Java

Question 1

What is Java Development Kit & what does it include?

Answer 1

In order to start writing the code in Java, we need to install Java Dev. Kit
JDK includes:
- Compiler - will compile for Java a source code into java-byte code, which isn’t unique machine code & will be executed by Java Virtual Machine (JVM)
- Java Runtime Environment (JRE) contains JVM & core java libraries
- Tools for Java development (archiver, docs generator, etc.)

Question 2

What is a method?

Answer 2

A method is a set of codes which is referred to by name & runs when it’s called. Each method has its own name. We need method to reuse the code: define the code once & use it many times

Question 3

What is a main method?

Answer 3

Main method is a special method that actually runs the program. Everything in the main method will be executed when we run the program from top to bottom

Question 4

What is a constructor?

Answer 4

It’s a special method to create an object

Question 5

What’s a default constructor?

Answer 5

If we don’t define any constructors by ourselves java will provide one default (empty) constructor.

Question 6

What is the difference between the constructor & the method?

Answer 6

• Constructor doesn’t have a return type & its name must be the same as a class name
• Java provides the default constructor if we don’t define it
• Constructor isn’t inherited by child class
• Method has a return & its name may or may not be the same as a class name
• Method isn’t provided buy Java
• Methods are inherited by child class

Question 7

What is the difference between local & instance variables?

Answer 7

• Local variables are declared in methods, constructors, or blocks.
• Local variables are created when the method, constructor or block is entered and the variable will be destroyed once it exits the method, constructor, or block.
• Access modifiers cannot be used for local variables.
• Local variables are visible only within the declared method, constructor, or block.
• Instance variables are declared in a class, but outside a method, constructor or any block.
• Instance variables hold values that must be referenced by more than one method, constructor or block
• Instance variables can be declared in class level before or after use.
• Access modifiers can be given for instance variables.

Question 8

What are the OOP concepts in Java you know?

Answer 8

Object Oriented Programming. In Java, we have 4 concepts:

Encapsulation, Inheritance, Abstraction, Polymorphism.

Question 9

What’s an encapsulation in java and how do we achieve it?

Answer 9

It’s data protection(hiding) mechanism from the client code. We make instance variables as private and we provide public getters and setters. use pojo class

Question 10

What’s inheritance in java?
Can you give an example of using the inheritance in your project?

Answer 10

It is a process where one class can inherit visible properties and methods from another class. Parent-child relationship (super class - subclass relationship). Inheritance is used for not creating the object. Using extends keyword

Question 11

What’s abstraction in java?

Answer 11

When we focus on what object does instead of how it does it by hidding implementation details. We achvieve abstraction by using abstract method.

Question 12

What’s polymorphism in java?

Answer 12

Polymorphism is the ability of the object to take many different forms.

List list = new ArrayList<>();
list = newLinkedList<>();

Question 13

What’s POJO?

Answer 13

Plain Old Java Object. Usually, encapsulated objects referred as POJO.

Question 14

What’s super class?

Answer 14

It’s parent class in inheritance.

Question 15

What’s the class which every class will inherit from? Why?

Answer 15

java.lang.Object. To give generic behaviors for every object. For example, equals, hashCode, toString.

Question 16

How many classes we can extend at once?

Answer 16

Just one because java supports only single type inheritance.
*** Java doesn’t support multiple inheritance.

Question 17

Why Java doesn’t support multiple inheritance?

Answer 17

Java doesn’t support multiple inheritance in classes because of “Diamond Problem”.
Due to this Java does not support multiple inheritance i.e., you cannot extend more than one other class

Question 18

What’s the difference between method overriding and method overloading?

Answer 18

• Overloading is when we have multiple methods with the same name but different arguments
• Overriding is when we override the parent class or interface method in the child class.

1. Most of the method signatures should be the same
2. Child class access modifier should be the same or more visible
3. Return type should be the same or covariant
4. Child class cannot declare bigger exception than parent class

Question 19

What does ‘this’ and ‘super’ keyword mean in java?

Answer 19

• ‘super’ is used to access the parent class properties and methods.
• ’ this’ is used to access current object properties and methods.

