java

Question 1

JVM vs JRE vs JDK

Answer 1

Spec vs runtime vs runtime + dev tools

Question 2

Is java 100% oriented?

Answer 2

No - primitive types

Question 3

Wrapper classes

Answer 3

Wrap primitives to add some API / use in collection

Question 4

Constructor

Answer 4

A block of code to initialise an object. Same name as class, no return type

Question 5

Singleton

Answer 5

Only one instance in JVM. Private constructor

Question 6

ArrayList vs Vector

Answer 6

Fast, non-synchronised vs Slow and synchronised

Question 7

equals() vs ==

Answer 7

Equality of objects from business perspective vs memory address.

Question 8

Stack

Answer 8

LIFO, stack memory can be used by other threads, contains local, primitive and reference variables to objects in heap space, exists until the end of execution of the thread. -Xss, StackOverflow

Question 9

Heap

Answer 9

Memory lives until the end of app: objects. classes -Xms and -Xmx OutOfMemoryError

Question 10

What is a package in Java? Why?

Answer 10

A collection of related classes and interfaces to modularise code and avoid name clashes.

Question 11

Why pointers are not used in Java?

Answer 11

JVM is responsible for implicit memory allocation.

Question 12

What is JIT compiler in Java?

Answer 12

Compiler decides which parts of the code should be optimised.

Question 13

What are access modifiers in Java?

Answer 13

Keywords to restrict access to class method, field: public, protected, default, private.

Question 14

OOP

Answer 14

Programs are organised around objects rather than logic and functions.

Question 15

FP

Answer 15

Programs are organised around functions, avoiding shared state, mutable data, and side-effects (changing something somewhere - changing variable’s value, writing data to disk, enable a button in UI).

Question 16

PP

Answer 16

Procedures, also known as routines, subroutines, or functions, simply contain a series of computational steps to be carried out.

Question 17

What are the main concepts of OOPs?

Answer 17

Inheritance, Encapsulation, Abstraction, Polymorphism.

Question 18

Inheritance

Answer 18

A process where one class acquires the properties of another.

Question 19

Encapsulation

Answer 19

Data can be accessed only with accessors.

Question 20

Abstraction

Answer 20

Only the essential details are displayed to the user / also identifying only the required characteristics of an object ignoring the irrelevant details.

Question 21

Local vs instance variable

Answer 21

Used / scope inside a code block (controller, method) vs belongs to an object, declared on a class level.

Question 22

final

Answer 22

Variable (value can’t be changed once assigned), method (can’t be overridden), class (can’t be extended)

Question 23

break vs continue

Answer 23

Continue loop only, jumps to the next iteration

Question 24

this() vs super()

Answer 24

Instance of a current vs parent class.

Question 25

Java String pool.

Answer 25

Collection of Strings which are stored in heap memory

Question 26

Static vs non-static

Answer 26

Cannot acmes non-static vs can access static without creating a class.

Question 27

What is constructor chaining in Java?

Answer 27

Internal state remains constant after creation. side effects free, thread safe. Final to do. Examples: String, , wrappers. i = I+ 1 creates a new object.

Question 28

Collection

Question 29

Array vs ArrayList

Answer 29

Primitives allowed, fixed size, one type, no generics vs (different) objects, dynamic size generics

Question 30

Map

Question 31

Set

Answer 31

Unordered, does not allow duplicates.

Question 32

List

Question 33

overriding ve overloading

Answer 33

runtime polymorphism vs methods with different signature

Question 34

Runtime polymorphism / dynamic dispatch

Answer 34

Call to overriden method is resolved at runtime.

Question 35

abstract c vs intreface

Answer 35

a can extend only one class, can have instance variables, any visibility can have a constructor vs can extend many, static variables, public only

Question 36

Can you override private / static method?

Answer 36

2 x no - method hidding.

Question 37

Marker interface

Answer 37

e.g. Serializable, Cloneable- an interface with no methods.

Question 38

association vs aggregation vs composition

Answer 38

Own lifecycle / no owner. Own lifecycle, ownership and child object can not belong to another parent object. If parent object deletes all child object will also be deleted.

Question 39

Checked vs unchecked exceptions

Answer 39

Checked at compile-time - must be caught or rethrown.

Question 40

final, finally, and finalize

Answer 40

restrictions on class, method, and variable try / catch finalize - called by the Garbage Collector

Question 41

throw vs throws

Answer 41

Throw vs declare that a methods may throw an exception.

Question 42

Ways to create a thread

Answer 42

Extend Tread, implement Runable, executors - Runnable, Callable (return value), Future (get value later), scheduled executors

Question 43

How to synchronize access to mutable, shared variables

Answer 43

synchronized, locks, semaphore, atomic variables and ConcurentMap