Java Cheat Sheet

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May 23, 2021

5:46 am

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Java

Softwares to Install

JDK 11
https://www.oracle.com/in/java/technologies/javase-jdk11-downloads.html

Apache Tomcat Server (version 9)
https://tomcat.apache.org/download-90.cgi

Eclipse EE (2021)
https://www.eclipse.org/downloads/packages/release/neon/3/eclipse-ide-java-ee-developers

Xampp for MySQL (latest version)
https://www.apachefriends.org/download.html

Postman for REST API
https://www.postman.com/downloads/

Addition of Two Numbers

class Main {
	
	public static void main(String args[]) {
		System.out.println(args[0]);
		System.out.println(args[1]);
		
		int num1 = Integer.parseInt(args[0]);
		int num2 = Integer.parseInt(args[1]);

		int add = num1 + num2;
		
		System.out.println(String.format("%s + %s = %s", num1, num2, add));
	}
	
}

Inheritance

class Bank {
	
	protected int balance;
	
	public Bank(int bal) {
		this.balance = bal;
		System.out.println(String.format("Account opened with Balance: %s", this.balance));
	}
	
	public void deposit(int amt) {
		System.out.println(String.format("Deposit Amount: %s", amt));
		this.balance += amt;
	}
	
	public void withdraw(int amt) {
		System.out.println(String.format("Withdraw Amount: %s", amt));
		this.balance -= amt;
	}
	
	public void showBalance() {
		System.out.println(String.format("Available Balance: %s", this.balance));
	}
	
}

class HDFC extends Bank {
	public HDFC(int bal) {
		super(bal);
	}
}

class SBI extends Bank {
	public SBI(int bal) {
		super(bal);
	}
	
	public void withdraw(int amt) {
		super.withdraw(amt);
		super.balance -= 20;
	}
	
}

class Main {
	public static void main(String args[]) {
		Bank bobj = new Bank(1000);
		bobj.showBalance();
		bobj.deposit(500);
		bobj.showBalance();
		bobj.withdraw(300);
		bobj.showBalance();
		
		System.out.println("====================");
		
		HDFC hobj = new HDFC(2000);
		hobj.showBalance();
		hobj.deposit(500);
		hobj.showBalance();
		hobj.withdraw(300);
		hobj.showBalance();
		
		System.out.println("====================");
		
		SBI sobj = new SBI(3000);
		sobj.showBalance();
		sobj.deposit(500);
		sobj.showBalance();
		sobj.withdraw(300);
		sobj.showBalance();
	}
}

Exception Handling

class Main {
	public static void main(String args[]) {
		try {
			
			int num1 = Integer.parseInt(args[0]);
			int num2 = Integer.parseInt(args[1]);
			
			try {
				int div = num1 / num2;
				System.out.println(String.format("Division of %s and %s is %s", num1, num2, div));
			} catch (Exception e) {
				System.out.println(e);
			}
			
			
		} catch (ArrayIndexOutOfBoundsException aie) {
			System.out.println(aie);
			
		} catch (Exception e) {
			System.out.println(e);
		}
		
	}
}

Servlet Demo

Servlet Life Cycle

  1. form.html
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Addition of 2 numbers</title>
<link
	href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.1/dist/css/bootstrap.min.css"
	rel="stylesheet"
	integrity="sha384-+0n0xVW2eSR5OomGNYDnhzAbDsOXxcvSN1TPprVMTNDbiYZCxYbOOl7+AMvyTG2x"
	crossorigin="anonymous">
</head>
<body class="container">
	<form action="CalcServlet" class="m-5">
		<div class="row">
			<div class="col">Number 1</div>
			<div class="col">
				<input type="text" name="num1" placeholder="Enter number" autofocus>
			</div>
		</div>
		<div class="row">
			<div class="col">Number 1</div>
			<div class="col">
				<input type="text" name="num2" placeholder="Enter number">
			</div>
		</div>
		<div class="row">
			<div class="col">
				<input class="btn btn-primary" type="submit" value="Calculate" />
			</div>
		</div>
	</form>
</body>
</html>

