DEV Community: Aravind

Spring - H2, JPA starter ->

Aravind — Wed, 09 Aug 2023 05:09:54 +0000

1. Add H2, JPA dependencies

In case you are using gradle you can add the following to your build.gradle

implementation 'org.springframework.boot:spring-boot-starter-data-jpa'
runtimeOnly 'com.h2database:h2'

2. H2 jpa configration

set the jpa and H2 configs in

application.properties

spring.h2.console.enabled=true
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
spring.jpa.hibernate.ddl-auto=update
spring.datasource.driverClassName=org.h2.Driver

spring.datasource.url=jdbc:h2:file:./data/dcbappA
spring.datasource.username=user
spring.datasource.password=

3. Creating Entity

This entity will be added as a new table in the database.

model > Customer.java

@Entity
@AllArgsConstructor
@NoArgsConstructor
@Data
@Builder
public class Customer {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String name;
    private String Address;
    private Long phone;

}

4. Creating repository

Repository extendss the JPA to provide prebuild queries.

repository > CustomerRepository

@Repository
public interface CustomerRepository  extends JpaRepository<Customer,Long> {

}

Java recap - Espresso Guide ☕️

Aravind — Wed, 26 Jul 2023 11:28:42 +0000

Hi there,

This post features the basic concepts with dumb examples for quick reference.

1. How to print ?
2. How to get user input ?
3. How write a class ?
4. Wrapper Classes
5. Inheritance
6. Method overriding
7. Final and Static
8. interface
9. Abstraction
10. Exception handling
11. File handling

How to print ?

public class Main {   
   public static void main(String[] args) {   
      System.out.println("Hello world");  
   }   
}

you can also use

System.out.print("text"); - prints with out newline at end )
System.out.printf("%d text" , 55); - inserts value in the printed message

System.out.printf("%d text" , 55); //replaces %d with 55
//similarly for char,String,boolean we have
System.out.printf("%c text" , 55);
System.out.printf("%s text" , "hi");
System.out.printf("%b text" , true);

How to get user input ?

import java.util.Scanner;  // Import the Scanner class

class Main {
  public static void main(String[] args) {
   Scanner myObj = new Scanner(System.in);  // Create a Scanner object
   String input = myObj.nextLine();  // Read user input
  }
}

How write a class ?

A class is a "blueprint" for creating objects

class Car {
   int noOfWheel  = 6;
}

We can create an object of the class by

public static void main(String[] args){
   Car bmw = new Car();
}

Classes with constructor

public class Car {
   // Instance Variables
   String name;
   int year;
   String color;

   // Constructor 
   public Car(String name,int year,String color){
      this.name = name;
      this.year = year;
      this.color = color;
   }
}

Now the object can be created as

public static void main(String[] args)
   {
      Car bmw = new Car("bmw", 2022,"blue");       
   }

Wrapper Classes

Wrapper classes are reference of primitive datatypes.
Integer is the wrapper class of primitive int.
Boolean - boolean
Character - char, etc

autoboxing - automatic converison of primitive to wrapper
Integer a = 123;
unboxing - conversion of wrapper to primitive
if(a == 123){} // here the java unboxes the primitive to compare the values

Inheritance

Inheritance allows one object acquires all the properties and behaviors of a parent object.

class Worker{  
 float salary=40000;  
}  
class MailMan extends Worker{  
 int bonus=10000;  
 public static void main(String args[]){  
   MailMan m=new Mailman();  //m will have all characters of Worker class in addition to its own properties.
}  
}

Method overriding

Overriding the method of superclass in subclass

class Vehicle{  
  void run(){System.out.println("Vehicle is running");}  //method in parent
}  

class Bike extends Vehicle{  
  void run(){System.out.println("bike is running");} // method in child
}

Final and Static

Final classes cannot be inherited.
Final variable cannot be modified.
The static used to share the same variable or method of a given class. It can be accessed without an object instance.

class Student{  
   String name;  
   static String college ="abc";//static variable   
}  

public class Test{  
 public static void main(String args[]){  

   Student.college = "efgh"; // the static variable can be accessed without an object. it also changes the particular variable for all objects of the class.

