core.js:1673 ERROR Error: Uncaught (in promise): TypeError: instance[output.propName].subscribe is not a function

Hi Guys ,

Today I have faced this error :

core.js:1673 ERROR Error: Uncaught (in promise): TypeError: instance[output.propName].subscribe is not a function

Later I have found out that I have imported the EventEmitter from a different package in my component file :

import { EventEmitter } from 'events';

Later when I changed the package to @angular/core it worked fine .

import { EventEmitter } from '@angular/core';

The full stack trace is below for your reference :

ERROR Error: Uncaught (in promise): TypeError: instance[output.propName].subscribe is not a function
TypeError: instance[output.propName].subscribe is not a function
    at createDirectiveInstance (core.js:9183)
    at createViewNodes (core.js:10399)
    at callViewAction (core.js:10715)
    at execComponentViewsAction (core.js:10634)
    at createViewNodes (core.js:10427)
    at createRootView (core.js:10313)
    at callWithDebugContext (core.js:11344)
    at Object.debugCreateRootView [as createRootView] (core.js:10831)
    at ComponentFactory_.push../node_modules/@angular/core/fesm5/core.js.ComponentFactory_.create (core.js:8659)
    at ComponentFactoryBoundToModule.push../node_modules/@angular/core/fesm5/core.js.ComponentFactoryBoundToModule.create (core.js:3311)
    at resolvePromise (zone.js:831)
    at resolvePromise (zone.js:788)
    at zone.js:892
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:423)
    at Object.onInvokeTask (core.js:3811)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:422)
    at Zone.push../node_modules/zone.js/dist/zone.js.Zone.runTask (zone.js:195)
    at drainMicroTaskQueue (zone.js:601)
defaultErrorLogger @ core.js:1673
push../node_modules/@angular/core/fesm5/core.js.ErrorHandler.handleError @ core.js:1719
next @ core.js:4311
schedulerFn @ core.js:3551
push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.__tryOrUnsub @ Subscriber.js:196
push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.next @ Subscriber.js:134
push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber._next @ Subscriber.js:77
push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next @ Subscriber.js:54
push../node_modules/rxjs/_esm5/internal/Subject.js.Subject.next @ Subject.js:47
push../node_modules/@angular/core/fesm5/core.js.EventEmitter.emit @ core.js:3535
(anonymous) @ core.js:3842
push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke @ zone.js:391
push../node_modules/zone.js/dist/zone.js.Zone.run @ zone.js:150
push../node_modules/@angular/core/fesm5/core.js.NgZone.runOutsideAngular @ core.js:3779
onHandleError @ core.js:3842
push../node_modules/zone.js/dist/zone.js.ZoneDelegate.handleError @ zone.js:395
push../node_modules/zone.js/dist/zone.js.Zone.runGuarded @ zone.js:164
_loop_1 @ zone.js:694
api.microtaskDrainDone @ zone.js:703
drainMicroTaskQueue @ zone.js:608
Promise.then (async)
scheduleMicroTask @ zone.js:584
push../node_modules/zone.js/dist/zone.js.ZoneDelegate.scheduleTask @ zone.js:413
push../node_modules/zone.js/dist/zone.js.Zone.scheduleTask @ zone.js:238
push../node_modules/zone.js/dist/zone.js.Zone.scheduleMicroTask @ zone.js:258
scheduleResolveOrReject @ zone.js:879
ZoneAwarePromise.then @ zone.js:1012
push../node_modules/@angular/core/fesm5/core.js.PlatformRef.bootstrapModule @ core.js:4345
./src/main.ts @ main.ts:11
__webpack_require__ @ bootstrap:76
0 @ main.ts:12
__webpack_require__ @ bootstrap:76
checkDeferredModules @ bootstrap:43
webpackJsonpCallback @ bootstrap:30
(anonymous) @ main.js:1

ERROR Error: Uncaught (in promise): Error: Cannot match any routes.

Hi Guys ,

Today I have faced this error :

ERROR Error: Uncaught (in promise): Error: Cannot match any routes. 

We will face this error when we enter the URL of the angular app and the URL does not have a matching route configured . To verify that check your AppModule or AppRoutingModule . 

1. check the router array :

const appRoutes:Routes =[

{path:'cust-detail',component: CustDetailComponent},

{path:'mcq',component: MCQComponent},

{path:'sign-up',component: SignUpComponent}

];

if the path is missing add to it .

2. check the import array :

imports: [

BrowserModule,

  FormsModule,

  ReactiveFormsModule,

  RouterModule.forRoot(appRoutes),

],

You need to have the RouterModule.forRoot(appRoutes) present in your application.

The full stack trace is below for your reference :


ERROR Error: Uncaught (in promise): Error: Cannot match any routes. URL Segment: 'test'
Error: Cannot match any routes. URL Segment: 'test'
    at ApplyRedirects.push../node_modules/@angular/router/fesm5/router.js.ApplyRedirects.noMatchError (router.js:1384)
    at CatchSubscriber.selector (router.js:1365)
    at CatchSubscriber.push../node_modules/rxjs/_esm5/internal/operators/catchError.js.CatchSubscriber.error (catchError.js:34)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber._error (Subscriber.js:80)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.error (Subscriber.js:60)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber._error (Subscriber.js:80)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.error (Subscriber.js:60)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber._error (Subscriber.js:80)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.error (Subscriber.js:60)
    at TapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/tap.js.TapSubscriber._error (tap.js:61)
    at resolvePromise (zone.js:831)
    at resolvePromise (zone.js:788)
    at zone.js:892
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:423)
    at Object.onInvokeTask (core.js:3811)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:422)
    at Zone.push../node_modules/zone.js/dist/zone.js.Zone.runTask (zone.js:195)
    at drainMicroTaskQueue (zone.js:601)

Web services Interview Questions

Web Services Interview Questions.
This blog is just for revision.

