Constructor execution flow in Java

Introduction

Constructors help create objects by setting up their starting values. Understanding how constructors run helps you know what happens first when you make an object.

When you want to create a new object with specific starting values.

When you have a class with parent and child classes and want to know the order constructors run.

When debugging why some parts of an object are not set as expected after creation.

Syntax

Java

class ClassName {
    ClassName() {
        // constructor code here
    }
}

A constructor has the same name as the class.

It runs automatically when you create an object with new.

Examples

Simple constructor printing a message when an Animal object is created.

Java

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

Child class constructor calls parent constructor first, then runs its own code.

Java

class Dog extends Animal {
    Dog() {
        super(); // calls Animal constructor
        System.out.println("Dog constructor runs");
    }
}

If super() is not written, Java calls the parent constructor automatically first.

Java

class Cat extends Animal {
    Cat() {
        System.out.println("Cat constructor runs");
    }
}

Sample Program

This program shows the order constructors run when creating a Dog object. The Animal constructor runs first, then the Dog constructor.

Java

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

class Dog extends Animal {
    Dog() {
        System.out.println("Dog constructor starts");
    }
}

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

Important Notes

Java always runs the parent class constructor before the child class constructor.

If you don't write super(), Java adds it automatically as the first line in the child constructor.

Constructors help set up objects step-by-step from parent to child.

Summary

Constructors run automatically when creating objects.

Parent class constructor runs before child class constructor.

Understanding this flow helps you control object setup clearly.

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