Java Interview Qs

Question 1

Difference between JRE and JVM?

Answer 1

JRE is an implementation of the JVM - If you want to execute java you need the JRE Installed

Question 2

Can you have multiple inheritance in java?

Answer 2

Only through interfaces.

Interface can extent multiple interfaces

Question 3

write the main method?

Answer 3

Public static void main(String arg[])

//public and static so that Java can access it without initializing the class
//pass runtime arguments

Question 4

Java.lang(Object) imported by default?

Answer 4

yes

Question 5

what are the access modifiers for class?

Answer 5

for class - public and default

public: access from anywhere/class in other packages
default: can only be accessed from classes in that package

Question 6

what are access modifiers for members?

Answer 6

public: global access
private: only inside the class
protected: classes in the same packages and subclasses
default: classes in the same package

Question 7

how can we optimize SQL?

Answer 7

1. Specific columns to retrieve
2. use limit
   ex/ SELECT name, produce FROM products LIMIT 10
3. Use “add index” to a table
4. Don’t use select *
   ** Index - data structure improves the s[eed of operations
   Insert and Update take longer than selects with indexed tables.(Need to insert/update index values as well)

Question 8

ORM?

Answer 8

Object/Relational Mapping

Helps application achieve persistence. Keeping state- object live on beyond the scope

Question 9

what is the final keyword?

Answer 9

• used as a keyword so class cannot be extended
• using final on a method in a parent class so the child class cannot override it
• used with variables so it can only be assigned once

Question 10

what is the static keyword?

Answer 10

class level variables are global

methods can be static - access only static variables

Question 11

what is finalize?

Answer 11

called by the garbage collector in the object class

Question 12

what is a static block?

Answer 12

group of statements that gets executed when the class is loaded into memory by java class loader

Question 13

what is a class loader?

Answer 13

use custom if you want a class at runtime or FTP server or another web service at runtime

ex/ bootstrap, extensions, system

loads bytecode into memory when we want to access any class

Question 14

inheritance

Answer 14

• used for code reusability

- a subclass can inherit the contents of a superclass

Question 15

polymorphism

Answer 15

a concept where an object behaves differently in different situations

Question 16

compile time polymorphism

Answer 16

circle{
    public void draw();
    public void draw (int r);
}
overloading - different arguments same method name

Question 17

runtime polymorphism

Answer 17

"is-a" relationship
subclass method overrides the superclass method - different implementation, with the same name

Question 18

Abstraction

Answer 18

the concept of hiding the internal details and describing things in simple terms(Encapsulation and inheritance)

Question 19

Encapsulation

Answer 19

Hiding data with access modifiers so that it’s only visible to that class

Question 20

Interfaces

Answer 20

• provide a way to achieve abstraction
• interface can implement a superclass
• another class can implement an interface but another interface extends an interface
• • All attributes of interface are public, static, final

Question 21

Abstract class vs. Interface

Answer 21

abstract classes can have method implementations but interfaces cannot mostly used as a base for subclass

Question 22

Wrapper Classes

Answer 22

immutable/final
object representation of primitive types
byte,short,int,long,float, double, char, boolean

Question 23

super();

Answer 23

access superclass constructor

Question 24

this

Answer 24

reference to the current object

Question 25

garbage collecting

Answer 25

the process of checking the heap for objects that are no longer referenced and deleting them

Question 26

serialization

Answer 26

convert java object to a stream, the stream can be saved to a file stream data -> object is deserialization

Question 27

"InstanceofKeyword"

Answer 27

Checks if an object belongs to a class or not

Question 28

Is string a type or class

Answer 28

String is an object, its a class

Question 29

Pass by value

Answer 29

Java is passed by value | method parameter values are copied to another variable and then copied object is passed in

Question 30

Pass by reference

Answer 30

reference to the actual parameter is passed to the method

Question 31

what is stored on the stack?

Answer 31

LIFO - main method, reference variable, static varaibles