• What is SOAP Web Services

SOAP ( simple object access protocol) based web services (application exchange information over network)  .

• What is Contract first ?

WSDL or XSD is created first , then the java classes .

• What is WSDL ?

WS Description Language , it describes a web service .

• What are the parts of wsdl ?

1. types : data types used 
2. message : request and response
3. portType : operations available
4. binding : 
5. Operations:
6. Service :

Types

Schema

Contract namespace

Schema location xsd

Message can be multiple

Part

Name

Element

Type


Port type
Operation
Input, output,fault



Binding
Style document/ RPC transport
Operation
Input
Header, body
Output
Header,body
Fault
Fault

Service
Port
Binding
Location

• What is XSD ?

It defines schema which defines structure of xml.

• Explain wsdl2java

Use pluggin cfx-codegen-pluggin
Goal wsdl2java
Wsdl location
Binding files
Note : wsdl will have xsd,a java class for each xsd will be generated.

• What are the elements of soap message?

1. Envelope
2. Header
3. Body
4. Fault

•  What are SOLID principles in Rest Web services Development ?

1. Single Responsibility : Each class has only one function 
2. Open for Extension Closed for Modification : existing classes should not be modified , instead new classes should be created .
3. Liskov Substitution Principle :  App should work fine if we substitute subclass in place of super class .
4. Interface Segregation : large interfaces should be divided into smaller ones .
5. Dependency Inversion : loose coupling .

Hibernate interview questions

This blog is just for revision .

• What is ORM ?

Object Relational Mapping : Object to database tables .

• Explain Core Classes in hibernate .

1. Configuration : load xml file .
2. SessionFactory : thread safe
3. Session : get connection with database , not thread safe , used to perform CRUD operations .
4. Transaction :  commit and rollback operations . unit of work.
5. Query : common query language for any type of database.
6. Criteria : add restrictions and projections

• Explain Hibernate configuration file 

contains database details used to initialize session factory .

• Explain Hibernate Mapping file 

contains table and column details also contains the entity bean mapping .

• Explain state of the entity in the persistence context .

Session is the persistence context which is used to manage relationship of the entities .

1. transient : newly created object 
2. persistent : associated with session ( session.save)
3. detached : session closed 

• How to use hibernate in spring ?

In application context.xml file specify

1. the datasource bean  DriverManagerDataSource  
• specify the username /password or jndi
2. sessionfactory bean LocalSessionFactoryBean
3. transaction bean HibernateTransactionManager
4. tx:annotation driven 

• What is hibernate dialect.

Different dialect for different databases
MySqlDialect , Oracle Dialect ,Db2 Dialect.

• What are the different consistency anomalies in hibernate .

1. dirty read .reading uncommitted data
2. non repeatable read : second read returns different values for a row
3. phantom read : second read returns different number of rows 

• Explain isolation level .

1. Read uncommitted
2. Read committed :  

      4. Repeatable Read : second read will return same value 

      8. synchronized : does not allow concurrency

Isolation level is set using property hibernate.connection.isolation 

• Explain the locking strategy based on isolation level .

1. Read uncommitted : no lock 
2. Read committed : lock on committed data

      4. Repeatable Read : lock on a block of  SQL

      8. Synchronized : full lock on table .

• In which scenario repeatable read will return different data .

If the read is based on a condition and another thread inserts a row in database . Now for the second read we may get the new also if it satisfies the condition .

• What is Caching in hibernate ?

1. First level Catch : session cache of hibernate , enabled by default 
2. Second Level Cache : optional , eg : Ehcache 
3. Query Level Cache : cached query results . set hibernate.cache.use_query_cache property to true. 

• What are the concurrency strategies for second level cache?

1. Transactional : synchronized with the database .
2. Read-write : if the transaction is not committed , the entity is fetched from database and referred from the cache .
3. non strict read write : cache updated after data committed
4. read only : used when data never gets changed .

• What is dirty checking?

When hibernate loads an entity it keeps a copy of it in the memory, on tx.commit it compares the entity and the copy . If there are any changes, update statement is executed in the database.

• Explain session.get

1. Hits the database immediately.
2. If record is not available in the database it returns null.

• Explain session.load

1. Returns proxy object without hitting the database.
2. If the record is not available and when we try to access it through the proxy object we get Object Not Found Exception.

• What is the difference between get and load ?

in case of no records in the database get returns null and load throws an exception .

• What is the difference between save and persist ?

save return the id and persists returns nothing .persist marks the entity as persistent , does not generate Insert statement outside transaction boundaries . session.save on detached object will create a new row , session.persist on detached object will throw persistentobjectexception.

What is the difference between session.save and session.saveorupdate
save will insert the record if the Id is not available and throw exception if the Id is available. Saveorupdate will insert the record if the Id is not available, else it will update it.

• What is the difference between update and merge ?

Update is used to update the persistent entity and Merge is used to update a detached entity .

• What is the difference between session.flush and transaction.commit ?

flush synchronizes the persistent store with the state held  in memory .
commit commits the record to the database .

