package org.kodejava.example.util;
import java.util.Hashtable;
import java.util.Random;
public class HashtableGetRandom {
public static void main(String[] args) {
// Create a hashtable and put some key-value pair.
Hashtable<String, String> colors = new Hashtable<>();
colors.put("black", "#000");
colors.put("red", "#f00");
colors.put("green", "#0f0");
colors.put("blue", "#00f");
colors.put("white", "#fff");
// Get a random entry from the hashtable.
String[] keys = colors.keySet().toArray(new String[colors.size()]);
String key = keys[new Random().nextInt(keys.length)];
System.out.println(key + " = " + colors.get(key));
}
}
Category Archives: Core API
How to monitor file or directory changes?
package org.kodejava.io;
import java.io.IOException;
import java.nio.file.*;
import static java.nio.file.StandardWatchEventKinds.*;
public class FileWatchDemo {
public static void main(String[] args) {
try {
// Creates a instance of WatchService.
WatchService watcher = FileSystems.getDefault().newWatchService();
// Registers the logDir below with a watch service.
Path logDir = Paths.get("F:/Temp/");
logDir.register(watcher, ENTRY_CREATE, ENTRY_MODIFY, ENTRY_DELETE);
// Monitor the logDir at listen for change notification.
while (true) {
WatchKey key = watcher.take();
for (WatchEvent<?> event : key.pollEvents()) {
WatchEvent.Kind<?> kind = event.kind();
if (ENTRY_CREATE.equals(kind)) {
System.out.println("Entry was created on log dir.");
} else if (ENTRY_MODIFY.equals(kind)) {
System.out.println("Entry was modified on log dir.");
} else if (ENTRY_DELETE.equals(kind)) {
System.out.println("Entry was deleted from log dir.");
}
}
key.reset();
}
} catch (IOException | InterruptedException e) {
e.printStackTrace();
}
}
}
To get the created, modified or deleted file you can see the following example: How to get the file name when using WatchService?.
How to get the file name when using WatchService?
package org.kodejava.io;
import java.io.IOException;
import java.nio.file.*;
import static java.nio.file.StandardWatchEventKinds.*;
public class WatchServiceGetFilename {
public static void main(String[] args) {
try {
// Create a WatchService and register the logDir path with the
// WatchService for ENTRY_CREATE.
WatchService watcher = FileSystems.getDefault().newWatchService();
Path logDir = Paths.get("F:/Temp");
logDir.register(watcher, ENTRY_CREATE);
while (true) {
WatchKey key;
try {
key = watcher.take();
} catch (InterruptedException e) {
return;
}
for (WatchEvent<?> event : key.pollEvents()) {
if (event.kind() == ENTRY_CREATE) {
// Get the name of created file.
WatchEvent<Path> ev = cast(event);
Path filename = ev.context();
System.out.printf("A new file %s was created.%n",
filename.getFileName());
}
}
key.reset();
}
} catch (IOException e) {
e.printStackTrace();
}
}
@SuppressWarnings("unchecked")
private static <T> WatchEvent<T> cast(WatchEvent<?> event) {
return (WatchEvent<T>) event;
}
}
How to read file using Files.newBufferedReader?
In the snippet below you’ll learn to open file for reading using Files.newBufferedReader() method in JDK 7. This method returns a java.io.BufferedReader which makes a backward compatibility with the old I/O system in Java.
To read a file you’ll need to provide a Path and the Charset to the newBufferedReader() method arguments.
package org.kodejava.io;
import java.io.BufferedReader;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
public class FilesNewBufferedReader {
public static void main(String[] args) {
Path logFile = Paths.get("app.log");
try (BufferedReader reader =
Files.newBufferedReader(logFile, StandardCharsets.UTF_8)) {
String line;
while ((line = reader.readLine()) != null) {
System.out.println(line);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
How to write file using Files.newBufferedWriter?
To open a file for writing in JDK 7 you can use the Files.newBufferedWriter() method. This method takes three arguments. We need to pass the Path, the Charset and a varargs of OpenOption.
For example, in the snippet below we pass the path of our log file, we use the StandardCharsets.UTF_8 charset, and we use the StandardOpenOption.WRITE to open a file for writing. If you want to open a file and append its contents instead of rewriting it you can use the StandardOpenOption.APPEND.
package org.kodejava.io;
import java.io.BufferedWriter;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
public class FilesNewBufferedWriter {
public static void main(String[] args) throws Exception {
Path logFile = Paths.get("app.log");
if (Files.notExists(logFile)) {
Files.createFile(logFile);
}
try (BufferedWriter writer =
Files.newBufferedWriter(logFile, StandardCharsets.UTF_8,
StandardOpenOption.WRITE)) {
for (int i = 0; i < 10; i++) {
writer.write(String.format("Message %s%n", i));
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
Because we use the StandardOpenOption.WRITE we have to make sure that the file to be written is exists. If the file is not available we will get error like java.nio.file.NoSuchFileException.