HashMap vs HashTable
HashMap vs HashTable
HashMap | HashTable |
| Not synchronized | Synchronized |
| Allows one null key and many null values | Doesn't allow null values |
No order
| No order |
| No duplicates | No duplicates |
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ArrayList vs LinkedList vs Vector
June 27, 2021 Ravi Yasas
ArrayList vs LinkedList vs Vector
ArrayList | LinkedList | Vector | |
| Data structure | Index-based dynamic array | Doubly linked list | Growable array |
| Increment size | 50% | No initial size | 100% |
| Traverse | Uses iterator | Uses iterator | Uses enumeration |
| Memory usage | Less memory usage | More memory usage | |
| Accessibility | Random and fast | Sequential and slow | Random and fast |
| Order | Insertion order | Insertion order | Insertion order |
| Duplicates | Allow | Allow | Allow |
| Insert / Delete | Slow | Fast | Slow |
| Synchronized | No | No | Yes |
| Implements | RandomAccess interface | NA | RandomAccess interface & Serializable interface |
| Null values | Yes | Yes | Yes |
array / arraylist / beginner / collection framework / collections / devloper / typesafety / vector
Array vs ArrayList
June 27, 2021 Ravi Yasas
Array | ArrayList |
| Fixed-size | Size is not fixed |
| Not type-safe | Type-safe |
| Allow both primitives and objects | Doesn't allow primitives. But after Java 5 auto-boxing will convert primitives to objects |
Type safety means the compiler will validate the types while compiling and throw an error if you do anything wrong.
array / arraylist / beginner / collection framework / collections / devloper / typesafety
What is REST
June 26, 2021 Ravi Yasas
REST
Introduction
- Stands for Representational State Transfer.
- REST is a web standards-based architecture that uses the HTTP protocol (port 80) for data communication.
- Uses HTTP methods for data communication
- REST server simply provides access to resources and client access and presents the resource.
- REST is stateless, so there is no session.
- REST uses various representations like TXT, JSON, XML...etc to represent resources.
Methods in REST
- POST
- Sends data to the server for creating a new resource, maybe an entity. Often used when uploading a file or submitting a web form.
- GET
- Retrieves data from the server. Should have no other effect.
- PUT
- Similar to POST, but used to replace an existing entity.
- PATCH
- Similar to PUT, but used to update only certain fields within an existing entity.
- DELETE
- Removes data from the server.
- TRACE
- Provides a way to test what the server receives. It simply returns what was sent.
- OPTIONS
- This allows a client to get information about the request methods supported by a service.
- The relevant response header is Allow with supported methods.
- HEAD
- This returns only the response headers.
- CONNECT
- Used by browser when it knows it talks to a proxy and the final URI begins with https://.
- The intent of CONNECT is to allow end-to-end encrypted TLS sessions, so the data is unreadable to a proxy.
Directives in Angular 11
June 26, 2021 Ravi Yasas
Directives
- Directives are instructions in the DOM
- It adds additional behaviors to the elements
- Using inbuilt directives we can manage forms, lists, styles, and what users see.
- There are a few types
- Component directives
- Attribute directives
- Structural directives
Component directives
- These directives are used in the main class.
- Declared by @Component
@Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css']
})
Attribute directives
- Listen and modify the behavior of other HTML elements, attributes, properties, and components
- NgClass
- This allows to set the CSS classes dynamically.
- NgStyles
- This directive lets you to set a given DOM element's style properties.
<div [ngStyle]="{'background-color':'green'}"></<div>
- Instead of this you can use following too.
[style.color]="getColor(person.country)"
- NgModel
- This bind inputs, select, text areas...etc and store the required user value in a variable.
- Add two-way data binding
- This allows to create new directives too.
Structural directives
- These directives manipulate the DOM element or change the structure of the DOM element
- These directives begin with *
- Ex: *ngIf, *ngFor
<ul *ngFor="let age of ages">
<li *ngIf="age > 20">
{{age}}
</li>
</ul>
angular / angular 11 / angular tutorial / attribute / beginners / component / directives / learning / structural / tutorial
Access token vs refresh token
June 26, 2021 Ravi Yasas
Access token vs refresh token
Access token
- This is short-lived
- Send API request, with the access token
- If the access token is invalid, fail and ask the user to re-authenticate
- There are few types of access tokens
- Bearer tokens
- JWT tokens
- Opaque token
Refresh token
- These tokens are long-lived
- Refresh tokens are used to retrieve access tokens
- If the access token is invalid, try to update it using the refresh token
- If the refresh request passes, update the access token and re-send the initial API request
- If the refresh request fails, ask the user to re-authenticate
States of an object in Hibernate
June 26, 2021 Ravi Yasas
Hibernate object states
There are three states- Transient
- If the object just created, no primary key and not associate with a session, then the object is in the transient state
- Persistent
- If a session is open, the object is in the persistent state
- Detached
- If a session is closed, the object is in the detached state
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