• Explain Criteria

Used to filter record

cr =Session.createCriteria
cr.list();

• Explain Restrictions

Used to add condition s to the query to compare the columns
Cr.add( Restrictions.eq("id" , 10));

• Explain Projections

Used to call an aggregate method . Min , max , count .
Cr.setProjection( Projections.max("id"));

• What is Declarative transaction management  ?

 transaction handling is not a part of business logic . 

using @Transactional :

• What is Programmatic transaction management ?

transaction management is a part of business logic . 

• Explain Different transaction manager used in spring .

1. DataSourceTransactionManager : uses datasource 
2. HibernateTransactionManager : uses sessionfactory
3. JpaTransactionManager : uses entitiymanagerfactory 

• Explain transaction propagation ?

using @Transactional( propagation=Propagation.

1. REQUIRED : (default) if transaction is available use it otherwise create a new one. 
2. REQUIRES_NEW : create a new transaction , commit / rollback is independent .
3. NESTED : uses outer transaction but creates a savepoint to allow partial rollback and commit .

• What is readOnly in @Transactional ?

used for only read operations

• What are Value Types .

1. Basic 
2. Composite / Embedded
3. Collection : for one to many .

• Explain Many to Many .

Tables : Person , Project, PersonProject 

Entities :

Person : on projects set attribute add

1. @ManyToMany 
2. @JoinTable ( name=PersonProject , joinColums=@JoinColum , inverseJoinColumns=@JoinColum  )

Project: on persons set attribute add

1.  @ManyToMany(mappedBy = "projects")

• Explain One to Many

Tables : Person , Address

Entities :

•   Explain Cascade type :

In case of mapping like one to one,or one to many , cascade type defines how changes in the entity will cascade to the mapped entity .
@ oneToMany cascade = Cascade Type.ALL

1. All
2. Persist
3. Merge
4. Refresh
5. Remove
6. Detach

• How to lazy load the mapped classes

@oneToMany fetch = Fetch type.Lazy

• What are the fetching strategies ?

1. Join Fetching 
2. Select Fetching 
3. Sub select Fetching 
4. Batch Fetching 

• How to handle multiple databases ?

1. @Table (schema= "")
2. create two sessionfactory , use @Qualifier

• How to handle distributed databases ?

use XA datasource , if  transaction is failed in one database , the entire transaction is roll backed .

• What is the difference between jdbc and JTA?

2 phase commit can be handled using JTA only .

•  Explain different ways of creating  EntityManager ?

1. Container Managed : 

@PersistenceContext

EntityManager entityManager; 

2. Application Managed 

EntityManagerFactory emf = Persistence.createEntityManagerFactory("org.model.student");

• Explain  Anomalies in DBMS

1. Update : multiple rows needs to be updated to update a data .

2. Insert : cannot insert a row without supplementary data.

3. Delete : unable to delete a data as this will cause the important information to be deleted.

• Explain normalization.

Normalization is a process of organizing the data in database to avoid data redundancy, insertion anomaly, update anomaly & deletion anomaly.

Terminology :

1. attribute : column

2. candidate key : primary key ( simple or composite )

3. prime attribute : column which is part of primary key

4. non prime attribute : column not part of primary key .

5. super key : any set of attributes which identifies a unique record .

6. functional dependency : B depends on A 

7. transitive dependency : B is dependent of A . C is dependent of B . So C has a transitive dependency on A .

1NF : attribute should be atomic.

2NF : non prime attribute should not depend on a subset of primary key

3NF : non prime attribute should not have transitive dependency with the super key .

BCNF : any functional dependency should be a super key .

Java interview questions

Most Common Java interview questions

• String vs StringBuffer vs StringBuilder 

1. String : immutable 
2. StringBuffer : mutable , synchronized
3. StringBuilder : mutable , not tread safe .

• Explain toString method.

getClass().getName() + '@' + Integer.toHexString(hashCode())

• Explain hashcode

it is a native method in object Class , returns integer code .
Two unequal objects can have same hashcode

• Explain finalize

called before Garbage Collection .

• How to create a immutable class ?

1. make the class as final
2. attributes as private and remove setters .
3. getters should return a copy of object .

• what is transient ?

Transient attributes are not serialized .

• What is volatile ?

Changes made by one thread is immediately reflected on other thread .

• What is static inner class 

1. can be accessed without creating object of outer class , does not have access to instance variables .

• What is Stack

LIFO 
1. push : add element on the top of stack
2. pop : remove element from the top of stack
3. peek : returns element at top of stack without removing it .

• What is Queue 

FIFO .

1. add : adds to last , in case of capacity restriction throws IllegalStateException.
2. remove : returns and remove the element on the top of the queue , throws NoSuchElementException if the queue is empty .
3. element : returns the element on the top of the queue , throws NoSuchElementException if the queue is empty .
4. offer :  adds to last , in case of capacity restriction returns false .
5. poll: returns and remove the element on the top of the queue , returns null if the queue is empty 
6. peek: returns  the element on the top of the queue , returns null if the queue is empty .

• What is Dqueue ?

supports insertion and removal from both ends .

1. push  (addFirst)  : add element on the top of stack
2. pop : remove element from the top of stack
3. peek : returns element at top of stack without removing it .
4. add : adds to last , in case of capacity restriction throws IllegalStateException.
5. remove : returns and remove the element on the top of the queue , throws NoSuchElementException if the queue is empty .
6. element : returns the element on the top of the queue , throws NoSuchElementException if the queue is empty .
7. offer :  adds to last , in case of capacity restriction returns false .
8. poll: returns and remove the element on the top of the queue , returns null if the queue is empty 
9. peek: returns  the element on the top of the queue , returns null if the queue is empty .

