Java snippets

Factorial.java

import static java.lang.Math.*;

//TODO: use BigInteger instead of long
public class Factorial {

    public long recursiveFactorial(long n) {
        if (n == 0) {
            return 1;
        }
        return n * recursiveFactorial(n - 1);
    }

    public long analyticFactorial(long n) {
        return round(exp(Gamma.logGamma(n + 1)));
    }

    /**
     * @see http://introcs.cs.princeton.edu/java/91float/Gamma.java.html
     */
    public static class Gamma {

        static double logGamma(double x) {
            double tmp = (x - 0.5) * log(x + 4.5) - (x + 4.5);
            double ser = 1.0 + 76.18009173    / (x + 0)   - 86.50532033    / (x + 1)
                             + 24.01409822    / (x + 2)   -  1.231739516   / (x + 3)
                             +  0.00120858003 / (x + 4)   -  0.00000536382 / (x + 5);
            return tmp + log(ser * sqrt(2 * PI));
        }

        static double gamma(double x) { 
            return exp(logGamma(x)); 
        }
    }

    public static void main(String[] args) {
        Factorial f = new Factorial();
        for (int i = 0; i < 100; i++) {
            long a = f.analyticFactorial(i);
            long r = f.recursiveFactorial(i);
            if (a != r) {
                System.out.println(String.format("different values for i=%d: %d != %d", i, a, r));
            }
        }
    }
}

Fibonacci.java

import static java.lang.Math.*;
 
public class Fibonacci {
 
    public long recursiveFibonacci(long n) {
        if (n == 0) {
            return 0;
        }
        if (n == 1) {
            return 1;
        }
        return recursiveFibonacci(n - 1) + recursiveFibonacci(n - 2);
    }
 
    public long analyticFibonacci(long n) {
        return round((pow(.5 + .5 * sqrt(5.0), n) - pow(.5 -.5 * sqrt(5.0), n)) / sqrt(5.0));
    }
}

HttpPostWithBasicAuthentication.java

    // using http://hc.apache.org/
    public void httpPostWithBasicAuthentication(byte[] byteArray) throws IOException {
        HttpPost httpPost = new HttpPost();
        httpPost.addHeader(BasicScheme.authenticate(new UsernamePasswordCredentials("user", "password"), "UTF-8", false));
        httpPost.setHeader("Content-Type", "application/octet-stream");
        httpPost.setEntity(new ByteArrayEntity(byteArray));

        HttpClient httpClient = new DefaultHttpClient();
        httpClient.execute(httpPost);
    }

HttpServer.java

import java.io.*;
import java.net.*;
import java.nio.file.*;
import com.sun.net.httpserver.*;

public class Main {

    public static void main(String[] args) throws IOException {
        int port = Integer.parseInt(System.getProperty("app.port", "8080"));
        System.out.println(String.valueOf(port));

        HttpServer server = HttpServer.create(new InetSocketAddress(port), 0);
        server.createContext("/", new HttpHandler() {
            @Override
            public void handle(HttpExchange exchange) throws IOException {
                String uri = exchange.getRequestURI().toString();

                switch (uri) {
                    case "/": {
                        byte[] body = "Hello World!".getBytes();
                        exchange.getResponseHeaders().add("Content-Type", "text/html");
                        exchange.sendResponseHeaders(200, body.length);
                        exchange.getResponseBody().write(body);
                        break;
                    }
                    case "/html": {
                        byte[] body = Files.readAllBytes(Paths.get("app", "index.html"));
                        exchange.getResponseHeaders().add("Content-Type", "text/html");
                        exchange.sendResponseHeaders(200, body.length);
                        exchange.getResponseBody().write(body);
                        break;
                    }
                    default: {
                        exchange.sendResponseHeaders(404, 0);
                    }
                }

                exchange.close();
            }
        });
        server.start();
    }
}

ReadFile.java

    // Java < 7
    public void readFile(String pathname) throws IOException {
        File file = new File(pathname);
        BufferedReader bufferedReader = new BufferedReader(new FileReader(file));
        String line;
        try {
            while ((line = bufferedReader.readLine()) != null) {
                //do something
            }
        } finally {
            bufferedReader.close();
        }
    }
    
    // Java >= 7
    public void readFile(String pathname) throws IOException {
        File file = new File(pathname);
        try (BufferedReader bufferedReader = new BufferedReader(new FileReader(file))) {
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                //do something
            }
        }
    }

ReadFileInClasspath.java

InputStream inputStream = this.getClass().getClassLoader().getResourceAsStream("path/to/file");
 
//with Spring
InputStream inputStream = new ClassPathRessource("path/to/file").getInputStream();

WriteToFile.java

    // Java < 7
    public void writeToFile(String pathname, List<Output> outputs) throws IOException {
        File file = new File(pathname);
        BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(file));
        try {
            for (int i = 0; i < outputs.size(); i++) {
                Output output = outputs.get(i);
                bufferedWriter.write(output.toString());
                bufferedWriter.newLine();
            }
        } finally {
            bufferedWriter.close();
        }
    }
    
    // Java >= 7
    public void writeToFile(String pathname, List<Output> outputs) throws IOException {
        File file = new File(pathname);
        try (BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(file))) {
            for (Output output : outputs) {
                bufferedWriter.write(output.toString());
                bufferedWriter.newLine();
            }
        }
    }