How do I append data to a text file?

One of the common task related to a text file is to append or add some contents to the file. It’s really simple to do this in Java using a FileWriter class. This class has a constructor that accept a boolean parameter call append. By setting this value to true a new data will be appended at the end of the file when we write a new data to it.

Let’s see an example.

package org.kodejava.io;

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

public class AppendFileExample {
    public static void main(String[] args) {
        File file = new File("user.txt");

        try (FileWriter writer = new FileWriter(file, true)) {
            writer.write("username=kodejava;password=secret" + System.lineSeparator());
            writer.flush();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

How do I change the cursor shape in Swing?

Using the following code snippet you can change the shape of mouse cursor in your Java Swing desktop application. The cursor is represented by the java.awt.Cursor class. Create an instance of Cursor using the new operator and pass the cursor type to the Cursor class constructor.

We can change Swing’s objects (JLabel, JTextArea, JButton, etc) cursor using the setCursor() method. In the snippet below, for demonstration, we change the cursor of the JFrame. Your mouse pointer or cursor shape will be changed if you positioned inside the frame.

A collections of cursor shape defined in the java.awt.Cursor class, such as:

  • Cursor.DEFAULT_CURSOR
  • Cursor.HAND_CURSOR
  • Cursor.CROSSHAIR_CURSOR
  • Cursor.WAIT_CURSOR
  • etc
package org.kodejava.swing;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;
import java.awt.Cursor;

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

            // Here we create a hand shaped cursor!
            Cursor cursor = new Cursor(Cursor.HAND_CURSOR);
            frame.setCursor(cursor);
            frame.setVisible(true);
        });
    }
}

What’s needed to be prepared for learning Java programming?

At the time you decided to start to learn Java Programming you can start by downloading the Java Development Kit (JDK) from Java Download website. There are three different types of JDK, the Java SE (Java Standard Edition), Java EE (Java Enterprise Edition), Java ME (Java Mobile Edition).

From the website you can also download the Java API documentations which will sure be your first companion when learning the language. It is better also to download the Java Tutorial Series that was written by the Java experts.

In the tutorial you can learn from the basic of Java programming, introduction of the fundamental of Object-Oriented Programming (OOP) which is Java all about. Next you can also find trails in each subject of the API (Application Programming Interface) provided by Java library, such as the core package, how to communicate with a database, Java GUI programming, image manipulation, RMI, Java Beans Framework, etc.

When you want to write a code, you might wonder what editor or IDE you will need to use to start learning. A good text editor that supports a coloring will be a good candidate, colorful screen is better than just a black and white, isn’t it?

There are a lot of good text editor available today such as the VIM, NotePad++, TextPad, Editplus, UltraEdit. If you already have your preferred editor, you can use it of course.

If you’re ready for the big stuff, a bigger homework project, you might consider using an IDE (Integrated Development Environment) as you’ll be working with lots of Java classes, configuration files and build script for examples. There are many great IDE on the Java world from the free to the commercial product.

What IDE to use is really a developer decision, use whatever tools that can help you to improve your learning and programming activities. You can find IDE such as NetBeans, Eclipse, IntelliJ IDEA, etc.

Beside learning from the Java tutorials there are also many forums on the internet where you can discuss your doubts or your problems. Forums like JavaRanch, Stack Overflow are great forums with Java gurus that can help you to clarify your doubts and help you to solve your problem. Remember one thing when you ask for help, be polite, elaborate your problem clearly.

Good Java books on your desktop are good resources to study Java, from good books you can learn the nuts and bolts of the Java programming languages. When you have all your arsenal you can get the best out of you in learning Java. Have fun!

How do I delete record from a table?

In this example we are showing you how to delete a record from table in the database. We use a standard JDBC library for this purpose. For the database we use MySQL, you can use any type of database you want. All you need to do is to find the JDBC driver for the database and configure it accordingly.

