Java FileNotFoundException While File is Existing There

Sometimes you my encounter a FileNotFoundException in java while you can see that the specified file is there. This can happen for several reasons.The java code may throw a  FileNotFoundException even when the file exists. This can be for different reasons.

Reason 1

The file does not exist really. Although you can see the file, the path you may have entered may be incorrect. For example, the following code may throw exception in some operating systems:

File f=new File("D:\myfile.txt");

This is because the file separator character is '\' in windows os. since \ is used to display non graphical characters like '\n' you should use its corresponding escape sequence. So "D:\\myfile.txt" would work on windows. If your java application is a cross platform program, you should consider using File.separator instead of '\\' because not all operating systems use '\\' as file separator character. So, the best approach is ;

File f=new File("D:"+File.separator+"myfile.txt");

File.separator is the system-dependent default name-separator character, represented as a string for convenience. This string contains a single character, namely File.separatorChar. If you want to get this character as a Character itself, you can use File.separatorChar. There can be other mistakes in the path.

Reason 2

The name you entered as a file may be actually a directory. So an exception can be thrown in java when you try to read from or write to such File object.

Reason 3

You cannot open it for reading or writing for some other reasons. You can check the status of file by calling the methods like f.exists(), f.canRead(), f.canWrite(), f.isFile() and f.isDirectory(). All these methods return a boolean. I hope these java method names describe well what they are intended for.

How to Filter Filetypes using File Extensions in JFileChooser - Java File Chooser

Screenshot of following example java program

In java, to browse files or folders from the file system, we use the JFileChooser class. You can use  the JFileChooser object in two modes, save mode or open mode. Here we discuss about filtering file type by file extensions for opening the file. By default, the JFileChooser object will have an AcceptAllFileFilter. This file filter will allow the user to select file of any file type. The file filter will display text All files. If you want to restrict the file chooser to open only one type of file, you should first remove the AcceptAllFileFilter. To do this, call the following method:

myfilechooser.removeChoosableFileFilter(createoropenchooser.getAcceptAllFileFilter());

What Does Modal and Modeless (Non Modal) Mean in Java

In java, the dialogue boxes in JOptionPane class are very popular. Now when we use dialogue boxes, whether it is message dialogue box, input dialogue or any other,  we may sometimes need to make the parent container (often JFrame or JWindow) not focusable. In another words, we may have to prevent a (parent) component taking input from the user while one of its member component is active. (You may have noticed that when error message boxes are displayed, you cannot click its parent window on many software until you dismiss it by clicking 'OK' or 'close'). Such a property of a member or child component, is called modality. In java, you can use JDialog with or without modality. A JDialog can be either modal or modeless.

A Modal dialog box is a dialog box that blocks input to some other top-level windows in the application. The modal dialog box captures the window focus until it is closed, usually in response to a button press.

A Modeless dialog box is a dialog box that does not block input to any other top level window while it is shown.

See following java code example:

How to Get User Related Informations in Java

In java, there are some methods that will help you to know some user related informations. Here also,

System.getProperty() 

method can give some informations about the user like:

User account name
User Working directory
User Home directory

To get the username of current user on the computer just call

System.getProperty("user.name")

It will return the current username as a string.

To get the working directory of current user on the computer call

System.getProperty("user.dir")

It will return the path of working directory of current user as a string.

To get the home directory (user's home folder) of current user on the computer call

System.getProperty("user.home")

It will return the path to home directory (folder) of current user as a string.

How to Get Operating System Information in Java

Using java, you can gather some information about the operating system of the computer system on which the code is running. That us java provides some API classes that will help you to know about the operating system of the computer on which your application program is running. You can use System.getProperty() method for this purpose. There are some system property keys (strings) which can be used to get the respective information. The parameter to the System.getProperty() method is a string, the property key. The following values for the parameter string will give operating system related informations.

"os.arch"
Operating system architecture

"os.name"
Name of the operating system.

"os.version"
Version of Operating system.

