Core Java - Interview Questions and Answers for 'Core Java' | Search Interview Question

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Q41. Which keyword is used to provide explicit access of a code block to single thread ?

a. Transient
b. Final
c. Explicit
d. Synchronized

Core Java

Ans. Synchronized

Sample Code for Synchronized

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Usually asked in relation to casting and ClassCastException.

Q42. What is instanceOf operator ? Explain it's use ?

Core Java

Ans. The operator instanceOf is used to verify if the specified object is the instance of specified class or interface.

Syntax if(x instanceOf ABC)

where x is an object reference and ABC could be a class name or interface name. The above statement will be true if x holds an object that is an instance of ABC or any of the child class of ABC or if x holds an object that implements ABC.

instanceOf operator is used to verify in case of downcasting. For ex -

DerivedClass extends BaseClass

x is the reference of BaseClass but holds DerivedClass object ( Polymorphism )

There is an operation that is defined in Derived Class, let's say derivedClassMethod()

We cannot call derivedClassMethod() directly using x as x is reference of BaseClass and not DerivedClass and hence can only access methods that are defined in BaseClass and overridden in derived class.

Though we can cast it to DerivedClass as following
((DerivedClass)x).derivedClassMethod();

But it may throw ClassCastException in case x doesn't hold an instance of DerivedClass at that point.

So before casting it to DerivedClass we may like to make sure that it is an instance of DerivedClass and hence won't throw ClassCastException.

So we make a check for it

if(x instanceOf DerivedClass) {
((DerivedClass)x).derivedClassMethod();
}

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.

Q43. Difference between TreeMap and HashMap ?

Core Java

Ans. They are different the way their elements are stored in memory. TreeMap stores the Keys in order whereas HashMap stores the key value pairs randomly.

Sample Code for treemap

Sample Code for hashmap

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Q44. What are concepts introduced with Java 5 ?

Core Java

Ans. Generics , Enums , Autoboxing , Annotations and Static Import.

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Q45. Describe what happens when an object is created in Java ?

Core Java

Ans. 1. Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data.
2. The instance variables of the objects are initialized to their default values.
3. The constructor for the most derived class is invoked. The first thing a constructor does is call the constructor for its superclasses. This process continues until the constructor for java.lang.Object is called,as java.lang.Object is the base class for all objects in java.
4. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last.

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Advanced level question usually asked to senior developers , leads and architects.

Q46. How does volatile affect code optimization by compiler?

Core Java

Ans. Volatile is an instruction that the variables can be accessed by multiple threads and hence shouldn't be cached. As volatile variables are never cached and hence their retrieval cannot be optimized.

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Frequently asked to fresh graduates and less experienced.

Q47. Why do we write public static void main ? Can we use some other syntax too for main ?

Core Java

Ans. 1. public is the access modifier that makes the method accessible from anywhere, static is the keyword that makes it accessible even without creating any object, void means it doesn't return anything , String args[] is the array of argument that the method receives.

2. If we use main without the string args , it will compile correctly as Java will treat it as just another method. It wont be the method "main" which Java looks for when it looks to execute the class and hence will throw

Error: Main method not found in class , please define the main method as:
public static void main(String[] args)

3. Main is not a keyword but a special string that Java looks for while initiating the main thread.

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Q48. Can constructors be synchronized in Java ?

Core Java

Ans. No. Java doesn't allow multi thread access to object constructors so synchronization is not even needed.

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Q49. Difference between Static and Singleton Class ?

Core Java

Ans. 1. Static class is a class which cannot be instantiated and all its members are static whereas Singleton is the class that only permit creation of single object and then the object is reused.

2. As there is no object in Static class, it cannot participate in runtime Polymorphism.

3. As Static class doesnt allow creating objects and hence it cannot be serialized.

4. Static class body is initialized eagerly at application load time whereas Singleton object can be initiated eagerly using static blocks or lazily on first need.

5. Its not recommended to use pure static class as it fails to use many OOPs concepts.

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Q50. How to find whether a given integer is odd or even without use of modulus operator in java?

Core Java

Ans. public static void main(String ar[])
{
int n=5;
if((n/2)*2==n)
{
System.out.println("Even Number ");
}
else
{
System.out.println("Odd Number ");
}
}

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Q51. Can we declare static variables as transient ?

Core Java

Ans. It's weird that compiler doesn't complain if we declare transient with static variable because it makes no sense. At least a warning message saying "transient is useless in this situation" would have helped with code cleaning.

Static variables are never serialized and transient is an indication that the specified variable shouldn't be serialized so its kind of double enforcement not to serialize.

It could be that as it makes no different to the variable behavior and hence using both keywords with a variable are permitted.

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Q52. Which of the following are valid declarations

1. void method(int... x){};
2. void method(int.. x){};
3. void method(int.. .x){};
4. void method(int ...x){};
5. void method(int... x){};
6. void method(int ... x){};
7. void method(int x, int... y){};
8. void method(int... x, int y){};
9. void method(int... x,int... y){};

Core Java

Ans. 1st is a valid and standard declaration.

2nd results in compilation error as only 2 dots are there.

3rd results in compilation error as three dots are not consecutive and broken.

4 through 6 may not be standard and ideal way of declarations but they are valid and will compile and work fine.

7 is valid declaration.

8 and 9 will result in compilation error as var args can only be provided to last argument.

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Q53. How is string object immutable if we can concat a string to it ?

Core Java

Ans. Because it doesn't make the change in the existing string but would create a new string by concatenating the new string to previous string. So Original string won't get changed but a new string will be created. That is why when we say

str1.concat("Hello");

It means nothing because we haven't specified the reference to the new string and we have no way to access the new concatenated string. Accessing str1 with the above code will still give the original string.