• What is the difference between ArrayList and Vector?

Vector increase of capacity by double of its current capacity , ArrayList increase its capacity by 50%.

• What is the difference between ArrayList and LinkedList ?

1. LinkedList implements Dqueue has additional methods .
1. decendingIterator
2. getFirst
3. getLast
4. addFirst
5. addLast
2. Add and remove is faster , get and set is slower in LinkedList.
3. ArrayList stores data in contiguous locations LinkedList store data using pointers and reference 
4. LinkedList is better if we need to perform data manipulation , arrayList is better if we just want to send the data.

• Explain Legacy collection in  java ?

All are synchronized

1. Vector , Stack (LIFO ) :  dynamic array
2. Dictionary (abstract) , Hashtable (concrete ), Properties ( string key and value , loaded to/from file)  :  key value pair ,null key not allowed 
3. Enumeration : Iterate over legacy collection

• How to make ArrayList thread safe.

1. Use Collections.synchronizedList
2. CopyOnWriteArrayList : can be modified during iteration but modified values will not be reflected .

• How do HashMap handles different key with same hashcode ?

takes the hashcode of the key and check if it matches a with the hashcode of existing key . If the same hashcode exists then it compares both the keys using equals method . If both the keys are same it replaces the existing value with the new one . If both the keys does not match then it adds a new entry  in the same bucker .

• What is BlockingQueue

Thread-safe , does not accept null value . FIFO . It is used to implement producer / consumer issues .

1. put : add the element to the queue , if space is not available it waits for the space to be available.
2. take : returns element from head of the queue , if no elements wait for the element to come.

• What are the new features added in Java 7?

1. Try with resources : the resources will be closed automatically (ARM), 
• should implement AutoCloseable introduced in Java 7 .
• to support ARM Suppressed Exceptions are introduced .
• the exceptions originating at try block is propagated and the exceptions occurring in finally block is suppressed . 
• addSuppressed and getSuppressed methods added in Throwable class
• Closeable interface now extends AutoClosable interface .
2. Multiple Exception in single catch separated by | . 
3. Underscore in numeric literals
4. Binary literals 0b,0B
5. Switch case with string
6.  Type interface for generic instance.using <> .
7. Java.nio.file package : used to create, delete , copy , move etc. files and directories.
8. @SafeVarargs : used by programmer to mark that no unsafe operations are performed in the constructor with var args .
9. String constant pool is stored in heap instead of perm gen.

• New features in Java 8

1. Perm gen is removed.
2. Functional interface
3. Lambda support
4. Interface default and static 
5. For each in iterator
6. Java stream API.
7. Optional class : avoid null pointer

• Explain interface default method :

1. The implementing class does need not implement the default method.
2. Sub interfaces can override or make it abstract .

• Explain method reference

Static method 

Class name :: method name

Instance method

Object name:: method name

• How does multiple inheritance issue for default method is handled .

It is mandatory for the implementing class to implement a default method in case of multiple inheritance issue .

• What is optional class in Java 8

Pre defined action instead of throwing null pointer exception.

• Explain functional interface

@FunctionalInterface annotation will add a compile time check for an interface to have only one abstract method. It will be used for lambda expressions.

• Explain memory management in Java.

1. Heap : stores object data.
2. Permgen : stores class data ,method local initialization , string contact pool.
3. Stack : store data required while returning from method

• How to manage memory ?

1. System.gc() or Runtime.gc() : garbage collector may be called .
2. Runtime.freeMemory() : free memory 
3. Runtime.totalMemory() : total memory available
4. Runtime.maxMemory() : maximum memory that JVM is going to use

• What is just in time compiler(JIT)

converts byte code to native code to boost performance .

• Explain Creational Design Patterns .

1. Factory 
2. Abstract Factory
3. Builder : create complex object , method chaining .
4. Singleton
5. Prototype

• Explain Structural Design Patter .

1. Adapter
2. Bridge
3. Composite
4. Decorator
5. Facade
6. Flywheel
7. Proxy

• Explain Behavioral Design Patter .

1. Chain of Responsibility
2. Command
3. Interpretor : how to include language  elements
4. Iterator
5. Mediator : simplify communication between objects
6. Observer : multiple objects notification
7. State : modify the behavior on state change
8. Template : abstract definition of algorithm
9. Visitor : add polymorphic functions to class noninvasively .

• How can we make singleton class thread safe.

Null check for instance and creation of new object should be done inside a synchronized block.

• What is executor service?

It provides a pool of threads and API to assign task for it.Simplifies task execution in async mode.

• What is Thread.yeald?

Thread Scheduler checks if any other thread available with same or higher priority and transfer the processor to it .

• Explain serialization

1. Serializable interface should be implemented .
2. Serializable is a marker interface for identification .
3. sub class must be serializable ,  super class need not be serializable .
4. Transient and static attributes are not serialized.
5. readresolve method can be implemented and same instance can be returned, while deserialization of singleton class.
6. Write replace is called before write object .

• Explain serialVersionUID .

1.  if it not present java compiler adds it .
2.  it is static final and long . 
3. It is used to check the sender and receiver are compatible if there is mismatch in the serialVersionUID InvalidClassException is thrown .
4. If there is no serialVersionUID and we have serialized an object  , we added some field and now if we want to deserialize it we get InvalidClassException .

• Explain Externalizable interface 

used for custom serialization 

1. readExternal
2. writeExternal

• Explain classloaders 

they load classes during runtime dynamically .

