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SoftwareTestingo » Interview Questions » Java Questions » Java Constructor Interview Questions

Java Constructor Interview Questions

Last Updated on: April 15, 2023 By Softwaretestingo Editorial Board

What We Are Learn On This Post

  • Constructor Interview Questions In Java
  • Private Constructor

Java Constructor Interview Questions: A constructor is a special method in Java that initializes objects of a class. Java Constructor Interview Questions evaluate a candidate’s knowledge and proficiency in using constructors in Java programming. These interview questions assess a candidate’s understanding of constructors, including their types, purpose, and best practices.

Java Constructor Interview Questions require a candidate to demonstrate their expertise in creating and using constructors to initialize objects of a class, with proper exception handling and encapsulation. In this context, this article presents some commonly asked Java Constructor Interview Questions to help Java developers prepare for interviews and have a comprehensive understanding of constructors in Java programming.

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Constructor Interview Questions In Java

What is Constructor in Java?

  • It is a special type of method that is used to initialize an object
  • Every class has a constructor which is invoked at the time of object creation and provides values
  • As this provides values at the time of object creation that is why it is called as a constructor (constructing default/initial values for an object)

Define Constructor in Java?

  • The constructor is a special type of method that is used to initialize an object. Every class has a constructor which is invoked at the time of object creation and provides values

Is it mandatory to define a constructor in a class?

  • It is not mandatory to define a constructor in class because the compiler inserts a default no-arg constructor during compilation
  • Note: compiler inserts default no-arg constructor when there is no constructor explicitly defined by a programmer in class

What are the rules for defining constructors in a class?

  • Name of the constructor should be the same as that of a class name
  • A constructor doesn’t have any return type unlike methods (not even void)

Generally, what is the name of the constructor in Java class and why it is so?

  • The name of the constructor should be (must be) same as that of a class name
  • It is just syntax or convention followed in Java and requires no extra keyword

What are the types of constructors in Java?

There are two types of constructor

  • Default constructor (no-arg constructor)
  • Parameterized constructor

What is a no-arg constructor?

  • Constructor which takes zero parameters is called as default constructor
  • Or, a constructor with no argument is known as a default constructor
  • It is also known as no-arg constructor

What is the default constructor and why it is called the default constructor? Reasons?

  • Constructor which takes zero parameters is called as default constructor
  • The compiler always inserts a no-arg constructor during compilation process if there is no other constructor defined explicitly by the programmer
  • During such a compilation process, compiler initializes all instance data members to default values like
    0 for int
    null for String
    false for Boolean
  • Since it provides default values, it is alternatively called as default constructor (other is a no-argument constructor)

What is parametrized, constructor?

  • Constructor which takes one or more parameters is called as a parameterized constructor
  • Or, a constructor with arguments is known as parameterized constructor

What happens if we don’t specify any constructor explicitly in class?

  • If there is no constructor defined explicitly by the programmer, the compiler inserts a default no-arg constructor during compilation

When compiler provides a default, constructor?

  • If there is no constructor defined explicitly by a programmer, the compiler inserts a default no-arg constructor during compilation

Will the compiler provides a default no-argument constructor when we explicitly defined a constructor in a class?

  • The compiler won’t provide/inserts default no-arg constructor during compilation process if a programmer defines constructor explicitly (whether it is default or parametrized constructor)

Will compiler provides default no-argument constructor when we explicitly defined a parameterized constructor in a class?

  • The compiler won’t provide/inserts default no-arg constructor during compilation process if a programmer defines constructor explicitly (whether it is default or parametrized constructor)

What happens when there is one parameterized constructor explicitly defined?

  • Compilation succeeds
  • But, if we try to create a new object using default constructor then compiler throws an error

If we define a constructor in a class, then will it have a default constructor?

  • No, there won’t be any default constructor
  • The programmer needs to define explicitly if required

Can we have both default constructors and parameterized constructors in Java class?

  • Yes, constructor overloading is possible

Why return type is not allowed for the constructor?

  • When we add a return type to a constructor, the compiler treats this as a method with method name same as that of the class name
  • Error: Compilation error will be thrown

Whether class compiles successfully if we add return-type to a constructor?