Question 20

Access modifiers

Answer 20

• public - the member accessible from anywhere in the project
• protected - accessible within the same package & inside subclasses
• default - accessible within the same package only
• private - accessible within the same class only

Question 21

Method Arguments

Answer 21

Arguments are used to provide the data to the method.
Methods can have multiple arguments with different types

Question 22

What is variable?

Answer 22

Variable is a container that can hold piece of information. There are different type of variables for different data types in Java.

Question 23

What kind of logical operators do you know? And how do they work?

Answer 23

Logical operators are used to check whether an expression is true or false. They are used in decision making.
&& (Logical AND) expression1 && expression2
true only if both expression1 and expression2 are true

|| (Logical OR) expression1 || expression2
true if either expression1 or expression2 is true

! (Logical NOT) !expression
true if expression is false and vice versa

Question 24

What’s a static keyword in Java?

Answer 24

• Static variables and methods belong to the class, not to a specific object.
• We don’t need to create an object to call them, we use them by class name.

public FileHelper {
public static String path = “ABC”;
public static String readContent() {
…
return

Question 25

What is final keyword in Java?

Answer 25

- final variable cannot be changed (reassinged) - final class cannot be inherited - final method cannot be overridden

Question 26

How to prevent your method to be ever overridden?

Answer 26

- make it final

Question 27

What is static block in Java?

Answer 27

Block of code that will be executed one time when this class is used static { . . . . . . }

Question 28

What is initializer block?

Answer 28

Block of code that run before constructor

Question 29

What is the relationship between parent class and child class?

Answer 29

Child class inherits visible variables and methods from the parent class

Question 30

Can one class extend multiple classes at the same time in Java?

Answer 30

No, we cannot

Question 31

Can one class implement multiple interfaces at the same time?

Answer 31

Yes, we can

Question 32

What is difference between abstract class and interface?

Answer 32

- Interface can have only abstract methods (except static and default) and abstract class can have abstract and regular methods and properties - We can extend one abstract class and implement multiple interfaces - Abstract class can have abstract and non-abstract methods. - Interface can have only abstract methods. Since Java 8, it can have default and static methods also. - Abstract class doesn't support multiple inheritance. - Interface supports multiple inheritance. -Abstract class can have final, non-final, static and non-static variables. - Interface has only static and final variables. - Abstract class can provide the implementation of interface. - Interface can't provide the implementation of abstract class. - The abstract keyword is used to declare abstract class. - The interface keyword is used to declare interface. - An abstract class can extend another Java class and implement multiple Java interfaces. - An interface can extend another Java interface only. - An abstract class can be extended using keyword "extends". - An interface can be implemented using keyword "implements". - A Java abstract class can have class members like private, protected, etc. - Members of a Java interface are public by default. Example: Example: public abstract class Shape{ public interface rawable{ public abstract void draw(); void draw(); } } An Interface in Java programming language is defined as an abstract type used to specify the behavior of a class. An interface in Java is a blueprint of a class. A Java interface contains static constants and abstract methods. The interface in Java is a mechanism to achieve abstraction https://www.javatpoint.com/difference-between-abstract-class-and-interface

Question 33

Can you give me an example of using abstraction in your project?

Answer 33

- I don't use abstraction or - I use abstract templates to locate elements in my framework and we implement them in my steps class

Question 34

Can you give me an example of using polymorphism in your project?

Answer 34

WebDriver driver = new ChromeDriver(); WebDriver driver = new FirefoxDriver();

Question 35

What is the helper class for array and collection (for sorting, and searching)?

Answer 35

Arrays is a helper class for arrays Arrays.sort();

Question 36

How to get number of elements in the array?

Answer 36

.length -> without parentheses int[] nums = {1, 2, 3, 4, 5, 6}; System.out.println(nums.length);

Question 37

What is collection framework in Java?

Answer 37

It is framework provided by Java to work with collection of data List, Set, Queue, Map (it is not part of the framework)

Question 38

How does ArrayList work internally?

Answer 38

ArrayList works internally with array

Question 39

What is the initial capacity (size of internal array) of ArrayList?

Answer 39

10

Question 40

What is Set in Java?

Answer 40

It is unique collection of data, doesn't allow duplicates The List is an ordered collection of objects and the List can contain duplicate values SetTable synchronized doesn't maintain insertion order

Question 41

What is Map in Java?