1. Bootstrap : parent of all , first to be loaded
2. extension : load all classes from .jar from  java.ext.dirs
3. system : loads the classes at java.class.path or classes specified at -classpath .

Spring Interview Question

Most Common Spring Interview Questions : This blog is just for a revision .

• What is dependency injection ?

Spring IoC container takes care of object creation and wiring them together.

1. Constructor constructor-arg : for mandatory dependencies
2. Setter ref : for optional dependencies
3. using @Autowired

• Types of IoC container?

1. BeanFactory
2. ApplicationContext

• What are the advantages of ApplicationContext?

1. Internationalization
2. Annotation support
3. Load all beans on start 
4. lazy-init for lazy loading
5. No need to register AutowiredBeanPostProcessor
6. Automatic BeanPostProcessor , BeanFactoryPostProcessor registration
7. Easy MessageSource access (for i18n)
8. ApplicationEvent publication

• Advantages of spring IoC ?

1. Less code
2. Easy testing
3. Loose coupling
4. Lazy loading

• What is spring bean ?

java class created by spring IoC container .

• Explain Spring bean life cycle.

1. Check the configuration
2. Initialization
3. Dependency Injection
4. set bean name
5. set bean class loader
6. set bean factory
7. set resource loader
8. set application event publisher
9. set message source 
10. set Application Context
11. set Servlet context
12. postProcessBeforeInitialization
13. afterPropertiesSet
14. init method
15. postProcessAfterInitialization
16. destroy
17. destroy

• Explain scope of bean .

1. singleton : default , one instance in the container 
2. prototype : new instance every time the getBean is called
3. request : one bean for life cycle of http request 
4. session : one bean for life cycle of http session
5. global session : one bean for life cycle of ServletContext

• What is BeanPostProcessor ?

implementation can define a callback method .

1. postProcessBeforeInitialization  before init method
2. postProcessAfterInitialization after init method

• How to create ApplicationContext ?

1. XML based :ClassPathXmlApplicationContext("xml file")
2. Annotation based : in web-servlet.xml use  <context:component-scan base-package= > .
3. Java based :  @Configuration ,  @Bean , @Scope, @Import , @ComponentScan

• Explain  JSR 250  annotations used in spring .

1. @PostConstruct : alternative to init 
2. @PreDestroy : alternative to destroy
3. @Resource(name ="")   : similar to @Autowired @Qualifier("") 

• Explain other annotations in spring to be used with annotation based configuration

1. @Required:applied on setter , the value should be provided in xml 
2. @Component parent of annotations
• @Controller , 
• @Repository , 
• @Service 

•  Explain Autowiring .

Enables us to inject dependency implicitly, using autowire in xml

1. no : explicit bean references 
2. byName : by the bean name in xml file.
3. byType : matches the type of the bean 
4. contructor : check the type of the parameter in constructor 

• Explain inner bean 

A bean used as property of another bean . A <bean> inside <property> .

• What are the new features in Spring 4 .

1. Added path to RequestMapping
2. MVC test framework
3. @RestController and AsyncRestTemplate is used.
4. instread of response it returns ListenableFuture 
5. uses java 8 , minimum requirement is Java 6.
6. define external bean configuration using a Groovy
7. Added @Description
8. @Lazy can be used in bean injection and definition .
9. @Autowired @Value can be used as meta annotations
10. Introduced ordering for autowired , using @Order and Ordered interface .
11. @Bean in @Configuration Class
12. Conditional filtering of beans added for @Conditional .
13. Generic type as qualifier for injecting bean
14. create custom annotation to expose attribute of the source annotation .

• New features added in Spring 5.

1. bean scopes : singleton, prototype , request , session , application , websocket (one bean for lifecycle of Websocket )

• Explain the components of Spring MVC.

1. Dispatcher Servlet
2. Handler Mapping
3. Controller
4. Model And View
5. View Resolver
6. View

• Explain Dispatcher Servlet

Front Controller for all incoming Request.

entry added in web.xml for dispatcher servlet

• org.springframework.web.servlet.DispatcherServlet
• loadOnStartUp = 1

Create {servlet-name}-servlet.xml,specify

• context:component-scan
• mvc:annotation-driven

• What is handler mapping

Dispatcher servlet checks with handler mapping and  redirects to the appropriate controller .

• What is view resolver ?

It determines the view to which the request has to be transferred .

1. InternalResourceViewResolver : based on URL , suffix and prifix
2. XmlViewResolver : view definition in  a dedicated XML .
3. ResourceBundleViewResolver : view definition in a property file .

• What is ServletContext ?

One per application , created once the web application is deployed .

To access in spring context , the controller must implement ServletContextAware   ,add either of the following code  
1. 

@Autowired

ServletContext context; 

2.
private ServletContext context;

public void setServletContext(ServletContext servletContext) {
    this.context = servletContext;
}

Methods :

1. getInitParameter () : returns value set in <context-param> <param-name>

• What is servletConfig?

One per servlet

1. getInitParameter(String name): returns value set in <init-param><name>  
2. getInitParameterNames(): returns all <init-param>
3. getServletContext(): returns ServletContext
4. getServletName(): returns value set in <servlet><servlet-name>

• What is servlet context listener ?

gets notified when servlet context is initialized or destroyed .

• Explain  Spring MVC annotations?

