Constructor overloading in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of overloaded constructors with different parameters

What is the output of this Java program using constructor overloading?

Java

public class Box {
    int width, height, depth;

    Box() {
        width = height = depth = 1;
    }

    Box(int w, int h, int d) {
        width = w;
        height = h;
        depth = d;
    }

    void display() {
        System.out.println("Volume: " + (width * height * depth));
    }

    public static void main(String[] args) {
        Box b1 = new Box();
        Box b2 = new Box(2, 3, 4);
        b1.display();
        b2.display();
    }
}

A

Volume: 0
Volume: 0

B

Volume: 0
Volume: 24

C

Volume: 1
Volume: 9

D

Volume: 1
Volume: 24

💡 Hint

Check how the default constructor sets all dimensions to 1.

The default constructor sets width, height, and depth to 1, so volume is 1*1*1=1. The overloaded constructor sets them to 2,3,4, so volume is 24.

❓ Predict Output

intermediate

Which constructor is called?

Given this Java class with overloaded constructors, what will be printed when creating an object with new Person("Alice")?

Java

public class Person {
    String name;
    int age;

    Person() {
        name = "Unknown";
        age = 0;
        System.out.println("Default constructor called");
    }

    Person(String n) {
        name = n;
        age = 18;
        System.out.println("Constructor with name called");
    }

    Person(String n, int a) {
        name = n;
        age = a;
        System.out.println("Constructor with name and age called");
    }

    public static void main(String[] args) {
        Person p = new Person("Alice");
    }
}

A Constructor with name called

B Constructor with name and age called

C Default constructor called

D Compilation error

💡 Hint

Look at the constructor signatures and which matches the argument.

The constructor Person(String n) matches the call new Person("Alice"), so it prints "Constructor with name called".

🔧 Debug

advanced

Identify the compilation error in constructor overloading

Why does this Java class fail to compile?

Java

public class Car {
    String model;
    int year;

    Car(String m) {
        model = m;
    }

    Car(String m, int y) {
        model = m;
        year = y;
    }

    Car(String m, int y) {
        model = m;
        year = y + 1;
    }
}

A Cannot overload constructors with different parameter counts

B Missing return type in constructors

C Duplicate constructor with same parameter types

D Constructor names do not match class name

💡 Hint

Check if any constructors have identical parameter lists.

Two constructors have the same parameter types (String, int), which causes a duplicate method error.

❓ Predict Output

advanced

Output of constructor chaining with this()

What is the output of this Java program that uses constructor chaining?

Java

public class Animal {
    String type;
    int legs;

    Animal() {
        this("Unknown");
        System.out.println("No-arg constructor");
    }

    Animal(String t) {
        this(t, 4);
        System.out.println("One-arg constructor");
    }

    Animal(String t, int l) {
        type = t;
        legs = l;
        System.out.println("Two-arg constructor");
    }

    public static void main(String[] args) {
        Animal a = new Animal();
    }
}

A

No-arg constructor
One-arg constructor
Two-arg constructor

B

Two-arg constructor
One-arg constructor
No-arg constructor

C

One-arg constructor
Two-arg constructor
No-arg constructor

D Compilation error due to constructor chaining

💡 Hint

Remember that this() calls must be the first statement in a constructor.

The no-arg constructor calls the one-arg constructor, which calls the two-arg constructor. The print statements execute after each call returns, so the order is two-arg, one-arg, no-arg.

🧠 Conceptual

expert

Why constructor overloading improves code design?

Which of the following best explains why constructor overloading is useful in Java?

A It enables creating objects with different initial data by providing multiple constructor options.

B It allows creating multiple constructors with the same parameters but different names.

C It forces all constructors to have the same number of parameters for consistency.

D It automatically generates default constructors if none are defined.

💡 Hint

Think about how constructors help initialize objects in different ways.

Constructor overloading lets you create objects with different sets of initial data by defining multiple constructors with different parameter lists.

Practice

1.

What is constructor overloading in Java?

easy

A. Having multiple constructors with different parameter lists in the same class.

B. Using the same constructor name with the same parameters multiple times.

C. Creating constructors only with no parameters.

D. Defining constructors outside the class.

Solution

1. Step 1: Understand constructor overloading definition

   Constructor overloading means having more than one constructor in a class, each with a different set of parameters.

2. Step 2: Check options against definition

   Having multiple constructors with different parameter lists in the same class. correctly states this. Options B, C, and D are incorrect because they describe invalid or unrelated concepts.

3. Final Answer:

   Having multiple constructors with different parameter lists in the same class. -> Option A

4. Quick Check:

   Constructor overloading = multiple constructors with different parameters [OK]

Hint: Look for multiple constructors with unique parameter lists [OK]

Common Mistakes:

• Thinking constructors must have different names
• Confusing overloading with overriding
• Believing constructors cannot have parameters

2.

