Constructor Interview Questions In Java: 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.
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
