Interview Questions and Answers for 'Object' | Search Interview Question

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Q61. Does it make sense to clone an object which is supposed to be immutable ?

Ans. It make sense only if we intend to modify either of the object and would like to preserve original state in other. Otherwise we can reuse the original object by making it singleton.

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Q62. Can we access private members of the parent class ? i.e Are private members of the class visible in the derived class ?

Core Java

Ans. No, If both Parent and Derived are outer classes.

public class Vehicle {
private static String manufacturingDate = "2016";
}

public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // error - The field Vehicle.manufacturingDate is not visible
}
}

Yes, If derived is the inner class of Parent.

public class Vehicle {
private static String manufacturingDate = "2016";
static public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // no problem
}
}
}

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Q63. Difference between multiple and multi level inheritance ?

Core Java

Ans. Multiple Inheritance refers to the concept of a class inheriting multiple classes. Example - Class C extends Class A ,Class B. This is not allowed in Java.

Multilevel Inheritance refers to the concept of Inheritance in a chain. Example - Class B extends Class A, Class C extends Class B. This is permitted in Java.

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Q64. What is the difference between = and == in Java ?

Core Java

Ans. = is the assignment operator that assigns the result of the expression on the right to the variable on the left, whereas

== is the operator to check object equality to see if the reference on left and right are pointing to the same object. For primitive types, its used to check if both variables holds the same value.

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Q65. How is == operator different for objects and primitive types ?

Core Java

Ans. For objects or references, == operator check if the reference on left and right points to the same object.

For primitive types or variables, == operator check if the variable on left and right holds the same value.

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Q66. What is the default value of a declared object reference ?

Core Java

Ans. Null

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Q67. Is there a way to know size of an object in Java ?

Core Java

Ans. No, Java doesn't have a sizeOf operator. In C / C++ , its required to determine how much memory allocation is required which is not the case with Java. Java handles memory allocation and deallocation intrinsically.

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Q68. Difference between shallow copy and object cloning ?

Core Java

Ans. Shallow copy is one of the way for object cloning, other being deep copy.

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Q69. What is a class and object ?

or

How would you explain a fresher the concept of class and object in simple terms ?

Core Java

Ans. Class is a template using which objects are created in memory. It's kind of a mold using which objects with body / state are made.

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Q70. What are the benefits of creating immutable objects ?

Core Java

Ans. 1. Security and Safety - They can be shared across multiple threads as they are thread safe. Moreover, it protects then from bad state due to interception by the other code segment. One such problem due to mutability and access by alternate code segment could be the change of hash code and then the impact on its search with hash collections.

2. Reuse - In some cases they can be reused as only one copy would exist and hence it can be relied upon. For example - String Pool

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Q71. How are the concept of Association related to Composition and Inheritance ?

Core Java

Ans. Composition and Inheritance are the different types of Associations for Classes.

Composition is a has-a association between classes.
Inheritance is a is-a association between classes.

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Q72. What is ACL ?

Operating System

Ans. Access Control List or ACL is the list of permissions attached to an object in the File System.

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Q73. What could be the possible cause for following exception ?

org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect):

What could be the way to fix it ?

Hibernate

Ans. This looks like the case for optimistic locking wherein hibernate suspects that the information in table was updated by some other transaction after the entity was loaded by current transaction.

One way is to have synchronized entity state and don't detach the entity. Other could be to merge the entity with the table record rather than just directly persisting the entity.

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Q74. Which of the two - compile time and run time polymorphism - requires signature of the method to be different ?

Core Java

Ans. runtime polymorphism or method overriding doesn't require method name and signature to be different whereas compile time polymorphism or method overloading requires method name to be same but the signature to be different.

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Q75. What is "dynamic method dispatch" in java ?

Core Java

Ans. dynamic method dispatch is a process in which a call to an
overridden method is resolved at runtime rather than at compile-time. This is used to achieve runtime polymorphism in java.

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Q76. What will be the output of following code

Integer x = 1;
Integer y = 2;
System.out.println(x == y);

What if you change 1 to "1" and Integer to String?