So here is the code example for deleting records from a table in a database.

package org.kodejava.jdbc;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;

public class DeleteRecordExample {

    public static void main(String[] args) {
        // This is our connection url to MySQL database, where jdbc is the
        // prefix for all jdbc connection. The mysql sub telling that we
        // are using MySQL database. Localhost is where our database is
        // resided and kodejava is our database name.
        String url = "jdbc:mysql://localhost/kodejava";

        // To connect to a database we will need a username and a password
        // for the database server to allow us to manipulate its data.
        String username = "kodejava";
        String password = "s3cr*t";

        // Then we ask a connection from the DriverManager by passing
        // the connection URL, the username and the password.
        try (Connection connection =
                     DriverManager.getConnection(url, username, password)) {

            // To delete records from tables we create an SQL delete command.
            // The question mark that we used in the where clause will be the
            // holder of value that will be assigned by PreparedStatement
            // class.
            String sql = "DELETE FROM book WHERE isbn = ?";
            String isbn = "9781617294945";

            // Create a statement object. We use PreparedStatement here.
            PreparedStatement statement = connection.prepareStatement(sql);

            // Pass a value of an ISBN that will tell the database which
            // record in the database to be deleted. Remember that when
            // using a statement object the index parameter is start from
            // 1 not 0 as in the Java array data type index.
            statement.setString(1, isbn);

            // Tell the statement to execute the command. The executeUpdate()
            // method for a delete command returns number of records deleted
            // as the command executed in the database. If no records was
            // deleted it will simply return 0
            int rows = statement.executeUpdate();

            System.out.println(rows + " record(s) deleted.");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Maven dependencies

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

Maven Central

How do I reverse a string?

Below is an example code that reverse a string. In this example we use StringBuffer.reverse() method to reverse a string. In Java 1.5, a new class called StringBuilder also has a reverse() method that do just the same, one of the difference is StringBuffer class is synchronized while StringBuilder class is not.

And here is the string reverse in the StringBuffer way.

package org.kodejava.lang;

public class StringReverseExample {
    public static void main(String[] args) {
        // The normal sentence that is going to be reversed.
        String words =
                "Morning of The World - The Last Paradise on Earth";

        // To reverse the string we can use the reverse() method in
        // the StringBuffer class. The reverse() method returns a
        // StringBuffer so we need to call the toString() method to
        // get a string object.
        String reverse = new StringBuffer(words).reverse().toString();

        // Print the normal string
        System.out.println("Normal : " + words);
        // Print the string in reversed order
        System.out.println("Reverse: " + reverse);
    }
}

Beside using this simple method you can try to reverse a string by converting it to character array and then reverse the array order.

And below is the result of the code snippet above.

Normal : Morning of The World - The Last Paradise on Earth
Reverse: htraE no esidaraP tsaL ehT - dlroW ehT fo gninroM

How do I convert string to char array?

Here we have a small class that convert a string literal into an array, a character array. To do this we can simply use String.toCharArray() method.

package org.kodejava.lang;

public class StringToArrayExample {
    public static void main(String[] args) {
        // We have a string literal that contains the tag line of this blog.
        String literal = "Kode Java - Learn Java by Examples";

        // Now we want to convert or divided it into a small array of char.
        // To do this we can simply used String.toCharArray() method. This
        // method splits the string into an array of characters.
        char[] temp = literal.toCharArray();

        // Here we just iterate the char array and print it to our console.
        for (char c : temp) {
            System.out.print(c);
        }
    }
}

How do I format a number?

If you want to display some numbers that is formatted to a certain pattern, either in a Java Swing application or in a JSP file, you can utilize NumberFormat and DecimalFormat class to give you the format that you want. Here is a small example that will show you how to do it.

In the code snippet below we start by creating a double variable that contains some value. By default, the toString() method of the double data type will print the money value using a scientific number format as it is greater than 10^7 (10,000,000.00). To be able to display the number without scientific number format we can use java.text.DecimalFormat which is a sub class of java.text.NumberFormat.