Which of the following constructor declarations is correct for overloading?

public class Car {
    public Car() { }
    public Car(String model) { }
    public Car(int year) { }
    public Car(String model, int year) { }
}

easy

A. Only constructors with one parameter are allowed.

B. Constructors must have different names, so this is incorrect.

C. All constructors have different parameter lists, so all are correct.

D. Constructors cannot have parameters, so only the first is correct.

Solution

1. Step 1: Check parameter lists of constructors

   Each constructor has a unique parameter list: no parameters, one String, one int, and two parameters.

2. Step 2: Verify constructor overloading rules

   Constructors can share the same name but must differ in parameters. This code follows that rule.

3. Final Answer:

   All constructors have different parameter lists, so all are correct. -> Option C

4. Quick Check:

   Different parameters = valid overloading [OK]

Hint: Check if parameter lists differ, not constructor names [OK]

Common Mistakes:

• Thinking constructors need different names
• Believing constructors cannot have parameters
• Confusing method overloading rules with constructors

3.

What will be the output of the following code?

class Box {
    int width, height;
    Box() {
        width = 10;
        height = 10;
    }
    Box(int w, int h) {
        width = w;
        height = h;
    }
    void display() {
        System.out.println(width + "," + height);
    }
}

public class Test {
    public static void main(String[] args) {
        Box b1 = new Box();
        Box b2 = new Box(5, 15);
        b1.display();
        b2.display();
    }
}

medium

A. 10,10
5,15

B. 5,15
10,10

C. 10,10
10,10

D. Compilation error due to constructor overloading

Solution

1. Step 1: Identify which constructors are called

   b1 uses the no-argument constructor setting width and height to 10. b2 uses the two-parameter constructor setting width=5 and height=15.

2. Step 2: Understand display output

   b1.display() prints "10,10" and b2.display() prints "5,15" on separate lines.

3. Final Answer:

   10,10 5,15 -> Option A

4. Quick Check:

   Constructor sets values correctly, output matches [OK]

Hint: Match constructor parameters to object creation [OK]

Common Mistakes:

• Assuming default constructor sets zero values
• Mixing order of printed outputs
• Thinking overloading causes errors here

4.

Find the error in the following code snippet:

class Person {
    String name;
    int age;
    Person(String n) {
        name = n;
    }
    Person(String n, int a) {
        name = n;
        age = a;
    }
    Person() {
        this("Unknown");
        this(0);
    }
}

medium

A. Missing return type in constructors.

B. Cannot call two constructors using 'this()' in the same constructor.

C. Constructor names must be different for overloading.

D. No error; code is correct.

Solution

1. Step 1: Analyze constructor chaining rules

   In Java, a constructor can call only one other constructor using 'this()' and it must be the first statement.

2. Step 2: Identify the error in the no-argument constructor

   The no-argument constructor calls 'this("Unknown")' and then 'this(0)', which is not allowed.

3. Final Answer:

   Cannot call two constructors using 'this()' in the same constructor. -> Option B

4. Quick Check:

   Only one 'this()' call allowed per constructor [OK]

Hint: Only one 'this()' call allowed as first line in constructor [OK]

Common Mistakes:

• Trying to call multiple constructors with 'this()'
• Thinking constructor names must differ
• Forgetting 'this()' must be first statement

5.

Consider a class Employee with overloaded constructors:

class Employee {
    String name;
    int id;
    double salary;

    Employee(String name) {
        this.name = name;
        this.id = 0;
        this.salary = 0.0;
    }

    Employee(String name, int id) {
        this(name);
        this.id = id;
    }

    Employee(String name, int id, double salary) {
        this(name, id);
        this.salary = salary;
    }

    void display() {
        System.out.println(name + "," + id + "," + salary);
    }
}

public class Main {
    public static void main(String[] args) {
        Employee e = new Employee("Alice", 101, 75000.0);
        e.display();
    }
}

What will be the output when running Main?

hard

A. Alice,101,0.0

B. Alice,0,0.0

C. Compilation error due to constructor chaining

D. Alice,101,75000.0

Solution

1. Step 1: Trace constructor calls for Employee("Alice", 101, 75000.0)

   The three-parameter constructor calls the two-parameter constructor with ("Alice", 101), which calls the one-parameter constructor with ("Alice").

2. Step 2: Understand field assignments

   The one-parameter constructor sets name="Alice", id=0, salary=0.0. The two-parameter constructor updates id=101. The three-parameter constructor updates salary=75000.0.

3. Step 3: Output values

   After all calls, name="Alice", id=101, salary=75000.0, so display prints "Alice,101,75000.0".

4. Final Answer:

   Alice,101,75000.0 -> Option D

5. Quick Check:

   Constructor chaining updates fields stepwise [OK]

Hint: Follow constructor calls step-by-step to track field values [OK]

Common Mistakes:

• Assuming fields reset after each constructor call
• Thinking constructor chaining causes errors
• Ignoring order of field assignments