Open index.js (in your case it may be app.js or server.js) and set the request header

var express = require('express');
var app = express();
...
...

//CORS support settings
app.use(function(req, res, next){
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Headers', 'Origin, X-Requested-With, Content-Type, Accept');
    res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
    next();
});


...

....

app.get('/', (req, res) => {
    res.sendFile(__dirname + '/index.html');
});

app.use('/api/user', user);

...

Move the rout definition from the index.js or app.js

Folder structure

index.js file

var express = require('express');
var app = express();

app.use(express.static('public'));

//Routes
app.use(require('./routes'));  //http://127.0.0.1:8000/    http://127.0.0.1:8000/about

//app.use("/user",require('./routes'));  //http://127.0.0.1:8000/user  http://127.0.0.1:8000/user/about

//you can create more routs
//app.use("/admin",require('./adminroutes')); //http://127.0.0.1:8000/admin http://127.0.0.1:8000/admin/about

var server = app.listen(8000, function () {

  var host = server.address().address
  var port = server.address().port

  console.log("Example app listening at http://%s:%s", host, port)

})

routes.js

var express = require('express');
var router = express.Router();

//Middle ware that is specific to this router
router.use(function timeLog(req, res, next) {
  console.log('Time: ', Date.now());
  next();
});


// Define the home page route
router.get('/', function(req, res) {
  res.send('home page');
});

// Define the about route
router.get('/about', function(req, res) {
  res.send('About us');
});


module.exports = router;

you can add more routs like admin routs (*i am not added this in my project so it will not see int the above folder structure)

adminroutes.js

var express = require('express');
var router = express.Router();

//Middle ware that is specific to this router
router.use(function timeLog(req, res, next) {
  console.log('Time: ', Date.now());
  next();
});


// Define the home page route
router.get('/', function(req, res) {
  res.send('Admin dash board');
});

// Define the about route
router.get('/profile', function(req, res) {
  res.send('About profile');
});


module.exports = router;

ref: https://expressjs.com/en/guide/routing.html

Installing Express with nodejs project dependency

1. Create project folder and open the folder using  command prompt
2. execute the following  commands
3. Create project using  npm init command

4. Now install Express in the app directory and save it in the dependencies list using :-  npm install express --save (To install Express temporarily and not add it to the dependencies list, omit the --save option)

you can see the express is added to the project dependency

5. You can install the following supporting libraries also

npm install body-parser --save

npm install cookie-parser --save

npm install multer --save

body-parser – This is a node.js middleware for handling JSON, Raw, Text and URL encoded form data.

cookie-parser – Parse Cookie header and populate req.cookies with an object keyed by the cookie names.

multer – This is a node.js middleware for handling multipart/form-data.

6. Create a sample code

var express = require('express');
var app = express();

app.get('/', function (req, res) {
   res.send('Hello World');
});

var server = app.listen(8000, function () {

  var host = server.address().address
  var port = server.address().port

  console.log("Server started and listening at http://%s:%s", host, port)

});

7 Run the server using node index.js open http://127.0.0.1:8000/ in your browser 

Done

There a node package nodemon, “Nodemon is a utility that will monitor for any changes in your source and automatically restart your server. Perfect for development”

npm install -g nodemon

Running our project with nodemon

nodemon index.js

How to use express generator for creating project https://wiki.workassis.com/nodejs-express-application-generator/

Install Node.js and NPM

You will get the single installation package for Nodejs and NPM from the node website nodejs.org.

For testing the installation open command prompt

node -v

If it is showing node version then the NodeJs installation success

For testing npm installation use

npm -v

For a simple program

create a test.js file put some log and run this file from command prompt using node (node <filename.js>)

console.log('Welcome to Node programming');

node test.js

String

String objects are immutable ( once created can not be changed ) and it will stored in the  Constant String Pool , so if you chose to do a lot of manipulations with String objects, you will end up with a lot of abandoned String objects in the String pool. To avoiding this memory loss we can use StringBuilder or StringBuffer and can be modified over and over again without causing memory loss

StringBuilder example

StringBuffers are thread-safe, they have synchronized methods to control access so that only one thread can access a StringBuffer object’s synchronized code at a time. Thus, StringBuffer objects are generally safe to use in a multi-threaded environment where multiple threads may be trying to access the same StringBuffer object at the same time.

        String from = "From : ";
        String to = " To : ";

        StringBuilder builder = new StringBuilder();
        builder.append(from);
        builder.append("Manager");
        builder.append(to);
        builder.append("Developer");

        Log.i("Result : ", builder.toString());

StringBuffer example

StringBuilder’s access is not synchronized so that it is not thread-safe. The performance of StringBuilder can be better than StringBuffer. Thus, if you are working in a single-threaded environment, using StringBuilder instead of StringBuffer may result in increased performance.

        String from = "From : ";
        String to = " To : ";

        String str = new StringBuffer()
                .append(from)
                .append("Manager")
                .append(to)
                .append("Developer")
                .toString();


        Log.i("Result : ",str);

ref:
https://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html

https://docs.oracle.com/javase/tutorial/java/data/buffers.html

https://docs.oracle.com/javase/7/docs/api/java/lang/StringBuffer.html

In Android we can set click listener by three ways

1. Setting individual click listener for each element.
2. Implementing OnClickListener in activity level.
3. Creating a separate function for handling button click and add this function in xml

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.bm.buttonclickeg.MainActivity">

    <Button
        android:id="@+id/btn_one"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button 1" />

    <Button
        android:id="@+id/btn_two"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button 2" />

    <Button
        android:id="@+id/btn_three"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="myBtnClick"
        android:text="Button 3" />

</LinearLayout>

In the above example btn_three I have added a property android:onClick=”myBtnClick” myBtnClick is a function I have defined in activiy

MainActivity.java

package com.example.bm.buttonclickeg;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //option 1
        
        Button btn1 = (Button) findViewById(R.id.btn_one);

        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Toast.makeText(MainActivity.this, "Created individual click listener", Toast.LENGTH_SHORT).show();
            }
        });


        //option 2
        Button btn2 = (Button) findViewById(R.id.btn_two);

        btn2.setOnClickListener(this);

    }


    //option 2
    @Override
    public void onClick(View v) {

        switch (v.getId()) {
            case R.id.btn_two:
                Toast.makeText(MainActivity.this, "Implemented click listener in activity", Toast.LENGTH_SHORT).show();
                break;
        }

    }


    //option 3
    // This function I have added in the btn_tree xml
    public void myBtnClick(View v) {
        Toast.makeText(MainActivity.this, "Created a separate function for handling button click and added this function in button xml", Toast.LENGTH_SHORT).show();
    }
}