2. CalcServlet

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class AddServlet
 */
@WebServlet("/CalcServlet")
public class CalcServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public CalcServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		//response.getWriter().append("Served at: ").append(request.getContextPath());
		int num1 = Integer.parseInt(request.getParameter("num1"));
		int num2 = Integer.parseInt(request.getParameter("num2"));
		
		response.getWriter().write(String.format("%s + %s = %s", num1, num2, num1+num2));
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		//doGet(request, response);
		int num1 = Integer.parseInt(request.getParameter("num1"));
		int num2 = Integer.parseInt(request.getParameter("num2"));
		
		response.getWriter().write(String.format("%s * %s = %s", num1, num2, num1*num2));
	}
	
	protected void doPut(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		//doGet(request, response);
		int num1 = Integer.parseInt(request.getParameter("num1"));
		int num2 = Integer.parseInt(request.getParameter("num2"));
		
		response.getWriter().write(String.format("%s - %s = %s", num1, num2, num1-num2));
	}

	protected void doDelete(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		//doGet(request, response);
		int num1 = Integer.parseInt(request.getParameter("num1"));
		int num2 = Integer.parseInt(request.getParameter("num2"));
		
		response.getWriter().write(String.format("%s / %s = %s", num1, num2, num1/num2));
	}

	protected void doOptions(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		//doGet(request, response);
		int num1 = Integer.parseInt(request.getParameter("num1"));
		int num2 = Integer.parseInt(request.getParameter("num2"));
		
		response.getWriter().write(String.format("%s %% %s = %s", num1, num2, num1%num2));
	}

}

DATABASE SQL

CREATE DATABASE

CREATE DATABASE company;

CREATE TABLE

CREATE TABLE users(
    id INT,
    NAME VARCHAR(100) NOT NULL,
    age TINYINT NOT NULL,
    city VARCHAR(200) NOT NULL
);

ALTER TABLE

ALTER TABLE
    users MODIFY id INT PRIMARY KEY AUTO_INCREMENT;

MySQL JDBC

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;

public class CRUDLDemo {
	public static void main(String args[]) {
		String conn_str = "jdbc:mysql://localhost:3306/company";
		String dbusername = "root";
		String dbpassword = "";

		try {
			Class.forName("com.mysql.cj.jdbc.Driver");
			Connection conn = DriverManager.getConnection(conn_str, dbusername, dbpassword);

			String select_sql = "SELECT * FROM users";
			Statement stmt = conn.createStatement();

			ResultSet rs = stmt.executeQuery(select_sql);

			while (rs.next()) {
				System.out.println(rs.getInt("id"));
				System.out.println(rs.getString("name"));
				System.out.println(rs.getInt("age"));
				System.out.println(rs.getString("city"));
			}

//			String insert_sql = "INSERT INTO users VALUES (NULL, 'Priyanka', 30, 'Mumbai', NOW(), NOW())";
//			Statement stmt = conn.createStatement();
//			int result = stmt.executeUpdate(insert_sql);
//			System.out.println(result);

//			String delete_sql = "DELETE FROM users WHERE id = 3";
//			Statement stmt = conn.createStatement();
//			int result = stmt.executeUpdate(delete_sql);
//			System.out.println(result);

//			String update_sql = "UPDATE users SET city = 'Panvel' WHERE id = 2";
//			Statement stmt = conn.createStatement();
//			int result = stmt.executeUpdate(update_sql);
//			System.out.println(result);

		} catch (Exception e) {
			e.printStackTrace();
		}

	}

}

select.jsp

<%@page import="java.sql.ResultSet"%>
<%@page import="java.sql.Statement"%>
<%@page import="java.sql.DriverManager"%>
<%@page import="java.sql.Connection"%>
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
	pageEncoding="ISO-8859-1"%>
<% 

