Constructor execution flow in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this constructor execution?

Consider the following Java code with inheritance and constructors. What will be printed when new Child() is executed?

Java

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

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

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

A

Parent constructor
Child constructor

B

Child constructor
Parent constructor

C Child constructor

D Parent constructor

💡 Hint

Remember that the parent constructor runs before the child constructor.

When creating a Child object, Java first calls the Parent constructor implicitly, then the Child constructor. So the output is Parent constructor followed by Child constructor.

❓ Predict Output

intermediate

What is the output when constructors call each other?

Analyze the constructor calls and determine the output.

Java

class A {
    A() {
        this(5);
        System.out.println("A no-arg");
    }
    A(int x) {
        System.out.println("A int: " + x);
    }
}

public class Test {
    public static void main(String[] args) {
        new A();
    }
}

A

A no-arg
A int: 5

B

A int: 5
A no-arg

C A int: 5

D Compilation error

💡 Hint

Look at how this() calls another constructor before printing.

The no-argument constructor calls the one-argument constructor first, which prints A int: 5. After that finishes, it prints A no-arg.

🔧 Debug

advanced

What error occurs in this constructor chaining?

Examine the code and identify the error that occurs when compiling.

Java

class B {
    B() {
        this();
        System.out.println("B no-arg");
    }
}

public class Test {
    public static void main(String[] args) {
        new B();
    }
}

A NullPointerException at runtime

B StackOverflowError at runtime

C No output, program runs silently

D Compilation error: recursive constructor call

💡 Hint

Check how this() is used inside the constructor.

The constructor calls itself recursively with this() without a base case, causing a compile-time error for recursive constructor invocation.

❓ Predict Output

advanced

What is the output of this multi-level constructor chain?

Given the classes below, what will be printed when new C() is executed?

Java

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

class B extends A {
    B() {
        System.out.println("B constructor");
    }
}

class C extends B {
    C() {
        super();
        System.out.println("C constructor");
    }
}

public class Test {
    public static void main(String[] args) {
        new C();
    }
}

A

A constructor
B constructor
C constructor

B

B constructor
A constructor
C constructor

C Compilation error

D

A constructor
C constructor
B constructor

💡 Hint

Remember the order of constructor calls in inheritance.

When C() is called, it calls B() via super(), which implicitly calls A(). So the order is A, then B, then C.

🧠 Conceptual

expert

How many times is the constructor of class A called?

Consider the following code. How many times is the constructor of class A executed when new D() is called?

Java

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

class B extends A {
    B() {
        System.out.println("B constructor");
    }
}

class C extends A {
    C() {
        System.out.println("C constructor");
    }
}

class D extends B {
    D() {
        super();
        System.out.println("D constructor");
    }
}

public class Test {
    public static void main(String[] args) {
        new D();
    }
}

A 2

B 3

C 1

D 0

💡 Hint

Trace the inheritance chain and constructor calls carefully.

Class D extends B, which extends A. When D() is called, it calls B(), which calls A(). Class C is unrelated here and its constructor is not called. So A() runs once.

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