Usage examples:

The following java program displays the Operating system related informations using getProperty() method.

System.out.println("Operating system architecture:"+System.getProperty("os.arch") ); System.out.println("Operating system Name:"+System.getProperty("os.name") ); System.out.println("Operating system version:"+System.getProperty("os.version") );

How to Get JRE installation Directory in a Java Application

To get the java runtime environment (JRE) installation directory of a computer, we can use the getProperty() method in System class. The key to be used for System.getProperty() method to get the java JRE installation directory is "java.home". System.getProperty("java.home") will return the java runtime environment(JRE) installation directory path as a string. the following code will display the jre installation directory path:

System.out.println(System.getProperty("java.home"));

How to Get Class Path in a Java Application

Class Path is the Path used to find directories and JAR archives containing class files. Elements of the class path are separated by platform-specific characters. To get class path from a running java application, you may use the getProperty() method in System class. The following java code will show you how to get class path in java.

String classpath=System.getProperty("java.class.path");
System.out.println("Class path="+ classpath);

Why FileOutputStream or BufferedWriter Does Not Write New Line to File

The new-line character is system dependent. It depends on operating system. It can be \n, \r, \r\n or something else depending on the operating system. So, when we are writing a cross platform java program, it is important to take care when we use a line separator or new line character. It is always safe to use system specific line separator provided by JVM. You can get the system specific newline character in different ways.

Method 1:

System.lineSeparator() instead of new line character. The java method Sytem.lineSeparator() returns the newline character (line separator) as a strong. If you are appending a string on another with a line separator in between, it can be done as follows.

line1+System.lineSeparator()+line2

Method 2:

You can also get the line separator using the getProperty() method with property key line.separator.

System.getProperty("line.separator");

Method 3:

If your are using a BufferedReader object to write text into a file, you can use the newLine() method to write a newline into the file. If br is a BufferedReader object, br.newLine() will write a newline (line separator) into the file.

Method 4:

If you want to get the line separator as a character, you may use the static member Character.LINE_SEPARATOR. It returns the Unicode newline character.

How to Make a Timer Controlled Progress Bar in Java

timer controlled progress bar in java

For some applications we require to use timer and progress-bars. Java has a swing class called
JProgressBar to create progress bars. It can have horizontal or vertical orientation. We can set the maximum and minimum values for JProgressBar. It can also have an indeterminate state in which the progressbar does not show a definite value and plays an animation to show that the process is in progress. JProgressBar can be used in indeterminate mode when neither can you determine how long a process would take to complete nor can you track its progress. In some other cases, you may have to use timers. Java provides a Timer class (Java.util.Timer). But you can create a very simple and flexible timer if you want. If the timer does not have crucial importance, you can use a thread that sleeps for every one second as a timer. I have been using such threads as timers for a long time and never failed. Such timers may fail (although very rarely) when the processor has very heavy workload.

Whatever, in a simple example, we will see how to create a timer using thread and how to use JProgressBar. In this example we control the JProgressBar using the timer. The progress in JProgressBar is set calling the method setValue(). In this example we are using a 1 minute timer. The progress bar should show the progress of timer. The java code for timer controlled JProgressBar is as follows:

Problem With JProgressBar setValue Method? JProgressBar Value Not Updating - Solution

I had a problem with a JProgressBar. I called setValue() method of the java swing class JProgressBar for an object but the JProgressBar is still sowing zero progress.This problem may occur for many people. People use the words:

Jprogressbar setvalue not upadting

jprogressbar.setvalue not working

jprogressbar setvalue problem

for googling this problem. All these keyword combinations have a presumption that the problem is with java not on our part, Actually, the problem is on our part but it is a silly mistake. I know two reasons for such problem.