Core Java

Ans. It will print "true" with integers as well as strings. The reason is "Integer constant pool" and "String pool"

String pool maintains pool of string literals. When a string literal is used for the first time, a new string object is created and is added to the pool. Upon it's subsequent usage , the reference for the same object is returned. Similarly java uses integer constant pool.

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Q77. How can we destroy an object ?

Core Java

Ans. We cannot explicitly initiate destruction of an object as no destroy method is available in java. Only Garbage collection can destroy an object.

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Q78. When does an object cleaned or destroyed by garbage collection ?

Core Java

Ans. When it is no longer referenced or it looses all references which were earlier pointing to it.

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Q79. What is the difference between reference and object ?

Core Java

Ans. Object is an entity in Java , i.e which has a state ( instance variables ) and methods attached to it ( static or non static , through class definition ). References are the identifiers that are used to point to objects.

For example -

Employee emp = new Employee();
emp = new Employee();

In this code, emp is the reference that gets assigned to the new object created by the new operator. In the second line , we have assigned the same reference to another object. So with these 2 lines of code, we have 2 objects in memory with reference emp now pointing to second object.

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Q80. What happens when you assign a pre initialized reference some other object ?

Core Java

Ans. Now the reference points to a new object in memory. If that was the only reference for the previous object , it will be marked for garbage collection.

Foe example -

Object obj = new Object();
obj = new Object();

object created in first line will be eligible for garbage collection after line 2 as it looses all it's handlers.

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Q81. What are the examples of immutable objects in Java ?

Core Java

Ans. Following Classes in Java SE creates immutable objects

String class
Wrapper Classes like Integer, Float etc.
StackTraceElement
Most Enum classes
File Class
Locale Class

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Q82. What is dynamic dispatch in Java ?

Core Java

Ans. Dynamic dispatch in java is also known as runtime polymorphism.It is a process in which a call to an overriden method is resolved at runtime.

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Q83. What happens to private variables during inheritance ?

Core Java

Ans. Private variables are not inherited. They are simply ignored and not made part of the derived object body.

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Q84. How can we hide a class in Java ?

Core Java

Ans. By encapsulating it within another class and declaring it private. In such a case, it will only be accessible through parent class or parent class object.

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Q85. How would you explain the statement "Java Programs are collection of objects" ?

Core Java

Ans. Java Programs are collection of objects that communicates with each other to get a task accomplished. To add to those objects, there are common spaces ( static i.e common for objects belonging to a class ) that are used too.

We can visualize objects as departments of an organization in real world. Just like Task gets initiated in one department and then files are moved across different departments to get work done. In a similar fashion, a task is initiated in one object ( having main method ) and then information ( through POJOs / DTOs ) is moved across objects to accomplish a task.

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Very frequently asked. Usually among very first few questions.

Q86. Define encapsulation in Java ?

Core Java

Ans. Encapsulation is a feature of OOP's that binds the data and it's associated methods together as a single unit and facilitate protection and data hiding by providing minimal interface to outside. For example - member variables are declared private and are accessed through public methods. Moreover we have private methods that can only be used internally and hence providing minimal interface to outside class through use of public methods.

Sample Code for encapsulation

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Q87. Explain Inheritance and Polymorphism.

Core Java

Ans. Getting parent class object properties into child class is called Inheritance.

The process of representing one form into multiple forms is called polymorphism.

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Q88. What will be the output of following code

class TestMain {
public static void main(String[] args) {
String s1 = "ravi";
String s2 = new String("ravi");
Integer i = 10;
Integer a1 = new Integer("10");
System.out.println(s1 == s1);
System.out.println(i == a1);
}
}

Core Java

Ans. false
false

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Q89. How can we track of all objects created in an application ?

Design

Ans. We can store the references in a collection by adding to those objects in the collection. We can create a class "ObjectRegistry" with a collection or multiple collections with a search algorithm to look for the already collected objects.

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Q90. Is Runtime Polymorphism possible without Inheritance ?

Core Java

Ans. Yes, Runtime Polymprohism requires either Class inheritance or Interface implementation.

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