Constructor execution flow in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the parent class constructor from the child class.

Java

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

class Child extends Parent {
    Child() {
        [1];
        System.out.println("Child constructor");
    }
}

Drag options to blanks, or click blank then click option'

Asuper()

Bthis()

CParent()

Dparent()

Attempts:

3 left

💡 Hint

Common Mistakes

Using this() instead of super()

Calling Parent() like a method

Forgetting to call the parent constructor

In Java, super() calls the parent class constructor.

2fill in blank

medium

Complete the code to call another constructor in the same class.

Java

class Example {
    Example() {
        this(10);
        System.out.println("Default constructor");
    }

    Example(int x) {
        System.out.println("Constructor with value: " + [1]);
    }
}

Drag options to blanks, or click blank then click option'

A10

Bthis

Cx

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using this instead of the parameter name

Using the literal 10 instead of the parameter

Using an undefined variable

The parameter name x is used to print the passed value.

3fill in blank

hard

Fix the error in the constructor chaining code.

Java

class Test {
    Test() {
        [1];
        System.out.println("No-arg constructor");
    }

    Test(int a) {
        System.out.println("Constructor with int: " + a);
    }
}

Drag options to blanks, or click blank then click option'

Athis(5)

Bthis()

CTest(5)

Dsuper(5)

Attempts:

3 left

💡 Hint

Common Mistakes

Using super(5) when no parent constructor with int exists

Calling Test(5) like a method

Using this() which calls no-arg constructor causing recursion

Use this(5) to call another constructor in the same class with an int argument.

4fill in blank

hard

Fill both blanks to complete the constructor chaining and print the correct messages.

Java

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

class Derived extends Base {
    Derived() {
        [1];
        System.out.println("Derived constructor");
    }

    Derived(int x) {
        System.out.println("Derived with int: " + [2]);
    }
}

Drag options to blanks, or click blank then click option'

Athis(5)

Bsuper()

Cx

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using super() instead of this(5) in the no-arg constructor

Printing the literal 5 instead of the parameter x

Not chaining constructors properly

The no-arg constructor calls this(5) to chain to the int constructor, which prints the parameter x.

5fill in blank

hard

Fill all three blanks to create a class with constructor chaining and parent constructor call.

Java

class Alpha {
    Alpha() {
        System.out.println("Alpha default");
    }

    Alpha(String msg) {
        System.out.println(msg);
    }
}

class Beta extends Alpha {
    Beta() {
        [1];
        System.out.println("Beta default");
    }

    Beta(String msg) {
        [2];
        System.out.println("Beta with message: " + [3]);
    }
}

Drag options to blanks, or click blank then click option'

Athis("Hello")

Bsuper(msg)

Cmsg

Dsuper()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling super() instead of this() in the no-arg constructor

Not calling super(msg) in the string constructor

Printing a variable not defined or the wrong variable

The no-arg constructor calls this("Hello") to chain to the string constructor.
The string constructor calls super(msg) to call the parent constructor with the message.
Finally, it prints the message using the parameter msg.

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