Java Fundamentals: JDBC, File I/O, and Network Socket Programming
Java Database Connectivity (JDBC) Implementation
This example demonstrates establishing a connection to a MySQL database, executing a complex SQL query, and processing the resulting data set. Note the use of standard Java naming conventions and proper exception handling.
JDBC Connection and Data Retrieval Code (Cs391Jdbc1.java)
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
import java.sql.SQLException;
public class Cs391Jdbc1 {
public static void main(String[] args) {
Connection conn = null;
try {
// 1. Load the JDBC Driver (Legacy method)
Class.forName("com.mysql.jdbc.Driver").newInstance();
// 2. Database Credentials and URL
String dbUser = "cs391";
String dbPass = "abc123";
String dbUrl = "jdbc:mysql://localhost/test";
// 3. Establish Connection
conn = DriverManager.getConnection(dbUrl, dbUser, dbPass);
System.out.println("Connection established successfully.");
// 4. Create Statement and Execute Query
Statement s = conn.createStatement();
// SQL query fixed to use standard syntax and 'AND' operator
String sqlQuery = "SELECT * FROM students, classes, studentinclass " +
"WHERE students.sid = studentinclass.sid AND classes.cid = studentinclass.cid;";
s.executeQuery(sqlQuery);
ResultSet rs = s.getResultSet();
int count = 0;
// 5. Process Results
while (rs.next()) {
String name = rs.getString("name");
String prof = rs.getString("prof");
// Assuming 'coolnessfactor' is the intended column name
int coolnessFactor = rs.getInt("coolnessfactor");
System.out.println(name + '\t' + prof + "\t" + coolnessFactor);
count++;
}
// 6. Close Resources
rs.close();
s.close();
System.out.println("Found " + count + " rows.");
} catch (Exception e) {
// Improved error message
System.out.println("An error occurred with the database connection:\n" + e.toString());
} finally {
// 7. Ensure Connection Closure
if (conn != null) {
try {
conn.close();
System.out.println("Connection closed.");
} catch (SQLException e) {
// Ignore closing errors
}
}
}
}
}
Handling File Input and Output (I/O)
This program demonstrates how to use the Scanner class for reading user input and file content, and the PrintStream class for writing output to a file. It includes robust error checking for file existence and readability, and calculates the average of integers found in the input file.
File I/O Implementation (Cs391FileIo.java)
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
import java.util.Scanner;
public class Cs391FileIo {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
System.out.println("Enter a filename: ");
String filename = in.nextLine();
File f = new File(filename);
// Input validation loop: Ensure file exists and is readable
while (!f.canRead()) {
System.out.println("Bad filename! File cannot be read or does not exist.");
System.out.println("Enter a filename: ");
filename = in.nextLine();
f = new File(filename);
}
int count = 0;
int sum = 0;
Scanner infile = null;
try {
infile = new Scanner(f);
// Read integers from the file, calculate sum and count
while (infile.hasNextInt()) {
sum += infile.nextInt();
count++;
}
} catch (FileNotFoundException ex) {
System.out.println("Error: File not found during Scanner initialization.");
} finally {
if (infile != null) {
infile.close();
}
}
PrintStream out = null;
try {
// Open file for writing output
out = new PrintStream("average.txt");
if (count > 0) {
out.println("Average is: " + ((double) sum / count));
} else {
out.println("No numbers found in the input file.");
}
} catch (FileNotFoundException ex) {
System.out.println("Error: Could not create output file average.txt");
} finally {
if (out != null) {
out.close();
}
}
}
}
Basic TCP Socket Client: Making an HTTP Request
This code demonstrates fundamental TCP socket programming by connecting to a web server (www.poly.edu on port 80) and issuing a basic HTTP 1.0 GET request. It uses Scanner and PrintStream for stream handling over the socket.
Socket Client Implementation (CS391Socket1.java)
package cs391socket1;
import java.net.Socket;
import java.io.IOException;
import java.io.PrintStream;
import java.util.Scanner;
public class CS391Socket1 {
public static void main(String[] args) {
Socket s = null;
Scanner sin = null;
PrintStream sout = null;
try {
// Connect to the server (www.poly.edu on port 80)
s = new Socket("www.poly.edu", 80);
sin = new Scanner(s.getInputStream());
sout = new PrintStream(s.getOutputStream());
// Send HTTP GET request
sout.print("GET / HTTP/1.0\r\nhost: www.poly.edu\r\n\r\n");
// Read and print response lines
String line;
while (sin.hasNext()) {
line = sin.nextLine();
System.out.println(line);
}
} catch (IOException ex) {
System.out.println("Connection Failed: " + ex.getMessage());
} finally {
// Ensure socket closure
try {
if (s != null) {
s.close();
}
} catch (IOException ex) {
// Handle closure exception if necessary
}
}
}
}
Multithreaded TCP Socket Server Implementation
This example sets up a simple server listening on port 5190. It uses a continuous loop to accept incoming client connections and delegates the handling of each client to a separate thread (HandleClient class), allowing concurrent communication. The server logs client IP addresses and messages until the client sends the “QUIT” command.
Server Main Class (CS391Server1.java)
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
public class CS391Server1 {
public static void main(String[] args) throws IOException {
// Server listens on port 5190
ServerSocket ss = new ServerSocket(5190);
System.out.println("Server started, listening on port 5190...");
while (true) {
// Wait for a client connection
Socket client = ss.accept();
// Start a new thread to handle the client
new HandleClient(client).start();
}
}
}
Client Handler Thread (HandleClient.java)
import java.io.IOException;
import java.io.PrintStream;
import java.net.Socket;
import java.util.Scanner;
class HandleClient extends Thread {
private Socket client;
HandleClient(Socket news) {
this.client = news;
}
@Override
public void run() {
PrintStream out = null;
String ip = client.getInetAddress().getHostAddress();
try {
System.out.println("Connection established from: " + ip);
out = new PrintStream(client.getOutputStream());
Scanner in = new Scanner(client.getInputStream());
// Read the first line
String line = in.nextLine();
// Loop until the client sends "QUIT"
while (!line.equals("QUIT")) {
System.out.println(ip + ": " + line);
try {
// Attempt to read the next line
line = in.nextLine();
} catch (Exception e) {
// Connection closed abruptly
break;
}
}
} catch (IOException ex) {
System.err.println("I/O error handling client " + ip + ": " + ex.getMessage());
} finally {
// Ensure client socket closure
try {
client.close();
System.out.println("Connection lost from: " + ip);
} catch (IOException ex) {
// Ignore closure errors
}
}
}
}