Java Interview - Details

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It's in a fixed position in memory and its impossible for it to be instanced like with non-static fields, class or methods. the static variable can be used to refer to the common property of all objects (i.e. not unique for each object). Its good to use static to make your program more memory efficent.

Static

When there is no access modifier specified on a class. Are visible inside the same package only. Default access is also called package access. Default variables and methods from SuperClass are available only to Subclasses in same package.

Default Class Modifier

Typically used in variables and methods. They can only be access in the class they are declared. Private variables and methods from SuperClass are Not available in Subclass.

Private Access Modifier

Protected variables and methods can be accessed in the same package Classes. Protected Variables and Methods from SuperClass are available to SubClass in any package.

Protected Access Modifier

Public variables and methods can be accessed from every other Java Classes. Public variables and methods from SuperClass are all available directly in the SubClass.

Public Access Modifier

Final Class cannot be extended. Example of a Final class in java is String and Integer (or other wrapper classes). This is to make the class immutable. Final is used rarely as it prevents reusing the class.

Final Modifier on a Class

Final Methods cannot be overridden. Any subclass extending the class cannot override the final method. It is the final definition.

Final Modifier on a Method

Once Initialized, the value of a final variable cannot be changed.

Final Modifier on a Variable

Final arguments value cannot be modified.

Final Modifier on a Argument

List interface is the child interface of collections interface. It inhibits a list type data structure where we can store orded collection of objects. It can have duplicate values.

List Interface

Arraylist class implements the List interface. It uses a dynamic array to store the duplicate element of different data types. ArrayList Class maintains the insertions order and is non synchronized.

ArrayList Class

LinkedList implements the List interface. It can store duplicate elements. It maintains the insertion order and is not synchronized. Maniplation is fast because no shifting is required.

LinkedList Class

Implements List interfac. It is similar to ArrayList however it is synchronized and it contains many methods that are not part of the collections framework.

Vector

Queue Interface maintains the first-in-first-out order. Ordered List that is used to hold the elements which are about to be processed. It is implemented by PriortyQueue, Deque, and ArrayDeque

Queue Interface

PriortyQueue implements the Queue interface. It holds elements or objects that are about to be processed by their priorty. Does not allow null values. Is not synchronized

Priorty Queue

Deque Interface extends the Queue interface. We can remove and add elements from both the side. Deque stands for double ended queue which enables us to perform operations at both the ends.

Deque Interface

ArrayDeque class implements the Deque interface. It facilatates us to use the Deque. We can add or delete elements from both the sides. ArrayDeque is faster than Stack and ArrayList. It also has no capacity restrictions.

ArrayDeque

Set interface represents the unordered set of elements which doesnt allow us to store the duplicate itams. We can store at most one null value in Set. Set is implemented bny HashSet, LinkedHashSet and TreeSet

Set interface

HashSet Class implements the Set interface. It represents the collection that uses a hash table for storage. Hashing is used to store the elements in the hashSet. Contains unique items.

HashSet

LinkedHashSet class extends the HashSet class and implements the Set interface. Represents the LinkedList implemention of set Interface. It contains unique elements but maintains the insertion order and permits null elements.

LinkedhashSet

Hashmap allows for null keys where Hashtable doesnt allow null keys and values. Hashtable is synchronized but hashMap is not synchronized.

HashMap vs Hashtable

Break Statements breaks out of the inner most loop (can define loops using labels "outer: for(,,,) break outer;

Break

Continue Statements skips rest of the statements in inner most loop (can define loops using labels "outer: for(,,,) break outer;

Continue

Do while is used when its not clear how many times loop has to be executed. Code in do whole is executed alteast once.

Do while

For Loop: need to specify the loop bounds( minimum or maximum). For each: do not need to specify the loop bounds minimum or maximum. It repeats statements for each element in an array or an object collection.

For loop vs For Each*

While is used when it is not clear how many times loop has to be executed.Depending on the condition, code in while might not be executed at all

While loop

Collection interface has List, Queue and Set interfaces that extends it. You have ArrayList and LinkedList class implementing List. PriorityQueue class implementing Queue. HashSet and LinkedHashset classes implementing Set

Hierarchy of collections

Arrays.asList() convert to arrayList. I can use system out on the new list1 or

10000 items in an array how to pull them out and use efficiently

.put(key, element) and .get(key)

Learn map. How do you put and grab things from map. What is the method.

Super Keyword is used to access methods of the parent class while this is used to access methods of the current class. This and super both kewyords are reference variables that refer to some object of a class. So they both belonga instance of the class. while static method belong to the clas than instance of the class. So static method cant access this and super keyword in java

Super and This. Can you use This inside a main class.

To break it down. Public means you can call this method outside of the class we are currently in. This is important for Java runtime system which is not located in your current class. Static because when the JVM calls to the main method there is no object existing for the class being called. Void because java is platform independent language and if it will return some value then the value many mean different things to different platforms. Main, the name of the method. The name is fixed and its called by the JVM. String args[] this are arguments of type string that your java application accepts when you run it.

The whole (public static void main(String[] args) {)

Static methods belongs to the class rather than object of a class. Static methods can be inoked without create an instance of a class. Static method can access static data memeber and change the value of it.

What is Static, and what it means? If you have a static variable in a class can you reference it

Sets a = 100, single equal is an assignement operator used to assign the values to the variables.

Integer a = 100 and Integer b=100 what is a=b

System – is a final class in java.lang package. Out is a static member field of System Class and is of type PrintStream. Println is a method of PrintStream class.

What is System.out.print.ln (in each word)

Double equals checks if both objects point to the same memory location where as .equals compares the values in the objects.

When do you use double equals and .equals

Java Bytecode and having .class extension. A java class file is created by java compiler from .java files as a result of successful compilation.

What are the contents of a class file

For each loop meant for traversing items in a collection.

For each and for loop.

JDK is is for development purpose where as JRE is for running java programs. JDK and JRE both contains JVM so that we can run out java application with platform independence

JDK vs JRE vs JVM

Class is a blueprint from which objects are created. Objects is an instance of a class.

Class Vs Object

Fast fail throws and error when there is threat concurrency issues and fail safe makes sure there is no threat concurrency issues.

What is the difference between fail-fast and fail-safe iterators?

Abstraction is hiding implemention using things such as abstract classes and interface. Encapsulation is hiding a way of hiding data from other classes which can be done through access modifiers. polymorphism is the abilty of a object to take on many forms this can be done by using methods from the parent class and overwriting it with its own implementation. Inheretance is the idea of a child class inhereting methods from a parent class

OOP Concepts in Java

A singleton class means that only one instance of the class can be created at a time. This is done by setting the contructor as private.

What is singleton class in Java and how can we make a class singleton?

*ngFor and *ngIF, Directives with an asterisk, *, are structural directives. it is used for shape and reshape the DOM's structure by adding, removing and manipulating the elements to which they are attached.

Structural directive

Renders a property's value as text

Interpolation syntax {{ }}

Use property value in a template expression

Property binding [ ]

Lets your bind an event to a method. such as button (click) = "method()"

Event binding ( )

Components define areas of responsibilty with the UI that lets you reuse a set of UI functionality.

Component Class

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