1. @Controller : class is a web request handler 
2. @Component : auto-detection when using annotation-based 
3.  @RestController = @Controller + @ResponseBody
4. @RequestMapping method: get/put value : URL
5. @PathVariable input in request URL.
6. @RequestParam input in url with ?id=
7. @Transactional : can be applied to class or method.
8. @EnableWebMvc enable spring MVC through java configuration.

• What is WebMvcConfigurer .

To customise web MVC configuration.
Like add view controller
View resolver in java code .

• What is WebApplicationInitializer

used to configure servlet context programmatically , used as an alternative to configuration in web.xml

• Explain file upload in spring 

1. use CommonsMultipartResolver add dependency commons-fileupload
2. add bean configuration in servlet-config.xml file 
• "multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver"
3. input type = file in html
4. @RequestParam MultipartFile

• What is a profile

If we want enable a bean only during development and not in production we can assign the profile to the bean , using @Profile("dev")

• How to declare a profile ?

1. Set the servlet context init Param using WebApplicationInitializer
2. Set context Param in web.xml
3. Set the JVM system parameter
4. Set the environment variable
5. Set in maven profile
6.  Configurable environment set active profile

• Explain spring events 

1. event class should extend ApplicationEvent
2. publish event can be done using applicationEventPublisher.publishEvent(customSpringEvent);
3. listener has to implement ApplicationListener<MyEvent>
4. listener need to override onApplicationEvent(MyEvent myEvent)

• What are the events related to ApplicationContext

1. ContextRefreshedEvent
2. ContextStartedEvent
3. ContextStoppedEvent
4. ContextClosedEvent

• What is Join Point ?

An event occurred during the execution of a program.

1. method called 
2. state changed
3. exception occurred 

• What is an Point Cut ?

an expression

• What is an advice ?

Action taken (method ).

1. before
2. after
3. around : decides whether the method gets executed or not
4. afterReturning :executed only if method executed successfully .
5. afterThrowing : executed only if exception is occurred during method execution .

• What is aspect ?

class that implements cross cutting concerns .

1. logging
2. security
3. transaction management
4. exception handling
5. performance testing

• What is weaving?

Linking aspect with other objects/application types to create adviced object.

• What is proxy?

Object created after applying advice to target object.

• What are the features of spring boot?

1. Auto configuration : configure beans based on the jar available at classpath  and beans used in the project . Annotations used : @ConditionalOnClass, @ConditionalOnMissingBean .
2. Starters project
3. Cli using groovy
4. Actuator
5. Initializer

• Explain Spring Boot Annotations

1. @SpringBootApplication 
2. @EnableAutoConfiguration

• What is Spring Boot Initilizr ?

It is used to generate a new project .

• What is Spring Boot Actuator

adds production grade services to the application

1. info
2. health : up
3. trace
4. httptrace

• How do you create Logger in Spring boot .

org.slf4j.Logger logger = org.slf4j.LoggerFactory.getLogger(MyClass.class);

configure logger level , file path , pattern console profile in application.properties

create logback.xml

Angular interview Questions

Most Common Angular Interview Questions :

• What is Angular  ?

Client Side Framework based on node js and typescript

• What are the building blocks of Angular ?

Component , Template , Service , Dependency Injection , Data binding , Routing , Module

• How does Angular Application starts ?

main.ts -> platformBrowserDynamic().bootstrapModule(AppModule) -> AppModule

• What is a component ?

Basic building block , it has template which is loaded on using the selector .

• What is a template ?

it is the html part of component .
Two types are there
1. within the component.ts enclosed in back tick
2. in a separate html file .

• How to style a component ?

Three ways : 1. inside the component.ts using style
2. in a separate css using styleUrl
3 . multiple css files can be included using styles [] .

• What is providers for component ?

It specifies the Services to be used in the component .

• Explain model to view binding ?

Two ways : {{}} Interpolation : variables defined in component can be used in template .
[] property binding : a known property of html element is bound to a variable defined in component .

• Explain view to model binding ?

using () : event binding : binds an event to method defined in component .

• Explain two way binding ?

using [()] : it is a combination of event binding and property binding , it synchronizes the data between model and view . 
eg : <input [(ngModel)] = "myInput" > <span >you entered {{myInput}} </span>

• What is a module ?

Module groups related components , services , directives and pipes .
4 Parts :
1. declarations [] : list of components , pipes and directives
2. imports [] : list of modules , used to import routes
3. providers [] : list of services
4. bootstrap : the root component that Angular creates and inserts into the index.html.

• What is a root module ?

The module loaded on start of the app . This imports BrowserModule  . Only one per app .

• What are directives ?

Add to the DOM .

3 Types :
Components :directives with template.
Structural :  add / remove DOM elements. *ng-if , *ng-for
Attribute  : change the appearance or behavior of an element, component, or another directive. Eg : ngClass

• How to create custom attribute directive ?

using @Directive , in the constructor we can access the native element from ElementRef.nativeElement , using which we can manipulate the DOM.
we can also use @Input to get the input in the specified property .

• How to create custom structural directive ?

using @Directive , inject ViewContainerRef and TemplateRef , in ngOnChanges()  using this.container.createEmbeddedView  .

• How to pass data from parent and child component ?

Using @Input on a property in child component and pass the data to property in while it is called in parent component .

• How to pass data from child to parent component ?

1.  Using @ViewChild(Component class) add to a variable of Child component in parent  component  
2.  Using @Output and EventEmitter .eg :  @Output() voted = new EventEmitter<boolean>();            this.voted.emit(agreed);

Note: EventEmitter is subclass of Observable .

• How to pass data between unrelated component ?

