How do I read file contents to string using Commons IO?

By Wayan in Apache Commons, Commons IO 0 Comment

In this example we use FileUtils class from Apache Commons IO to read the content of a file. FileUtils have two static methods called readFileToString(File) and readFileToString(File, String) that we can use.

An example to read file contents and return the result as a string can be seen below.

package org.kodejava.commons.io;

import org.apache.commons.io.FileUtils;

import java.io.File;
import java.io.IOException;

public class ReadFileToStringSample {
    public static void main(String[] args) {
        // Here we create an instance of File for sample.txt file.
        File file = new File("sample.txt");

        try {
            // Read the entire contents of sample.txt
            String content = FileUtils.readFileToString(file, "UTF-8");
            System.out.println("File content: " + content);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.14.0</version>
</dependency>

Maven Central

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;
    }
}

How do I use Java NIO to copy file?

By Wayan in Core API, IO 0 Comment

The following code snippet show you how to copy a file using the NIO API. The NIO (New IO) API is in the java.nio.* package. It requires at least Java 1.4 because the API was first included in this version. The JAVA NIO is a block based IO processing, instead of a stream based IO which is the old version IO processing in Java.

package org.kodejava.io;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.nio.channels.FileChannel;
import java.nio.ByteBuffer;

public class CopyFileExample {
    public static void main(String[] args) throws Exception {
        String source = "medical-report.txt";
        String destination = "medical-report-final.txt";

        FileInputStream fis = new FileInputStream(source);
        FileOutputStream fos = new FileOutputStream(destination);

        FileChannel inputChannel = fis.getChannel();
        FileChannel outputChannel = fos.getChannel();

        // Create a buffer with 1024 size for buffering data
        // while copying from source file to destination file.
        // To create the buffer here we used a static method
        // ByteBuffer.allocate()
        ByteBuffer buffer = ByteBuffer.allocate(1024);

        // Here we start to read the source file and write it
        // to the destination file. We repeat this process
        // until the read method of input stream channel return
        // nothing (-1).
        while (true) {
            // Read a block of data and put it in the buffer
            int read = inputChannel.read(buffer);

            // Did we reach the end of the channel? if yes
            // jump out the while-loop
            if (read == -1) {
                break;
            }

            // flip the buffer
            buffer.flip();

            // write to the destination channel and clear the buffer.
            outputChannel.write(buffer);
            buffer.clear();
        }
    }
}

How do I split a string?

By Wayan in Core API, Lang Package 0 Comment

Prior to Java 1.4 we use java.util.StringTokenizer class to split a tokenized string, for example a comma separated string. Starting from Java 1.4 and later the java.lang.String class introduce a String.split(String regex) method that simplify this process.

Below is a code snippet how to do it.

package org.kodejava.lang;

import java.util.Arrays;

public class StringSplit {
    public static void main(String[] args) {
        String data = "1,Diego Maradona,Footballer,Argentina";
        String[] items = data.split(",");

        // Iterates the array to print it out.
        for (String item : items) {
            System.out.println("item = " + item);
        }

        // Or simply use Arrays.toString() when print it out.
        System.out.println("item = " + Arrays.toString(items));
    }
}

The result of the code snippet:

item = 1
item = Diego Maradona
item = Footballer
item = Argentina
item = [1, Diego Maradona, Footballer, Argentina]

How do I execute other applications from Java?

By Wayan in Core API, Lang Package 0 Comment

The following program allows you to run / execute other application from Java using the Runtime.exec() method.

package org.kodejava.lang;

import java.io.IOException;

public class RuntimeExec {
    public static void main(String[] args) {
        //String command = "/Applications/Safari.app/Contents/MacOS/Safari";
        String command = "explorer.exe";

        try {
            Process process = Runtime.getRuntime().exec(new String[]{command});
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

How do I get path / classpath separator?

By Wayan in Core API, Lang Package 0 Comment

OS platform has a different symbol used for path separator. Path separator is a symbol that separate one path element from the other. In Windows the path separator is a semicolon symbol (;), you have something like:

.;something.jar;D:/libs/commons.jar

While in Linux based operating systems the path separator is a colon symbol (:), it looks like:

.:something.jar:/libs/commons.jar

To obtain the path separator you can use the following code.

package org.kodejava.lang;

import java.util.Properties;

public class PathSeparator {
    public static void main(String[] args) {
        // Get System properties
        Properties properties = System.getProperties();

        // Get the path separator which is unfortunately
        // using a different symbol in different OS platform.
        String pathSeparator = properties.getProperty("path.separator");
        System.out.println("pathSeparator = " + pathSeparator);
    }
}

How do I make a centered JFrame?

By Wayan in Swing 0 Comment

If you have a JFrame in your Java Swing application and want to center its position on the screen you can use the following code snippets.