  • A compilation error will be thrown
  • To be precise, if we add return-type then compiler treats this as a method with method name same as that of the class name

Can constructor return any value, although there is no return type?

  • As such, there is no return type for the constructor and it doesn’t return values
  • But constructor return values in the form of instances after initialization

Private Constructor

What is a private constructor in Java?

  • Adding a private keyword to constructor makes constructor as a private constructor
  • Which implies except its own class, no other classes are not allowed to instantiate objects of this type

Can we add access to the modifier ‘private’ to the constructor?

  • Yes, very well
  • Access modifiers like private, protected and the public are allowed (even default is ok)
  • the private constructor is meant to create a singleton object (singleton design pattern)

How can we create objects, if we make the constructor as private?

  • Add a private keyword to the constructor which becomes a private constructor
  • By making, constructor as private makes difficult for other classes to instantiate objects of this type

Can we declare constructor as ‘protected’?

  • Yes, we can declare constructor as protected

Can we instantiate a subclass object, if superclass constructor defined is protected?

  • Yes, we can create an object of subclass type even if super class’s constructor is marked as protected

Constructor on non-access modifier:

Can a constructor be final?

  • No, we cannot mark constructor as final
  • non-access modifiers like final, static, transient, synchronized, volatile, strictfp are not allowed in the constructor

Q) Is it valid to add ‘final’ to a constructor in Java? (Non-access modifier)

  • The final keyword is not allowed in the constructor
  • non-access modifiers like final, static, transient, synchronized, volatile, strictfp are not allowed in the constructor

Explain static constructor in Java?

  • There is no such thing known as a Static constructor in Java

Can we declare constructor as ‘static’?

  • non-access modifiers like final, static, transient, synchronized, volatile, strictfp are not allowed in the constructor

Can we overload constructor in Java?

  • Yes, constructor overloading is possible

Why do we overload constructor?

  • Constructor provides a way to create an object implicitly for any class using ‘new’ keyword
  • So, overloaded constructor serves in many ways to create distinct objects using different types of data of the same class

Is overloading constructor an example of both polymorphism and inheritance?

  • Constructor provides a way to create a distinct object using different types of data
  • Polymorphism is achieved through method overloading (static polymorphism) and method overriding (dynamic polymorphism) but not with constructor overloading
  • Also, the constructor cannot be inherited rather it can access via constructor chaining process

How JVM differentiates between constructor and methods in Java during compilation?

  • With the help of return type
  • The constructor is a special type of method which has no return type

Difference between constructor and methods in Java?

Explain Constructor chaining in Java?

Explain this() keyword w.r.t constructor in Java?

  • To invoke one constructor from another constructor, we use this() constructor call

How to invoke one constructor from another constructor in the same class?

  • Using this() constructor call

Explain super() keyword w.r.t constructor in Java?

  • To invoke super class’s constructor from subclass constructor, we use super() constructor call

How to invoke superclass constructor from subclass constructor?

  • Using super() constructor call

Is it possible to call subclass constructor from the superclass constructor?

  • No, it is not possible
  • Using super() constructor call, we can invoke super class’s constructor from subclass constructor but reversely is not possible

Can we have both this() and super() inside the same constructor?

  • No, at any given point in time both this() and super cannot be present inside the same constructor
  • Either this() constructor call or super() constructor call is allowed, if present should be the 1st statement of a constructor

Can an abstract class have a constructor in Java?

  • Yes, defining constructor inside abstract classes is allowed
  • But the object of the abstract class cannot be instantiated rather we can use this as reference variable (rather inheritance concept)

Can an Interface have a constructor in Java?

  • defining constructors inside Interface is not allowed and compilation is thrown
  • Starting Java 8, new features are added but it doesn’t allow defining constructor
  • One such feature is the default method and static method inside Java Interface

Does Java provide default copy constructor?

  • Java doesn’t have any built-in copy constructor
  • But, yes programmer can write their own copy constructor in their own way, like below example

References: Link

    Filed Under: Java Questions

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