Here, we have listed the top 40 java constructor interview questions with the best possible answers.
These constructor interview questions can be asked in java technical tests and interviews from the fresher or 1 to 2 years of experience.
We have also covered some important constructor coding interview questions that you must practice cracking interviews. So, let’s start practicing.
Constructor Interview Questions in Java
1. What is a constructor in Java?
Ans: A constructor is a block of code, similar to a method that is used to initialize the state of an object (i.e. instance variable) in a class through a new operator. It is automatically called and executed at the time of object creation by JVM.
2. What is the main objective of a constructor in java?
Or, Why do we need a constructor in a class as a member?
Ans: The main objective/purpose of a constructor in java is to initialize instance variables in a class (or to set the initial state of an object).
3. When a constructor is called/invoked in Java?
Ans: The constructor of a class is called every time an object is created with a new keyword. For example, in the below code, two objects of class are created with the new keyword, therefore, constructor is called two times.
public class Test {
Test() {
System.out.println("Inside constructor");
}
public static void main(String[ ] args){
Test t1 = new Test();
Test t2 = new Test();
}
}
4. Is it possible for a class to have multiple constructors in java?
Ans: Yes, a class can have multiple constructors with different parameters. Which constructor gets called for object creation depends on the arguments passed while constructing different objects.
5. Does a constructor return any value?
Ans: The constructor can not have any return type even void also because if there is a return type then JVM would consider as a method, not a constructor.
6. Can a constructor be marked with the final keyword?
Ans: No, a constructor cannot be marked with the final keyword.
7. Is it possible to inherit a constructor?
Ans: No, a constructor cannot be inherited in java.
8. Can we use this() and super() inside the constructor?
Ans: No, this() and super() cannot be used together inside the constructor.
9. Will the below code compile successfully? If not, why?
class Test {
Test() {
}
public void display() {
Test();
}
}
Ans: On compilation of the above code, compile-time error will generate: The method Test() is undefined for the type Test. This is because it is illegal to invoke a constructor like this.
10. What are possible access modifiers that can be marked for a constructor?
Ans: The possible access modifiers for a constructor is as follows:
a) private: The constructor marked with private can be accessed only from its class.
b) protected: The constructor declared with protected access modifier is accessible from any class which resides in the same package.
c) public: The public constructor is accessible from any class within or outside the package.
11. Is it possible to invoke a constructor of a class more than once for an object?
Ans: No, it is not possible to call a constructor of a class more than once for an object. It is invoked only once per object at the time of object creation.
12. Does a constructor of the class get called, before or after creating an object?
Ans: A constructor gets called concurrently when the object creation is going on. JVM first allots memory space for the object in the heap and then executes the constructor to initialize instance variables. By the time object creation is completed, the execution of a constructor is completed.
13. How many types of constructors are in Java?
Ans: There are two types of constructors in java that are as follows:
- Default constructor (Non-parameterized constructor)
- Parameterized constructor
14. What is a default constructor?
Ans: A constructor that takes no parameter is called default constructor or parameterized constructor. For example:
A a = new A(); // It will create an object of class A by invoking default constructor.
15. What is a parameterized constructor?
Ans: A constructor that contains one or more parameters is called parameterized constructor. The parameterized constructor allows us to initialize different values to distinct objects.
16. What is the main purpose of default constructor in java?
Ans: The main purpose of default constructor is to initialize default values (null or zero value) to the objects.
Java compiler creates a default constructor at compile time only if there is no constructor in a class. After constructing the default constructor, default values are initialized to objects.
17. Is it necessary to define a constructor as the same name as the name of class?
Ans: Yes, a constructor must have the same name as that of class name. If the name of constructor is different, Java compiler will treat it as a normal method.
18. Give an example that proves the definition of constructor.
Ans:
public class Test
{
String name;
int age;
Test() // constructor function.
{
name = "John";
age = 20;
}
}
19. Write a program to define a constructor and pass data through parameters.
Ans:
public class Person
{
String name;
int age;
Person(String n)
{
name = n;
age = 25;
}
}
20. Will the below code compile successfully? If yes, what will be the output of the following program?
public class Test
{
Test() {
System.out.println("Calling default constructor");
}
Test(Test test) {
System.out.println("Calling parameterized constructor");
}
public static void main(String[] args)
{
Test t = new Test(new Test());
}
}
Ans: Yes, the above code will be compiled successfully. The output of the program is as follows:
- Calling default constructor
- Calling parameterized constructor
21. Is it possible to call a constructor from another constructor if multiple constructors are defined in a class?
Ans: If multiple constructors are defined inside a class, it is possible to call a constructor from another constructor using this keyword.
22. What will be the output of the following program?
public class Test
{
Test(Object object) {
System.out.println("Hello");
}
Test(Test test) {
System.out.println("World");
}
public static void main(String[] args)
{
Test t = new Test(null);
}
}
Ans: Output: world.
23. What is the use of constructor in java?
Ans: The use of constructor in java is:
a) to assign the default value of instance variables.
b) to execute a particular code at the time of object creation, we can write them inside the constructor.
24. Can we declare a constructor as private?
Ans: Yes, we can declare a constructor with a private access modifier. It is mainly done not to allow users to create an object of class from outside of the class. Basically, we use a private constructor in a singleton design pattern.
25. When does Java compiler define the default constructor?
Ans: Java compiler defines a default constructor only if there is no explicit constructor declared by the programmer.
26. Why a constructor defined by Java compiler is always called as default constructor?
Ans: A constructor defined by Java compiler is always called as default constructor because it obtains all its default properties from its class. They are:
a) Its access modifier is same as its class access modifier.
b) Its name is same as class name.
c) It has no parameters and logic.