The first way is to utilize java.awt.Toolkit class to get the screen size. The getScreenSize() method return a java.awt.Dimension from where we can get the width and height of the screen. Having this values in hand we can calculate the top left position of our JFrame as shown in step 2 of the code below.

package org.kodejava.swing;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;
import java.awt.Dimension;
import java.awt.Toolkit;

public class CenteredJFrame extends JFrame {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            // 1. Get the size of the screen
            Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();

            CenteredJFrame frame = new CenteredJFrame();
            frame.setTitle("Centered JFrame");
            frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
            frame.setSize(500, 500);

            // 2. Calculates the position where the CenteredJFrame
            // should be paced on the screen.
            int x = (screenSize.width - frame.getWidth()) / 2;
            int y = (screenSize.height - frame.getHeight()) / 2;
            frame.setLocation(x, y);
            frame.setVisible(true);
        });
    }
}

The second way which is better and simpler is to use the setLocationRelativeTo(Component) method. According to the Java API documentation of this method: If the component is null, or the GraphicsConfiguration associated with this component is null, the window is placed in the center of the screen.

If you call the JFrame.pack() method. The method should be called before the setLocationRelativeTo() method.

So we can rewrite the code above like this:

package org.kodejava.swing;

import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;

public class CenteredJFrameSecond {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            CenteredJFrame frame = new CenteredJFrame();
            frame.setTitle("Centered JFrame");
            frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
            frame.setSize(500, 500);

            // Place the window in the center of the screen.
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }
}

How do I get environment variables?

By Wayan in Core API, Lang Package 0 Comment

Environment variables are a set of dynamic values that can affect a running process, such as our Java program. Each process usually have their own copy of these variables.

Now we would like to obtain the available variables in our environment or operating system, how do I do this in Java? Here is a code example of it.

package org.kodejava.lang;

import java.util.Map;
import java.util.Set;

public class SystemEnv {
    public static void main(String[] args) {
        // We get the environment information from the System class. 
        // The getenv method (why shouldn't it called getEnv()?) 
        // returns a map that will never have null keys or values 
        // returned.
        Map<String, String> map = System.getenv();

        Set<String> keys = map.keySet();
        for (String key : keys) {
            // Here we iterate based on the keys inside the map, and
            // with the key in hand we can get it values.
            String value = map.get(key);
            System.out.println(key + " = " + value);
        }
    }
}

Here are some results on my machine.

...
M2 = C:\ProgramData\chocolatey\lib\maven\apache-maven-3.5.0\bin
JETTY_HOME = C:\ProgramData\chocolatey\lib\jetty\tools\jetty-distribution-9.4.31.v20200723
USERNAME = wsaryada
ProgramFiles(x86) = C:\Program Files (x86)
M2_REPO = C:\Users\wsaryada\.m2
SPRING_HOME = C:\ProgramData\chocolatey\lib\spring-boot-cli\spring-2.2.4.RELEASE
M2_HOME = C:\ProgramData\chocolatey\lib\maven\apache-maven-3.5.0
JAVA_HOME = C:\Program Files\Java\jdk1.8.0_161
OS = Windows_NT
COMPUTERNAME = KRAKATAU
CATALINA_HOME = C:\tools\apache-tomcat-10.0.11
...

How do I insert a record into database table?

By Wayan in JDBC 0 Comment

In this example you’ll learn how to create a program to insert data into a database table. To insert a data we need to get connected to a database. After a connection is obtained you can create a java.sql.Statement object from it, and using this object we can execute some query strings.

package org.kodejava.jdbc;

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

public class InsertStatementExample {
    private static final String URL = "jdbc:mysql://localhost/kodejava";
    private static final String USERNAME = "kodejava";
    private static final String PASSWORD = "s3cr*t";

    public static void main(String[] args) {
        try (Connection connection =
                     DriverManager.getConnection(URL, USERNAME, PASSWORD)) {

            // Create a statement object.
            Statement stmt = connection.createStatement();
            String sql = "INSERT INTO book (isbn, title, published_year) " +
                    "VALUES ('978-1617293566', 'Modern Java in Action', 2019)";

            // Call execute() method of the statement object and pass the
            // query.
            stmt.execute(sql);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Below is the script from creating the book table.

CREATE TABLE `book`
(
    `id`             bigint(20) unsigned                  NOT NULL AUTO_INCREMENT,
    `isbn`           varchar(50) COLLATE utf8_unicode_ci  NOT NULL,
    `title`          varchar(100) COLLATE utf8_unicode_ci NOT NULL,
    `published_year` int(11)                                       DEFAULT NULL,
    `price`          decimal(10, 2)                       NOT NULL DEFAULT '0.00',
    PRIMARY KEY (`id`)
) ENGINE = InnoDB
  DEFAULT CHARSET = utf8
  COLLATE = utf8_unicode_ci;

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.4.0</version>
</dependency>

Maven Central