C();
    }
}

What is the output, and if there is an error, how to fix it?

medium

A. ABC

B. Error: No default constructor in A

C. Error: Constructor call missing in B

D. BAC

Solution

1. Step 1: Check constructor chaining

   Each class has a default constructor that implicitly calls its parent constructor.

2. Step 2: Trace constructor calls

   Creating new C() calls C(), which calls B(), which calls A(), printing "A", then "B", then "C".

3. Final Answer:

   ABC -> Option A

4. Quick Check:

   Constructors chain up and print in order A-B-C [OK]

Hint: Default constructors chain automatically if no args [OK]

Common Mistakes:

• Expecting error without explicit constructors
• Thinking constructors don't chain automatically
• Mixing output order

5. Given these classes:

class Vehicle {
    Vehicle() {
        System.out.print("V");
    }
}
class Car extends Vehicle {
    Car() {
        super();
        System.out.print("C");
    }
}
class SportsCar extends Car {
    SportsCar() {
        System.out.print("S");
    }
}
public class Demo {
    public static void main(String[] args) {
        new SportsCar();
    }
}

What is the output, and why does it appear in that order?

hard

A. CSV

B. VSC

C. SVC

D. VCS

Solution

1. Step 1: Analyze constructor calls

   Creating new SportsCar() calls SportsCar(), which implicitly calls Car(), which calls Vehicle() via super().

2. Step 2: Trace output sequence

   Vehicle constructor prints "V", then Car prints "C", then SportsCar prints "S".

3. Final Answer:

   VCS -> Option D

4. Quick Check:

   Parent to child constructor output order is V-C-S [OK]

Hint: Parent constructors run before child, print in that order [OK]

Common Mistakes:

• Assuming SportsCar calls super() explicitly
• Mixing output order
• Forgetting implicit super() call in SportsCar