Default constructor in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a class with no explicit constructor

What is the output of this Java program?

Java

public class Test {
    int x;
    public static void main(String[] args) {
        Test obj = new Test();
        System.out.println(obj.x);
    }
}

A Compilation error

B null

C 0

D Runtime error

💡 Hint

Think about what Java does if you don't write any constructor.

Java provides a default constructor that initializes int fields to 0.

🧠 Conceptual

intermediate

Role of default constructor in inheritance

Consider these classes: class Parent {} class Child extends Parent {} What happens when you create new Child()?

A Runtime error due to missing constructor

B Child's default constructor does not call Parent's constructor

C Compilation error because Parent has no constructor

D Child's default constructor calls Parent's default constructor automatically

💡 Hint

Think about how constructors work in inheritance when none are defined.

If no constructor is defined, Java inserts a default constructor that calls super().

❓ Predict Output

advanced

Output when default constructor is overridden

What is the output of this code?

Java

public class Demo {
    int value;
    Demo() {
        value = 10;
    }
    public static void main(String[] args) {
        Demo d = new Demo();
        System.out.println(d.value);
    }
}

A 10

B 0

C Compilation error

D Runtime error

💡 Hint

Look at the constructor and what it does to the value field.

The constructor sets value to 10, so the output is 10.

❓ Predict Output

advanced

What error occurs without default constructor in subclass?

Given these classes: class A { A(int x) {} } class B extends A { B() {} } What happens when compiling?

A Compilation error: constructor A() is undefined

B Runtime error

C Compiles and runs fine

D Compilation error: constructor B() is undefined

💡 Hint

Think about what happens if superclass has no default constructor.

Subclass constructor must call superclass constructor explicitly if no default exists.

🧠 Conceptual

expert

How many constructors are created by default?

If you write a Java class with no constructors, how many constructors does the compiler create?

A No constructors are created automatically

B One default constructor with no parameters

C One constructor with parameters matching all fields

D Two constructors: one default and one copy constructor

💡 Hint

Think about what Java provides if you don't write any constructor.

Java automatically creates one no-argument default constructor if none is defined.

Practice

1. What is a default constructor in Java?

easy

A. A method that returns the default value of a class.

B. A constructor with no parameters that Java provides automatically if none is written.

C. A constructor that must always be written by the programmer.

D. A special method that runs only when a program ends.

Solution

1. Step 1: Understand what a constructor is

   A constructor is a special method used to create objects of a class.

2. Step 2: Identify the default constructor

   If no constructor is written, Java automatically provides a constructor with no parameters called the default constructor.

3. Final Answer:

   A constructor with no parameters that Java provides automatically if none is written. -> Option B

4. Quick Check:

   Default constructor = automatic no-parameter constructor [OK]

Hint: Default constructor has no parameters and is auto-created if missing [OK]

Common Mistakes:

• Thinking default constructor must be written manually
• Confusing default constructor with methods returning default values
• Believing default constructor runs at program end

2. Which of the following is the correct syntax for a default constructor in Java?

easy

A. public ClassName(void) { }

B. public void ClassName() { }

C. public ClassName() { }

D. void ClassName() { }

Solution

1. Step 1: Recall constructor syntax

   A constructor has the same name as the class and no return type.

2. Step 2: Check each option

   public ClassName() { } matches the class name and has no return type, so it's correct syntax.

3. Final Answer:

   public ClassName() { } -> Option C

4. Quick Check:

   Constructor syntax = class name + no return type [OK]

Hint: Constructor name = class name, no return type, parentheses empty [OK]

Common Mistakes:

• Adding void return type to constructor
• Using wrong parameter list syntax
• Using lowercase class name in constructor

3. What will be the output of this Java code?

class Car {
  String model;
}
public class Test {
  public static void main(String[] args) {
    Car c = new Car();
    System.out.println(c.model);
  }
}

medium

A. null

B. Empty string

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand default constructor usage

   No constructor is defined, so Java provides a default constructor that sets no initial values.

2. Step 2: Check default value of uninitialized String

   Instance variable 'model' is a String and defaults to null if not set.

3. Final Answer:

   null -> Option A

4. Quick Check:

   Uninitialized String = null by default [OK]

Hint: Uninitialized object fields default to null in Java [OK]

Common Mistakes:

• Expecting empty string instead of null
• Thinking default constructor sets values automatically
• Assuming compilation or runtime error

4. Identify the error in this Java class if any:

public class Book {
  String title;
  public Book() {
    title = "Java Basics";
  }
  public Book(String title) {
    title = title;
  }
}

medium

A. The second constructor does not set the instance variable correctly.

B. The default constructor is missing.

C. The class has no constructors.

D. The constructors have wrong return types.

Solution

1. Step 1: Analyze the second constructor

   It assigns parameter 'title' to itself, not to the instance variable.

2. Step 2: Understand correct assignment

   Use 'this.title = title;' to assign parameter to instance variable.

3. Final Answer:

   The second constructor does not set the instance variable correctly. -> Option A

4. Quick Check:

   Use 'this' to assign constructor parameters to fields [OK]

Hint: Use 'this.' to assign parameters to instance variables [OK]

Common Mistakes:

• Assigning parameter to itself instead of instance variable
• Missing default constructor (actually present)
• Adding return types to constructors

5. You want to create a class Person that sets the name to "Unknown" by default if no name is given. Which constructor code correctly implements this using a default constructor?

hard

A. public class Person {
String name;
public Person() {
name = name;
}
}

B. public class Person {
String name = "Unknown";
public Person(String name) {
name = name;
}
}

C. public class Person {
String name;
public Person(String name) {
this.name = name;
}
}

D. public class Person {
String name;
public Person() {
name = "Unknown";
}
public Person(String name) {
this.name = name;
}
}

Solution

1. Step 1: Check default constructor sets default value

   public class Person { String name; public Person() { name = "Unknown"; } public Person(String name) { this.name = name; } }'s default constructor sets name to "Unknown" correctly.

2. Step 2: Verify parameterized constructor sets name properly

   public class Person { String name; public Person() { name = "Unknown"; } public Person(String name) { this.name = name; } } uses 'this.name = name;' to assign parameter to instance variable.

3. Final Answer:

   public class Person { String name; public Person() { name = "Unknown"; } public Person(String name) { this.name = name; } } -> Option D

4. Quick Check:

   Default constructor sets default value, parameterized sets given value [OK]

Hint: Default constructor sets default values; use 'this' for parameters [OK]

Common Mistakes:

• Assigning parameter to itself without 'this.'
• Not setting default value in default constructor
• Missing default constructor entirely