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Q54. Difference between JVM, JRE and JDK ?

Core Java

Ans. JDK(Java Development kit) = Development Kit comprising of JVM , library and development tools for developers

JRE (Java Run time Environment) - Comprise of JVM and set of libraries

JVM(Java Virtual Machine) = Interpreter which reads the .class file line by line.

When we install JDK, JRE also get installed so we can write,compile and excute our code. Used by developer. Without JDK we can only execute the program using JRE.

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Very Frequently asked across all type of companies and across all levels.

Q55. Difference between Public, Private, Default and Protected ?

Core Java

Ans. Private - Not accessible outside object scope.

Public - Accessible from anywhere.

Default - Accessible from anywhere within same package.

Protected - Accessible from object and the sub class objects.

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Q56. Does Constructor creates the object ?

Core Java

Ans. New operator in Java creates objects. Constructor is the later step in object creation. Constructor's job is to initialize the members after the object has reserved memory for itself.

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Q57. What will this code print ?

String a = new String ("TEST");
String b = new String ("TEST");
if(a == b) {
System.out.println ("TRUE");
} else {
System.out.println ("FALSE");
}

Core Java

Ans. FALSE. == operator compares object references, a and b are references to two different objects, hence the FALSE. .equals method is used to compare string object content.

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Frequently asked question for intermediate developers. Frequently asked in HCL Technologies and EPAM.

Q58. What is Volatile keyword used for ?

Core Java

Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldnt be cached.

Sample Code for volatile

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Advanced level question. Recently asked in few Indian service companies ( Based on 3 inputs )

Q59. What are various types of Class loaders used by JVM ?

Core Java

Ans. Bootstrap - Loads JDK internal classes, java.* packages.

Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE

System - Loads classes from system classpath.

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Q60. How compiler handles the exceptions in overriding ?

Core Java

Ans. 1)The overriding methods can throw any runtime Exception , here in the case of runtime exception overriding method (subclass method) should not worry about exception being thrown by superclass method.

2)If superclass method does not throw any exception then while overriding, the subclass method can not throw any new checked exception but it can throw any runtime exception

3) Different exceptions in java follow some hierarchy tree(inheritance). In this case , if superclass method throws any checked exception , then while overriding the method in subclass we can not throw any new checked exception or any checked exception which are higher in hierarchy than the exception thrown in superclass method

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Q61. What things should be kept in mind while creating your own exceptions in Java?

Core Java

Ans. All exceptions must be a child of Throwable.

If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.

You want to write a runtime exception, you need to extend the RuntimeException class.

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Frequently asked to fresh graduates and less experienced.

Q62. Difference between Composition and Inheritance ?

Core Java

Ans. Inheritance means a object inheriting reusable properties of the base class. Compositions means that an abject holds other objects.

In Inheritance there is only one object in memory ( derived object ) whereas in Composition , parent object holds references of all composed objects.

From Design perspective - Inheritance is "is a" relationship among objects whereas Composition is "has a" relationship among objects.

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Q63. What will be the output of following code ?

public static void main(String[] args){
String name = null;
File file = new File("/folder", name);
System.out.print(file.exists());
}

Core Java

Ans. NullPointerException

at line: "File file = new File("/folder", name);"

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Not frequently asked as it was introduced with Java 8.

Q64. What is StringJoiner ?

Core Java

Ans. StringJoiner is a util method to construct a string with desired delimiter. This has been introduced with wef from Java 8.

Sample Code

StringJoiner strJoiner = new StringJoiner(".");

strJoiner.add("Buggy").add("Bread");

System.out.println(strJoiner); // prints Buggy.Bread

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Q65. Which access specifiers can be used with top level class ? a. public or default b. public or private c. public or protected d. protected or default

Core Java

Ans. public or default

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Q66. Which of the following can be marked static ?

a. Methods , Variables and Initialization Blocks.
b. Methods , Variables , Initialization Blocks and Outer Classes and nested Classes.
c. Methods , Variables , Initialization Blocks and Outer Classes.
d. Methods , Variables , Initialization Blocks and nested Classes

Core Java

Ans. Methods , Variables , Initialization Blocks and nested Classes

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Q67. Which of the following is false about main method ?

a. It should be declared public and static
b. it should have only 1 argument of type String array
c. We can override main method
d. We can overload main method

Core Java

Ans. We can override main method

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Q68. What is the difference between the following two code lines ?

1. new OuterClass().new InnerClass();

2. new OuterClass.InnerClass();

Core Java

Ans. In first case we are trying to initialize Inner class object using the instance of Outer Class whereas in second case we are trying to initialize the Inner class object directly using the Outer class name.

In second case , Inner class is "static inner class" as we cannot access "non static inner class" using Classname alone.

In first case, the inner class could be either "static inner class" or "non static inner class".

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Q69. What are the different uses of Interfaces in Java ?

Core Java

Ans. 1. Regulation / Enforcement / Policy / Contract , It's primary use

2. Use as a collection of utility methods(since java 8 through default methods)

3. Collecting constants together, As Interface are lighter, it makes sense to use them instead of classes if no getters and setters required.

4. Creation of Custom Annotations

5. Special services like marker interface

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Frequently asked in high end product companies. Frequently asked in Deloitte.

Q70. How is Hashmap internally implemented in Java ?

Core Java

Ans. https://medium.com/javarevisited/internal-working-of-hashmap-in-java-97aeac3c7beb#:~:text=HashMap%20internally%20uses%20HashTable%20implementation,the%20entries%20into%20the%20map.

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