   Student s1 = new Student();
   s1.name = "rahul"  //object must be created to access non-static variable
 }  
}

interface

It specifies what a class has or must do.
Inhereted classes can have only one superclass while interface allows to implement methods from different interfaces.

interface sayHello{  
   void hello();  
} 
interface sayHi{  
   void hi();  
}   

public class say implements sayHello , sayHi {
   @override
   void hello(){ // something };
   @override
   void hello(){ // something };
}

Abstraction

The abstract keyword can be used with a class or method.
Abstract classes cannot have an object, but their subclasses can.

abstract class Bike{  
  void run();  
}

Abstract methods specifies the method name without the method body. These methods must be ovveriden in the subclass.

class Bike{  
  abstract void run();  //no method body
}

class Honda extends Bike{  
   void run(){
     System.out.println("running safely"); // override
   }   
}

Exception handling

Exception handling helps to avoid program intreption by catching errors.

public class test{  
  public static void main(String args[]){  
   try{  
      //code that may raise exception  
   }catch(ArithmeticException e){
      //to do - in incase of exception
   }finaly{
     //to do - finaly  
  }  
}

`Exeception e` catches all exeptions but,its good practice to handle each exception individually. `e.printStackTrace` prints the error details.

File handling

File class can be used to create a file abstraction and FileReader will help with reading from files.

import java.io.File;
import java.io.FileReader;
public class file {
    public static void main(String[] args) {
        File f=new File("new.txt");
        try{
        if(f.exists())
        {
            FileReader fr=new FileReader("new.txt");
            int var=0;
            var=fr.read();
            while(var!=-1){
                System.out.println((char) var);
            var=fr.read();
            }
        }
        }
        catch(Exception e){
            System.out.println("Exception catched");

        }
    }
}

mysql Sequelize-cli in Node JS- beginers guide.

Aravind — Tue, 01 Feb 2022 22:16:59 +0000

Hi there amigos,

This post is intended to be a reference for using mysql - sequilize with nodeJS. We will be using sequelize cli for ease and a consistant project-structure. Express is used to demonstrate the working. _ yea its is gona be a silly demonstration ;) _

Fire up your ide, lets get started....

github repo

Setting up the node app.

run npm init to initiate node project in your folder.
Installing packages - just a couple of them
- run npm install sequelize mysql2 express
- run npm install nodemon if you don't have it already installed.

This will install all the necessery packages except the sequelize-cli.

Sequelize-cli

Sequelize-cli is used to create databases, generate models, migrate dbs, seed files etc.

Even if you are not doing any sort of migrations or seeding, sequelize-cli makes it much more easier to setup sequalize, helps to maintain a project structure (crucial as the app gets bigger) and its simply more fun to use.

You can install sequelize-cli
- globally using npm install -g sequelize-cli or
- as dev dependency by using npm install --save-dev sequelize-cli

Setting database using cli

Now that we have installed sequelize-cli *run sequelize init this will create some folders in project. For now we don't have to worry about migration and seeder, so after this you will end up with something like this ..

The config.json contains all the database configrations.
Change the development configrations to match your database, in my case it is ..

Now back to cli- (before that make sure your database is up and running, I am using phpmyadmin here)

lets start by creating the database
- run sequelize db:create - this will create the db specified in the config.json
creating a User model
- run sequelize model:generate --name User --attributes name:string

This will create a user.js with a ready to use model inside the models folder.

Finishing up

Basically thats it, now you have a model User with one column name of type string.

Modifying the model

currently your User model will look like .

You can add more atributes to User by changing User.init to

  User.init({
    name: {
      type: DataTypes.STRING,
      allowNull:false,
      validate:{
        notEmpty : true
      }
    }

  }, {
    sequelize,
    modelName: 'User',
  });

Also you can add more columns just by copy pasting the same for example.

Connecting DB (assuming that you are familiar with node)

you can create an app.js and use the model that we created.

const express = require ("express");
const app = express();
const db = require("./models");

// you can add more models like {User,Blog,..}
const { User } = require('./models')

app.get('/all',( req,res)=>{
    User.findAll()
        .then(result =>  res.send(result))  
        .catch(err => console.log(err));

});

app.post('/insert',( req,res)=>{
    User.create({
        name:"james",
        phone: 1234567890
    })
    .then(result => res.send(result))
    .catch(err =>console.log(err))
});

app.delete('/delete',( req,res)=>{
    User.destroy( {where:{id:2}})
    .then(result =>console.log(result))
    .catch(err => console.log(err))
});

app.post('/update',( req,res)=>{
    User.update(
        {phone:85487945211},
        {where:{id:1}}
        )
    .then(result =>console.log(result))
    .catch(err => console.log(err))
});

//database connection
db.sequelize.sync().then(req =>{ 
    app.listen(3000 , ()=>{
        console.log("server running");
    })
});

Lesgoo , nodemon app :D