Using a service , create a variable as BehaviorSubject type , another variable as observable for the variable  and a method to change the value . Subscribe the observable in both components , if value is changed from one component it will be reflected in other component .

• What is LifeCycle hook?

For a component Angular creates it, renders it, creates and renders its children, checks it when its data-bound properties change, and destroys it before removing it from the DOM. 

1. ngOnChanges()
2. ngOnInit()
3. ngDoCheck()
4. ngAfterContentInit()
5. ngAfterContentChecked()
6. ngAfterViewInit()
7. ngAfterViewChecked()
8. ngOnDestroy() 

Before these methods the constructor is called .

• What is a service ?

It is used to perform a task , like http put or get .

• What is dependency Injection ?

Angular creates instance of all  the services and makes it available for the component . Services are singleton .

1. constructor injection :dependencies are made available through constructor 
2. property injection :directly setting the object property .

• What is @optional

For optional dependencies , injector will inject null if dependencies are not found.

• What is injector?

resolves token into dependency .

• Explain   Hierarchical injector.

tree of injectors that parallels an app's component tree

• What is a provider ?

is an instruction to the DI system on how to obtain a value for a dependency .

• What is view provider

does not allow content be injected in the projected content

• How to create a service ?

using @Injectible

• When to use @providedin : root ?

when service is used anyway , if service is rarely used specify the service in provider [] of component .providedIn can also hold any module other than root .

• What is routing?

Loading of components based on URL .

• How to add routing to an angular app?

Create Routes[] in module , in imports add RouterModule.forRoot(appRoutes).Add router-outlet where the component should be loaded.

• What is RouterLink directive ?

generates hyperlink that can be embedded in anchor tags for navigation .

• What is redirectTo ?

It is used to match a wildcard and redirect to a different URL . The component associated with the redirected URL will be loaded .

• What is useHash:true

enables hash(#) based routing .

• What is Routing module?

AppRoutingModule is top level module dedicated for routing .In the ng module exports: [ RouterModule ]

• What is a feature module ?

Contains a group of components, directives pipes  with similar features  and imported in root module .

• What is lazy routing ?

Root module should be split into feature module and a routing module . Feature modules can have its own routing module . RouterModule.forRoot() is used in the app Routing Module and RouterModule.forChild() is used in the Routing Module of feature module . In the app routing module loadChildren is used instead of component .

• What is route Guards ?

tells the router whether or not to navigate to the requested route 

CanActivate

CanActivateChild

CanDeactivate

CanLoad

Resolve

• What is form ?

Handles user data . 2 Types :

1. Template driven :
2. Model Driven /Reactive

• Explain Template driven form .

Used for the following :

1.  two-way data binding : ngModel , ngModelGroup
2.  change tracking: pristine, dirty , valid 
3.  validation : required  , pattern , minlength 
4.  error handling  *ngIf

• Explain Model Driven / reactive Forms .

Using ReactiveFormsModule
1. FormGroup FormControl
2. FormBuilder
3. FormArray

• How to create a custom validator ?

Create a typescript class with a method
Add it in formbuilder . Group method for the name

• What is shadow DOM?

provides hidden area on the page for scripts HTML and CSS . Markup and styles in this area will not affect the other part of the page also will not be affected by other  part .

• What is view encapsulation ?

Added to a component using encapsulation attribute .

1. ViewEncapsulation.Native : lets angular to use the shadow DOM . isolates the component , default styles will not be applied to this component . 
2. ViewEncapsulation.Emulated : applied by  default 
3. ViewEncapsulation.None : the styles defined for this component is applied to all other component .

• Diffentiate observable and promise.

1. Promise is executed immediately, observable is lazy and executes only if it is subscribed.
2. Observable can be cancelled, promise can't.
3. Promise handles single event , observable is a stream and allows multiple events.

• What is async pipe?

It allows to bind with the observable directly in the template.
The advantage of using AsyncPipe is that we don’t need to call subscribe on our observable and store the intermediate data on our component.

• How to create a promise .

Promise handles single event when async operation completes or fails
let promise = new Promise((resolve, reject) => {

    //http call 

  });

return promise;

•  What is transpilation ?

Convert from one type of source code to another type of source code .

1. Build time : takes the type script files and compiles it to javascript files 
2. Runtime : happens at the browser at runtime . Typescript compiles is loaded in the browser which compiles the typescript files on the fly .

• What is webpack?

A tool to bundle application source code and load that code from server to browser .

• What are the special characters in angular ?

1. [] : property binding 
2. {{}} : interpolation
3. () : event binding
4. [()] : two way binding 
5. | : pipe
6. * : structural directive
7. ?  : safe navigation 
8. # : template reference variable 

• Explain folders in angular project ?

1. e2e : contain end to end tests
2. src : application code
3. assets : static content like images .
4. environments : configuration for developer and production environments .

• Explain the important files in Angular .

1. package.json
2. angular.json
3. main.ts
4. tslint.json
5. tsconfig.app.json
6. proxyconfig.json

• How do you change the path of node_modules?
• What are the symbols ~ and ^ in node modules 
• How do you build your application for production

ng build --prod
the bundles will be generated in dist folder

• What are polyfils

Polyfil scripts should be loaded for angular application to work on the supported browser .

• Explain angular binding Construct .

1. Interpolation
2. property binding
3. attribute binding
4. class binding
5. style binding
6. event binding

• What is XHR ?

XML Http Request built in object in browser to request data from server .

• How does angular safegaurd the app against XSS attacks ?

