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DavidApril 12, 2021 at 8:46 PM
This will be keep updated as I study along
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David: Hello, I'm a passionate student studying backend developing. I love algorithm, operating system and system design.

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Popular posts from this blog

Structures of JAVA

JAVA Features of Java Everything is a class in Java - Java is a pure Object Oriented Language The way how Java executes in the first place(without creating an object) is by defining a static function (usually we call it main) that will run even without creating an object. A package in java is a collections of classes Java is interpreted language (compiler + interpreter) Java can run on many different operating systems without modifying the source code This is why Java is called platform independent language unlike C/C++ Java is platform independent thanks to the JVM(Java Virtual Machine) Java generates .class files (C/C++ will generate .exe files) Java has garbage collection which handles automatic memory allocation and de-allocation by counting reference to an object. There are 3 interrelated components for developing and running Java programs JVM JRE JVM https://www.javatpoint.com/difference-between-jdk-jre-and-jv...

What is URI(URN, URL)

URI (Uniform Resource Identifier) URI is an identifier of a specific resource. Like a page, or book, or a document. For example, if you request a specific resource via https://www.google.com/resource/sample/1, you will retrieve resources such as sample.pdf, sample.docx etc. URL and URN are sub-types of URI. URL (Uniform Resource Locator) URL is a special type of identifier which tells you the location of specific resource on the server. If the location of resource changes in the server, you will not be able to access the resource via the previous URL URN (Uniform Resource Name) URN is the name of specific resource. Unlike the URL, even if the location of the resource changes on the server, you will still be able to access the resource using URN because URN points to the specified resource. ...

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Spring Boot JPA - What is JPA, Entity, Repository

Database Spring Boot uses JPA (Java Persistence API) to map Java objects to tables in the database. Mapping an object to a table in a database is called ORM(Object Relational Mapping) and JPA is one of Java's way of achieving ORM. JPA is not dependent on specified database language, so that users can associate many different databases using JPA. JPA Data persistence is the ability to maintain data between application executions. Persistence is vital for applications that are developed for enterprises. JPA is a collection of classes and methods to persistently store the vast amounts of data into a database which is provided by the Oracle corporation. To reduce the burden of writing codes for relational object management, a programmer follows the JPA framework, which allows easy interaction with the database instance. JPA standardizes the task of object-relational mapping by using annotations or XML to map objects into one or ...

Java Data Structures VS Python, C/C++ Data Structures

Data Structures (JAVA, PYTHON, C/C++) Arrays Arrays(fixed size) As for python, we use list to store series of data, and by default it's size can be dynamically adjusted. But arrays in JAVA and C/C++ have fixed size. Arrays(dynamic size) We can use dynamically adjustable arrays by using importing java.util.ArrayList in Java. In C/C++, we can #include <vectors>. Linked List In java.util.LinkedList, we can use LinkedList data structure (which is doubly linked list). In python doesn't have built in linked list data type. We will have to make it by ourselves (and it is pretty easy) In C/C++, we can use list to implement doubly linked list. Stack There is built libraries for stack in Java and C/C++. But there is no stack for python. We can just use the list for stack operation. ...

Design Pattern - Proxy

Proxy Definition A Proxy pattern is a software design that works as an interface to something else. The proxy can interface to anything, a network connection, a large object in memory, a file, or some other resource that is expensive or impossible to duplicate. In short, a proxy is a wrapper or agent object that is being called by the client to access the real serving object behind the scenes. Proxy can add extra functionality like caching. Types of Proxy pattern Remote proxy : Responsible for representing the object located remotely. Talking to real object might involve marshalling and unmarshalling of data and talking to the remote object. All the logic is encapsulated in these proxies and the client application need not worry about them. Virtual proxy : These proxies will provide some default and instant results if the real object is supposed to take some time to produce results. These proxies initi...

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What is REST API? REST is an abbreviation for Representational State Transfer. It is an way to transfer data's state. An API is an interface through which one program or web site talks to another. They are used to share data and services, and they come in many different formats and types. In short, a RESTful API is one of the many possible ways that programs, servers and web sites can share data and services. REST describes the general rules for how the data and services are represented through the API so that other programs will be able to correctly request and receive the data and services that an API provides. RESTful API uses HTTP URI to specify resources, and uses HTTP methods to apply CRUD operations. What is URI ?? Methods of RESTful API GET : retrieve a resource POST : create the resource PUT : change the state of or update a resource DELETE : remove a resource RESTful API Design and Architecture Constraint Use of a uniform i...

Design Pattern - Adapter

Adapter Definition Adapter pattern is a software design pattern (also known as wrapper, an alternative naming shared with the decorator pattern) that allows the interface of an existing class to be used as another interface. It is often used to make existing classes work with others without modifying their source code. How Adapter works Let's look at simple example where we have a hair dryer that can fit to 110V and a cleaner that can fit to 220V. What if we only have 110V port and want to use cleaner? We can use an adapter that can convert 110V to 220V. Adapter with code REFERENCE https://en.wikipedia.org/wiki/Adapter_pattern

Design Pattern - Singleton

Singleton Definition Singleton pattern is a software design pattern that restricts the instantiation of a class to one single instance. This is useful when exactly one object is needed to coordinate actions across the system. Common usage of Singleton pattern Single database object shared by different parts of the program Logging object. Replacement of global variables, since Singleton instances doesn't pollute namespace and has lazy initialization attribute. Singleton with code We will define a single Singleton instance that will be instantiated statically. We will prevent Singleton instance from being created by default constructor by private access modifier. That way, there is no way to create singleton instance using new keyword. There must be a way to retrieve Singleton instance. We will create a static method getInstance(), that will enable us to ...