How do I delete a file in Java?

By Wayan in Core API, IO 0 Comment

In this example you will see how to delete a file in Java. To delete a file we start by creating a File object that represent the file to be deleted. We use the exists() method to check if the file is exists before we try to delete it.

To delete the file we call the delete() method of the File object. If the file was successfully deleted the method will return a boolean true result, otherwise it will return false. Let’s try the code snippet below.

package org.kodejava.io;

import java.io.File;

public class FileDeleteExample {
    public static void main(String[] args) {
        // When want to delete a file named readme.txt
        File file = new File("write.txt");

        // Checks if the file is exists before deletion.
        if (file.exists()) {
            System.out.println("Deleting " + file.getAbsolutePath());
            // Use the delete method to delete the given file.
            boolean deleted = file.delete();
            if (deleted) {
                System.out.println(file.getAbsolutePath() + " was deleted.");
            }
        } else {
            System.out.println(file.getAbsolutePath() + " not found.");
        }
    }
}

How do I get IP address of localhost?

By Wayan in Core API, Networking 0 Comment

The example here show you how to get an IP or host address using the java.net.InetAddress class. To get an instance of InetAddress we call a static method of this class, the method is getLocalHost(), which return the local host address. Next, to get the IP address we can call the getHostAddress() method.

package org.kodejava.net;

import java.net.InetAddress;
import java.net.UnknownHostException;

public class LocalHostIpAddress {
    public static void main(String[] args) {
        try {
            InetAddress address = InetAddress.getLocalHost();
            String ip = address.getHostAddress();

            System.out.println("IP Address = " + ip);
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
    }
}

The result of this code snippet:

IP Address = 30.30.30.60

How do I create an empty collection object?

By Wayan in Core API, Util Package 3 Comments

Sometimes you need to return an empty collection from your Java methods. The java.util.Collections utility class have three different static constants for creating empty List, Set and Map.

  • Collections.EMPTY_LIST
  • Collections.EMPTY_SET
  • Collections.EMPTY_MAP

There are also methods when you want to create type-safe empty collections.

  • Collections.emptyList()
  • Collections.emptySet()
  • Collections.emptyMap()

Bellow it the code example.

package org.kodejava.util;

import java.util.*;

public class EmptyCollectionDemo {
    public static void main(String[] args) {
        List list = Collections.EMPTY_LIST;
        System.out.println("list.size()  = " + list.size());
        Set set = Collections.EMPTY_SET;
        System.out.println("set.size()   = " + set.size());
        Map map = Collections.EMPTY_MAP;
        System.out.println("map.size()   = " + map.size());

        // For the type-safe example use the following methods.
        List<String> strings = Collections.emptyList();
        System.out.println("strings.size = " + strings.size());

        Set<Long> longs = Collections.emptySet();
        System.out.println("longs.size() = " + longs.size());

        Map<String, Date> dates = Collections.emptyMap();
        System.out.println("dates.size() = " + dates.size());
    }
}

The output are:

list.size()  = 0
set.size()   = 0
map.size()   = 0
strings.size = 0
longs.size() = 0
dates.size() = 0

How do I read entries in a zip / compressed file?

By Wayan in Zip and GZIP 0 Comment 
package org.kodejava.util.zip;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Enumeration;
import java.util.zip.ZipEntry;
import java.util.zip.ZipFile;

public class ZipFileExample {
    public static void main(String[] args) {
        try {
            // Create an instance of ZipFile to read a zip file
            // called sample.zip
            ZipFile zip = new ZipFile(new File("data.zip"));

            // Here we start to iterate each entry inside
            // sample.zip
            for (Enumeration<?> e = zip.entries(); e.hasMoreElements(); ) {
                // Get ZipEntry which is a file or a directory
                ZipEntry entry = (ZipEntry) e.nextElement();

                // Get some information about the entry such as
                // file name, its size.
                System.out.println("File name: " + entry.getName()
                    + "; size: " + entry.getSize()
                    + "; compressed size: "
                    + entry.getCompressedSize());
                System.out.println();

                // Now we want to get the content of this entry.
                // Get the InputStream, we read through the input
                // stream until all the content is read.
                InputStream is = zip.getInputStream(entry);
                InputStreamReader isr = new InputStreamReader(is);

                char[] buffer = new char[1024];
                while (isr.read(buffer, 0, buffer.length) != -1) {
                    String s = new String(buffer);
                    // Here we just print out what is inside the
                    // buffer.
                    System.out.println(s.trim());
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

What is Autoboxing?

By Wayan in Basic, Core API 0 Comment

Autoboxing is a new feature offered in the Tiger (1.5) release of Java SDK. In short auto boxing is a capability to convert or cast between object wrappers (Integer, Long, etc) and their primitive types.

Previously when placing primitive data into one of the Java Collection Framework object we have to wrap it to an object because the collection cannot work with primitive data. Also, when calling a method that requires an instance of object rather than an int or long, we have to convert it too.

Now, starting from version Java 1.5 we have a new feature in the Java Language which automate this process, its call the Autoboxing. When we place an int value into a collection, such as List, it will be converted into an Integer object behind the scene. When we read it back, it will automatically convert to the primitive type. In most way this simplifies the way we code, no need to do an explicit object casting.

Here an example how it will look like using the Autoboxing feature:

package org.kodejava.basic;

import java.util.HashMap;
import java.util.Map;

public class Autoboxing {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();

        // Here we put an int into the Map, and it accepted
        // as it will be autoboxed or converted into the wrapper
        // of this type, in this case the Integer object.
        map.put("Age", 25);

        // Here we can just get the value from the map, no need
        // to cast it from Integer to int.
        int age = map.get("Age");

        // Here we simply do the math on the primitive type
        // and got the result as an Integer.
        Integer newAge = age + 10;
    }
}