Answer 41

Key value data collection and keys are unique in map put()

Question 42

How do you iterate over the map?

Answer 42

We can iterate over the map by using keySet() method that returns all keys. Once we have keys we can easily find out values

Question 43

What is the difference between HashMap and HashTable?

Answer 43

hashTable doesn't allow null key and null value, synchronized, doesn't maintain insertion order, thread-safe HashMap allows one null key and multiple null values,

Question 44

What is TreeMap? What is LinkedHashMap?

Answer 44

TreeMap - sorted by keys implementation of the map LinkedHashMap - maintains insertion order

Question 45

What is Queue data structure?

Answer 45

FIFO - First In, First Out data structure

Question 46

What is the helper class for collection framework in Java (for sorting, searching)?

Answer 46

Collections is a helper class for collection framework

Question 47

What is Stack data structure?

Answer 47

LIFO - Last In, First Out data structure

Question 48

Could you explain about upcasting vs downcasting?

Answer 48

Person <- Student // downcasting Person obj = new Student(); Student student = (Student)obj; Student.study(); // upcasting Student obj2 = new Student(); Person person = obj2;

Question 49

How to handle exceptions in Java?

Answer 49

By using try catch statement try { ... ... } catch (TypeOfException e) { // handle it }

Question 50

When do you get NullPointerException?

Answer 50

When object is null and we try to use some of the property or method to this project String str = null; System.out.println(str); // NullPointerException if (str == null) { }

Question 51

What is the difference between runtime and checked exceptions?

Answer 51

- we must handle checked exceptions but it is optional to handle runtime exceptions - all checked exceptions extend from Exception class (except RuntimeException)

Question 52

What does finally block do?

Answer 52

- it always runs (if there is exception or not) after try catch - usually it is used to close resources

Question 53

Can you catch multiple exceptions in the same try statement?

Answer 53

Yes, we can try { } catch (NullPointerException e) { } catch (RuntimeException e1) { }

Question 54

What is Error? Do you handle or throw errors in your project?

Answer 54

Error is system level failure. Errors are thrown by Java Virtual Machine and they should not be handles in application level

Question 55

final vs finally vs finalize()

Answer 55

- Final: - final is a keyword, it is used to apply restrictions on class method and variable - if a class is marked as final then this class can not be inherited by any other class - finally: - finally is a block that is used for exception handling along with try and catch blocks - Finalize: - finalize() method is protected method of java.lang.object class, it is inherited to every class you create in java & it is used to perform some clean-up operations on an object before it is removed from memory

Question 56

What is a StringPool?

Answer 56

It is a memory for String objects in the heap. Java will reuse the same String values from StringPool to save memory.

Question 57

String vs StringBuilder vs StringBuffer

Answer 57

STRING STRINGBUFFER STRINGBUILDER Storage constant HEAP Heap Area String pool Modifiable no(immutable) YES(mutable) YES(mutable) Thread Safe YES YES NO Performance FAST VERY SLOW FAST Immutable means values can not be changed once it's created:

Question 58

Most common Java Exceptions

Answer 58

NullPointerException. ArrayIndexOutOfBoundsException. IllegalStateException. ClassCastException. ArithmeticException. IllegalArgumentException.

Question 59

What are the wrapper classes in Java?

Answer 59

Object representations of the primitives in Java byte -> Byte short -> Short int -> Integer long -> Long float -> Float double -> Double char -> Character boolean -> Boolean String str = "123"; int num = Integer.parseInt(str);

Question 60

When do you use while loop? And when do you use for loop?

Answer 60

We use while loop when we don't know number of iterations in advance We use for loop when we know number of iterations in advance

Question 61

What is the difference between Array and ArrayList?

Answer 61

- Array is fixed size and ArrayList is resizable - Array can work with primitives and Objects but ArrayList works only with Objects

Question 62

What is Varags in Java?

Answer 62

- By using Varags we can create a method that can take dynamic number of arguments - it works inside the method exactly as Array - varag can be used in method argument only and only one varag per method public int sum(int . . . nums) { int sum = 0; for (int num : nums) { sum += num; } return sum; }