Java interview questions and answers on oops covered in this post:
What is a constructor? What are the types of constructors?
- Constructor is a function that is used to initialize the
object.
- Constructor doesn’t have a return type.
- Constructor name must be the same as a class name.
- We can have multiple constructors in one class.
- We can declare the constructor as private, public and
protected.
- There are three types of constructors.
1. Default Constructor: If you don’t provide any constructor compiler
will provide you with, default constructor.
2. No-arg constructor: If you have declared a constructor without a
parameter. That constructor is called the no-arg constructor.
3. Parameterized Constructor: If you declared a constructor with a
parameter then it is called the parameterized constructor.
Will the parent class constructor be invoked while you have invoked
the child class constructor without a super keyword?
- Yes. If you create an object of child class and call child class constructor. Child class constructor internally calls parent class constructor implicitly.
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package simplifiedjava.crackedInterview; public class OverrideOverloadedMethodDemo {
public static void
main(String[] args) {
Child c
= new
Child();
} } package simplifiedjava.crackedInterview; public class Parent {
public
Parent() {
System.out.println("Parent Class Constructor");
} } package simplifiedjava.crackedInterview; public class Child extends Parent{
public
Child() {
System.out.println("Child Class Constructor...");
} }
output :
Parent Class Constructor
Child Class Constructor... |
 
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What is the difference between method and constructor?
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Method |
Constructor |
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1. |
The method is used to implement the logic. |
Constructor is used to initializing the object. |
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2. |
The method can have a return type. |
A constructor cannot have a return type. |
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3. |
Method name can be anything or can be class name as well. |
The constructor name must be the same as the class name. |
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4. |
The method must invoke explicitly. |
A constructor can invoke automatically. |
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5. |
If you don’t have any method then the compiler won’t provide any
kind of method like a constructor. |
If you don’t have any constructor then the compiler will provide
a default constructor. |
|
6. |
The method can be overridden or overloaded. |
The constructor can be overloaded but cannot overridden. |
Can we have a constructor in abstract class? If we can't instantiate abstract class then why there is a need for a constructor in abstract class?
- Yes. We can declare a constructor inside the abstract class.
- Absolutely right we cannot instantiate abstract class but when its Implementation class or child class instantiate that time abstract class constructor will be called.
Can I declare the constructor as an abstract?
- Constructors cannot be abstract.
- Abstract keyword can be applied to class and method, not constructor or
blocks.
Can you declare constructor inside interface?
- Constructor cannot be declared inside interface.
- Constructor is mainly used to initialize the value. We cannot instantiate
the interface so we cannot declare the constructor inside an
interface.
Can you declare constructor static, final, abstract and
synchronized?
- We cannot declare constructor static, final, abstract and
synchronized.
- Purpose of all four keywords given above is different.
Why java doesn't allow static constructors?
- Static belongs to the class, not the object.
- Constructor belongs to object while initializing the value into it.
- When you create an object of child class then internally parent class
constructor gets called automatically.
- If you declare the constructor as static then the child class cannot call
the parent class constructor so it violates the inheritance mechanism.
Hence we cannot declare the constructor as static.
In your program, you have a static block, non-static block,
constructor and main(). What will be the sequence of execution?
- The sequence of execution is as follows.
1. Static Block: will be executed at the time of class loading into
memory.
2. Non-Static Block: Will be executed at the time of instantiation.
But make a note it depends if you are instantiating the class before
printing any statement in main. If you instantiate the class after
printing the statement.
3. Constructor: Will be executed at the time of object
initializing.
4. Main: Will execute last if you haven’t instantiated the class.
5. We can see more scenarios.
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Scenarios 1:
package simplifiedjava.crackedInterview; public class BlockExecutionSequenceDemo {
static {
System.out.println("Static Block Excecution");
}
{
System.out.println("Non Static Block Excecution"); }
public
BlockExecutionSequenceDemo() {
System.out.println("Constructor Excecution"); }
public static void
main(String[] args) {
System.out.println("Main Method Execution");
}
}
Output:
Static Block Excecution Main Method Execution
Scenarios 2: package simplifiedjava.crackedInterview; public class BlockExecutionSequenceDemo {
static {
System.out.println("Static Block Excecution");
}
{
System.out.println("Non Static Block Excecution"); }
public
BlockExecutionSequenceDemo() {
System.out.println("Constructor Excecution"); }
public static void
main(String[] args) {
BlockExecutionSequenceDemo demo
= new
BlockExecutionSequenceDemo();
System.out.println("Main Method Execution");
}
}
Output :
Static Block Excecution
Non Static Block Excecution
Constructor Excecution
Main Method Execution
Scenarios 3: package simplifiedjava.crackedInterview; public class BlockExecutionSequenceDemo {
static {
System.out.println("Static Block Excecution");
}
{
System.out.println("Non Static Block Excecution"); }
public
BlockExecutionSequenceDemo() {
System.out.println("Constructor Excecution");
}
public static void
main(String[] args) {
System.out.println("Main Method Execution");
BlockExecutionSequenceDemo demo
= new
BlockExecutionSequenceDemo();
} }
Output:
Static Block Excecution
Main Method Execution
Non Static Block Excecution
Constructor Excecution |
What is the purpose to make a constructor private?
- Private constructor ensures that only one object can be created at a
time.
- Cannot instantiate the class from outside the class.
- Generally for singleton design pattern uses a private constructor.
- Java interview questions and answers all MNC - Click here
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- Interview questions on multithreading in java for experienced - Click here
- Interview questions on serialization in java for experienced - Click here
- Interview questions on inner class in java for experienced - Click here
- Interview questions on Collections in java for experienced - Click here
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Thank you techies for visiting this blog. I hope you enjoyed this blog and got more technical knowledge. I have tried to cover all types of questions and provided examples tested on eclipse as much as I can. Guys, please don’t just mug up the questions and answers. Try to clear your concepts with examples. Try to write a code on eclipse once you read the concepts. It will help you to memorize the concepts for a very long time. Simultaneously you will be prepared for interview programs as well. It will help you to survive in the IT sector for a long time. It may be easy to crack an interview but it's really tough to survive in the IT industry with inadequate knowledge and skills. I have collected all the questions from actual interviews attended by my friends, colleagues, college mate and classmate. I have covered frequently asked questions as well as challenging questions. I have included many programs to understand the concept thoroughly. I will try to explain the concept with the help of a real-time program in eclipse. You can share more questions which are not covered in this blog or post. Always welcome your suggestions and queries. I will definitely try to resolve it. Please comment your queries and new set of questions under the comment sections. I will create a new post for those questions.
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