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.