Cross side scripting is injecting malicious code in web pages gain access and steal data .Sanitization : Inspection of untrusted value and turing it value that is safe to insert in DOM. Security Constructs are as below

1. HTML 
2. style
3. URL
4. resource URL

• What is DomSanitizer ?

helps preventing the cross side .

1. Sanitize :
2. bypassSecurityTrustHtml
3. bypassSecurityTrustScript
4. bypassSecurityTrustStyle
5. bypassSecurityTrustUrl
6. bypassSecurityTrustResourceUrl

• How does hot reloading works internally?

ERROR Error: Found the synthetic property @transitionMessages.

Hi Guys ,

Today I have faced this error :

ERROR Error: Found the synthetic property @transitionMessages.

The fix is to add the following modules to your ng module file .

------------app.module.ts---------------------------------------------------------

import { BrowserModule } from '@angular/platform-browser';

import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';

import { HttpModule } from '@angular/http';

import {MatSelectModule} from '@angular/material/select';

import { BrowserAnimationsModule } from '@angular/platform-browser/animations';

const appRoutes:Routes =[

{path:'cust-detail',component: CustDetailComponent},

{path:'mcq',component: MCQComponent},

{path:'sign-up',component: SignUpComponent}

];

@NgModule({

  declarations: [

    AppComponent,

    CustDetailComponent,

    MCQComponent,

    SignUpComponent

  ],

  imports: [

    BrowserModule,

FormsModule,

MatSelectModule,

BrowserAnimationsModule

  ],

  providers: [QuizServiceService],

  bootstrap: [AppComponent]

})

export class AppModule { }

----------------------------------------------------------------------------------------

This error comes if you are using mat-select and mat-option . This is required module so we need to import it to make the select functionality work .

The full stack trace is as below :

ERROR Error: Found the synthetic property @transitionMessages. Please include either "BrowserAnimationsModule" or "NoopAnimationsModule" in your application.

    at checkNoSyntheticProp (platform-browser.js:1141)

    at DefaultDomRenderer2.push../node_modules/@angular/platform-browser/fesm5/platform-browser.js.DefaultDomRenderer2.setProperty (platform-browser.js:1125)

    at DebugRenderer2.push../node_modules/@angular/core/fesm5/core.js.DebugRenderer2.setProperty (core.js:11505)

    at setElementProperty (core.js:8261)

    at checkAndUpdateElementValue (core.js:8212)

    at checkAndUpdateElementInline (core.js:8159)

    at checkAndUpdateNodeInline (core.js:10503)

    at checkAndUpdateNode (core.js:10469)

    at debugCheckAndUpdateNode (core.js:11102)

    at debugCheckRenderNodeFn (core.js:11088)

View_MatFormField_10 @ MatFormField.html:1

push../node_modules/@angular/core/fesm5/core.js.DebugContext_.logError @ core.js:11306

push../node_modules/@angular/core/fesm5/core.js.ErrorHandler.handleError @ core.js:1719

(anonymous) @ core.js:4578

push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke @ zone.js:391

push../node_modules/zone.js/dist/zone.js.Zone.run @ zone.js:150

push../node_modules/@angular/core/fesm5/core.js.NgZone.runOutsideAngular @ core.js:3779

push../node_modules/@angular/core/fesm5/core.js.ApplicationRef.tick @ core.js:4578

(anonymous) @ core.js:4462

push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke @ zone.js:391

onInvoke @ core.js:3820

push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke @ zone.js:390

push../node_modules/zone.js/dist/zone.js.Zone.run @ zone.js:150

push../node_modules/@angular/core/fesm5/core.js.NgZone.run @ core.js:3734

next @ core.js:4462

schedulerFn @ core.js:3551

push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.__tryOrUnsub @ Subscriber.js:196

push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.next @ Subscriber.js:134

push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber._next @ Subscriber.js:77

push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next @ Subscriber.js:54

push../node_modules/rxjs/_esm5/internal/Subject.js.Subject.next @ Subject.js:47

push../node_modules/@angular/core/fesm5/core.js.EventEmitter.emit @ core.js:3535

checkStable @ core.js:3789

onHasTask @ core.js:3833

push../node_modules/zone.js/dist/zone.js.ZoneDelegate.hasTask @ zone.js:443

push../node_modules/zone.js/dist/zone.js.ZoneDelegate._updateTaskCount @ zone.js:463

push../node_modules/zone.js/dist/zone.js.Zone._updateTaskCount @ zone.js:291

push../node_modules/zone.js/dist/zone.js.Zone.runTask @ zone.js:212

drainMicroTaskQueue @ zone.js:601

Promise.then (async)

scheduleMicroTask @ zone.js:584

push../node_modules/zone.js/dist/zone.js.ZoneDelegate.scheduleTask @ zone.js:413

push../node_modules/zone.js/dist/zone.js.Zone.scheduleTask @ zone.js:238

push../node_modules/zone.js/dist/zone.js.Zone.scheduleMicroTask @ zone.js:258

scheduleResolveOrReject @ zone.js:879

ZoneAwarePromise.then @ zone.js:1012

push../node_modules/@angular/core/fesm5/core.js.PlatformRef.bootstrapModule @ core.js:4345

./src/main.ts @ main.ts:11

__webpack_require__ @ bootstrap:76

0 @ main.ts:12

__webpack_require__ @ bootstrap:76

checkDeferredModules @ bootstrap:43

webpackJsonpCallback @ bootstrap:30

(anonymous) @ main.js:1