The Top 25 Constructor Interview Questions for Aspiring Developers

Constructors are a fundamental concept in object-oriented programming. As a developer, having a strong grasp of how to properly utilize constructors is essential for writing clean, efficient code. Constructors facilitate object creation by initializing an object’s state.

With the prevalence of Java, C++, and other OOP languages in the tech industry, constructors are a hot topic in coding interviews. Interviewers commonly use constructor questions to assess a developer’s technical skills and problem-solving abilities.

To help you ace your next coding interview here are the top 25 constructor interview questions that you need to be prepared for

1. What is a constructor in Java?

A constructor in Java is a special method that is invoked automatically when an object of a class is instantiated. The purpose of a constructor is to initialize the object’s properties upon creation. Constructors have the same name as the class do not have a return type and can be overloaded through method overloading.

Key facts about constructors in Java

• Shares the name of the class
• No return type
• Automatically invoked upon object instantiation
• Used to set default property values
• Can be overloaded like normal methods

2. Explain the difference between default and parameterized constructors.

A default constructor has no parameters and is invoked automatically by the compiler if no other constructors are defined. On the other hand, a parameterized constructor accepts parameters that can be used to initialize properties of the object. For example:

//default constructor public Person(){}//parameterized constructorpublic Person(String name){  this.name = name;}

Default constructors provide default values while parameterized constructors allow custom initialization.

3. What is constructor chaining in Java?

Constructor chaining refers to invoking one constructor from another constructor in the same class using the this() method. The this() call must be the first statement in a constructor. Constructor chaining is useful for:

• Avoiding duplicate initialization code
• Ensuring superclass initialization takes place
• Passing parameters to other constructors

For example:

public class Main{  int x;  Main(){    //default constructor  }  Main(int x){    this(); //invokes default constructor    this.x = x; //initializes x  }} 

Here, the parameterized constructor invokes the default one via chaining.

4. Can constructors be inherited in Java?

No, constructors are not inherited in Java. Each subclass has its own constructors that do not override the superclass constructors. However, the subclass constructor will implicitly call the superclass default constructor by default. To call a superclass parameterized constructor, the super() method is used.

5. Explain the use of this() and super() inside constructors.

• this() is used to invoke the current class constructor, mainly for constructor chaining.

• super() invokes the parent class constructor and must be the first statement in a subclass constructor.

For example:

public class Super{  public Super(){    //superclass constructor   }}public class Sub extends Super{    public Sub(){    super(); //calls Super() constructor  }}

6. What is a copy constructor in C++?

A copy constructor is a type of constructor used to create a new object as a copy of an existing object. It initializes the new object’s properties with the values of an existing object’s properties. Copy constructors are commonly passed objects by reference:

class Point {  int x;  int y;  //copy constructor   Point(const Point &p1){    x = p1.x;    y = p1.y;   }};

The copy constructor is useful for copying complex objects. In Java, copy constructors are not needed since we can directly assign one object to another.

7. Should constructors be declared virtual in C++? Why or why not?

No, constructors cannot be virtual in C++ because they are called automatically and not using dynamic binding. Making them virtual would go against the purpose of a constructor which is to initialize concrete instances of objects. Constructors establish an object’s identity.

8. How are C++ destructors different from constructors?

While constructors initialize objects, destructors free up resources used by the object before deletion. Key differences:

• Destructors are invoked automatically when an object goes out of scope
• Constructors have the same name as the class, while destructors have a tilde (~) before the class name
• Destructors have no parameters and don’t return anything
• Constructors allocate memory, destructors deallocate

9. Explain static constructors in C#.

Static constructors are used to initialize static members of a class. Key facts:

• Automatically invoked before the first instance is created or any static members are referenced
• Cannot be called directly
• No access modifiers – neither private nor public
• Cannot have parameters

For example:

static class Demo{    static Demo(){    //static constructor   }}

10. How are Java constructors different from methods?

• Constructors have the same name as the class, methods can have any name
• Constructors don’t have an explicit return type, methods must have a return type
• Constructors are automatically invoked, methods are explicitly called
• Constructors are used only for initialization, methods can do operations

11. Can constructors return a value in Java?

No, constructors do not return values in Java. They are only used to initialize objects and cannot return data like regular methods. Attempting to add a return type to a constructor will lead to a compile error.

12. When would you use private constructors in Java?

Private constructors are used in Singleton classes where only one instance of the class can exist. Making the constructor private prevents external classes from instantiating the Singleton class. Private constructors are also used when we have only static methods in a class and want to prevent instantiation.

13. Explain constructor injection in Spring framework.

Constructor injection initializes dependencies via the class constructor rather than setter methods. The main component class declares a constructor accepting all dependencies as arguments. In Spring config file, inject required dependencies using constructor-arg tag.

Benefits of constructor injection:

• Immutable object instances
• Required dependencies declared upfront
• No partial initialization of objects

14. What happens if no constructor is defined in a Java class?

If no constructor is explicitly defined in a Java class, the compiler automatically inserts a default no-argument constructor at runtime. This constructor initializes all member variables to default values like null, 0, etc.

15. Explain different ways to initialize arrays in a constructor.

There are several ways arrays can be initialized in a constructor:

• Initialize array inline – set values at time of declaration

int[] arr = {1,2,3}; 

• Initialize array in constructor body

int[] arr;public Constructor(){  arr = new int[3];  arr[0] = 1;  ..}

• Take array as constructor parameter

public Constructor(int[] inputArray){  arr = inputArray;}

• Call static method to initialize array

16. How can constructor ambiguity issues be resolved in Spring?

Constructor ambiguity happens when Spring cannot autowire a constructor due to multiple options available. Ways to resolve it:

• Use @Autowired on the intended constructor
• Add required=false to additional constructors
• Use @Primary on the bean intended for autowiring
• Have unique parameter counts for each constructor

17. What are the best practices for exception handling in constructors?

• Constructors should catch exceptions, release any acquired resources, do clean up before propagating exception with throw
• Follow RAII principle – acquire resources in constructor, release in destructor
• Don’t swallow exceptions – rethrow the exception after cleanup
• Document exceptions thrown using @throws annotation

18. Explain overloading and overriding of constructors in Java.

Overloading – Having multiple constructors in one class with different parameters

Overriding – Not applicable to constructors since they are not inherited

Constructors can be overloaded but not overridden.

19. How can you prevent instantiation of a class in Java?

Making the constructor private prevents other classes from instantiating it. This is used in Singleton classes to restrict instantiation.

public class Singleton{  private Singleton(){}}

Now only Singleton class can instantiate itself but not other classes.

20. Can constructors call virtual functions in base classes? Why?

No, constructors cannot call virtual functions in base classes because virtual functions rely on dynamic binding which only works after the object is fully constructed. Constructors are used to initialize the object’s state before it is ready for operations like virtual function calls.

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