String conn_string = "jdbc:mysql://localhost:3306/company";
String db_username = "root";
String db_password = "";

Connection conn = null;

try{
	Class.forName("com.mysql.cj.jdbc.Driver");
	conn = DriverManager.getConnection(conn_string, db_username, db_password);
} catch(Exception e) {
	out.println(e);
}

String select_sql = "SELECT * FROM users";
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery(select_sql);

while(rs.next()) {
	out.println(String.format("Id: %s | Name: %s | Age: %s | City: %s<br>", rs.getInt("id"), rs.getString("name"), rs.getInt("age"), rs.getString("city")));
}

%>

insert.jsp

<%@page import="java.sql.Statement"%>
<%@page import="java.sql.DriverManager"%>
<%@page import="java.sql.Connection"%>
<%
String conn_string = "jdbc:mysql://localhost:3306/company";
String db_username = "root";
String db_password = "";

Connection conn = null;

try{
	Class.forName("com.mysql.cj.jdbc.Driver");
	conn = DriverManager.getConnection(conn_string, db_username, db_password);
} catch(Exception e) {
	out.println(e);
}

String insert_sql = "INSERT INTO users VALUES (NULL, 'Priyanka', 30, 'Mumbai', NOW(), NOW())";
Statement stmt = conn.createStatement();
int result = stmt.executeUpdate(insert_sql);

if(result == 1) {
	out.println("Record inserted successfully...");
} else {
	out.println("Something went wrong...");
}
%>

update.jsp

<%@page import="java.sql.Statement"%>
<%@page import="java.sql.DriverManager"%>
<%@page import="java.sql.Connection"%>
<%
String conn_string = "jdbc:mysql://localhost:3306/company";
String db_username = "root";
String db_password = "";

Connection conn = null;

try{
	Class.forName("com.mysql.cj.jdbc.Driver");
	conn = DriverManager.getConnection(conn_string, db_username, db_password);
} catch(Exception e) {
	out.println(e);
}

String update_sql = "UPDATE users SET name = 'Palkar' WHERE id = 4";
Statement stmt = conn.createStatement();
int result = stmt.executeUpdate(update_sql);

if(result == 1) {
	out.println("Record updated successfully...");
} else {
	out.println("Something went wrong...");
}
%>

delete.jsp

<%@page import="java.sql.Statement"%>
<%@page import="java.sql.DriverManager"%>
<%@page import="java.sql.Connection"%>
<%
String conn_string = "jdbc:mysql://localhost:3306/company";
String db_username = "root";
String db_password = "";

Connection conn = null;

try{
	Class.forName("com.mysql.cj.jdbc.Driver");
	conn = DriverManager.getConnection(conn_string, db_username, db_password);
} catch(Exception e) {
	out.println(e);
}

String delete_sql = "DELETE FROM users WHERE id = 4";
Statement stmt = conn.createStatement();
int result = stmt.executeUpdate(delete_sql);

if(result == 1) {
	out.println("Record deleted successfully...");
} else {
	out.println("Something went wrong...");
}
%>

functions.jsp

<%@page import="java.sql.DriverManager"%>
<%@page import="java.sql.Connection"%>

<%!
public Connection getConnection() {

		String conn_string = "jdbc:mysql://localhost:3306/company";
		String db_username = "root";
		String db_password = "";

		Connection conn = null;

		try {
			Class.forName("com.mysql.cj.jdbc.Driver");
			conn = DriverManager.getConnection(conn_string, db_username, db_password);
		} catch (Exception e) {
			System.out.println(e);
		}

		return conn;
}

public void authorize(HttpSession session, HttpServletResponse response) {
	
	if(session.getAttribute("username") == null) {
		try {				
			response.sendRedirect("login_form.jsp?msg=Please login to access this page");
			
		} catch(Exception e) {
			e.printStackTrace();
		}
	}
}

%>

include functions and header

<%@include file="functions.jsp" %>
<jsp:include page="header.jsp" />

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