We create a formatter using DecimalFormat class with a pattern of #0.00. The # symbol means any number but leading zero will not be displayed. The 0 symbol will display the remaining digit and will display as zero if no digit is available.

package org.kodejava.text;

import java.text.DecimalFormat;
import java.text.NumberFormat;

public class DecimalFormatExample {
    public static void main(String[] args) {
        // We have some millions money here that we'll format its look.
        double money = 100550000.75;

        // Creates a formatter
        NumberFormat formatter = new DecimalFormat("#0.00");

        // Print the number using scientific number format and using our 
        // defined decimal format pattern above.
        System.out.println(money);
        System.out.println(formatter.format(money));
    }
}

Here is the different result of the code above.

1.0055000075E8
100550000.75

How do I convert string of time to time object?

You want to convert a string representing a time into a time object in Java. As we know that Java represents time information in a class java.util.Date, this class keep information for date and time.

Now if you have a string of time like 15:30:18, you can use a SimpleDateFormat object to parse the string time and return a java.util.Date object. The pattern of the string should be passed to the SimpleDateFormat constructor. In the example below the string is formatted as hh:mm:ss (hour:minute:second).

package org.kodejava.util;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;

public class StringToTimeExample {
    public static void main(String[] args) {        
        // A string of time information
        String time = "15:30:18";

        // Create an instance of SimpleDateFormat with the specified
        // format.
        DateFormat sdf = new SimpleDateFormat("hh:mm:ss");
        try {
            // To get the date object from the string just called the 
            // parse method and pass the time string to it. This method 
            // throws ParseException if the time string is invalid. 
            // But remember as we don't pass the date information this 
            // date object will represent the 1st of january 1970.
            Date date = sdf.parse(time);            
            System.out.println("Date and Time: " + date);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

The code snippet above print the following output:

Date and Time: Thu Jan 01 15:30:18 CST 1970

How do I use LineNumberReader class to read file?

In this example we use LineNumberReader class to read file contents. What we try to do here is to get the line number of the read data. Instead of introducing another variable; an integer for instance; to keep track the line number we can utilize the LineNumberReader class. This class offers the getLineNumber() method to know the current line of the data that is read.

package org.kodejava.io;

import java.io.FileReader;
import java.io.LineNumberReader;
import java.util.Objects;

public class LineNumberReaderExample {
    public static void main(String[] args) throws Exception {
        // We'll read a file called student.csv that contains our
        // student information data.
        String filename = Objects.requireNonNull(Thread.currentThread().getContextClassLoader()
                .getResource("student.csv")).getFile();

        // To create the FileReader we can pass in our student data
        // file to the reader. Next we pass the reader into our
        // LineNumberReader class.
        try (FileReader fileReader = new FileReader(filename);
             LineNumberReader lineNumberReader = new LineNumberReader(fileReader)) {
            // If we set the line number of the LineNumberReader here
            // we'll got the line number start from the defined line
            // number + 1

            //lineNumberReader.setLineNumber(400);

            String line;
            while ((line = lineNumberReader.readLine()) != null) {
                // We print out the student data and show what line
                // is currently read by our program.
                System.out.printf("Line Number %s: %s%n", lineNumberReader.getLineNumber(), line);
            }
        }
    }
}

The /resources/student.csv file:

Alice, 7
Bob, 8
Carol, 5
Doe, 6
Earl, 6
Malory, 8

And here is the result of our code snippet above:

Line Number 1: Alice, 7
Line Number 2: Bob, 8
Line Number 3: Carol, 5
Line Number 4: Doe, 6
Line Number 5: Earl, 6
Line Number 6: Malory, 8

How do I get user home directory name?

package org.kodejava.lang;

public class UserHomeExample {
    public static void main(String[] args) {
        // This is the key that we used to obtain user home directory
        // in the operating system
        String userHome = "user.home";

        // We get the path by getting the system property with the 
        // defined key above.
        String path = System.getProperty(userHome);

        // We print your home path here
        System.out.println("Your Home Path: " + path);
    }
}

On my machine it print something like:

Your Home Path: C:\Users\wsaryada

Or

Your Home Path: /Users/wsaryada