Java Program to Send Image between Computers using UDP and Display it

Java program to send image file over network using UDP

This was one of my experiments in Networking Lab. The aim was to write a java program to send a
picture (image) from a computer to another via network using UDP protocol and to display it at the receiving end. Simply, this is the java code to make a java application which can send images from one computer to another over a network. The program uses UDP (User Datagram Protocol). For both the computers (on either ends) the same code is enough. This app can act as a image sender or a receiver. The program uses a Progress bar (JProgressBar) to show the progress of the data transfer. Remember, transferring files over UDP is not easy because UDP does not guarantee ordered delivery, and not even successful delivery. So, here in this program, to ensure ordered delivery, the sender sends next chunk of data (DatagramPacket) only after receiving an acknowledgement for last sent packet. For acknowledgement, i used a string "ACK".

Chat Room Application using UDP in Java

Java Chat room application using UDP

The following is a Java program for chatting. This chatting application uses UDP (User Datagram
Protocol) for chatting. The application gives you to options. One is to create a new chat room and the other is to join a chat room. The one who creates the chat room acts as a server for the chatroom. Others can join this chatroom id they know his IP address. This simple java chat room application is enough for server and clients. The GUI is basic one and not too attractive. Also i have used JOptionPane to reduce code. You may make improvements to the code. The chat rooms are not password protected. Anyone who know the server's IP address are granted access to the chat room. If you wish, you may improve the program. This Group chat application using java can be used in Local area networks. This code for java group chat program can be modified to enable chatting in multiple chat rooms. Try it.

How to download a file from internet Using java if URL is Given

Let us see how to download a file from internet in java. It is so simple. The following java program is a program which will download any file if the URL to the file to be downloaded is given. It also asks the user for destination to save the file. It displays the file size also. It also prints (displays) the progress of download. It uses the classes HttpURLConnection, URL, FileOutputStream, JFileChooser etc. The following java code is a simple java program to download a file from internet using URL. Here in this case, user has to give the file name and extension.

How to Easily Copy a File to Another File or Location in Java

The Files class in java has many useful static methods to copy and move files, create temporary files or folders etc. To copy a file from one location to another in Java, there is a static method copy in Files class. The Files class is in java.nio.file package. The method Files.copy() takes three arguments (parameters) : Path of source file (as object of Path class), Path of destination file ( as object of Path class ) and an option parameter. The option parameter can be one of the following:

StandardCopyOption.REPLACE_EXISTING
If the target file exists, then the target file is replaced.

StandardCopyOption.COPY_ATTRIBUTES
Attempts to copy the file attributes associated with this file to the target file. The exact file attributes
that are copied is platform and file system dependent and therefore unspecified.

StandardCopyOption.NOFOLLOW_LINKS
Symbolic links are not followed. If the file is a symbolic link, then the symbolic link itself, not the target of the link, is copied.

You have to import java.nio.file.StandardCopyOption to use option parameters.

try{
File sourcefile=new File("audio.mp3");
File destination=new File(System.getProperty("java.io.tmpdir")+"\\temp.mp3");
Files.copy(sourcefile.toPath(),destination.toPath(),StandardCopyOption.COPY_ATTRIBUTES);
} catch (IOException ex) {
ex.printStackTrace();
}

For the copy method, there are two other overloaded method definitions. In one, instead of source file's path, inputstream is the first parameter and the rest are same. For the other overloaded definition, an outputstream object is the second parameter instead of destination path object and the rest are same.

Similarly, to move a file from one location to another, there is a method move() in Files class. The parameters of this method is also same as those of copy method.

How to Get Path to Temp Directory for Current User in Java

In some cases, you may need to create temporary files. It is preferred to create temporary files and folders in the current user's temporary folder (temp directory) because, the user will be able to clean unwanted files. To get the path to the temp folder of current user, java provides a property key to be used with

System.getProperty()

. it is

"java.io.tmpdir"

.

The following code displays the path to the temporary folder for the current user.

System.out.println("temp directory ="+System.getProperty("java.io.tmpdir"));

How to Get Center Point of Screen in Java

If you want position your JFrame in center of screen, there is method for that. Read this post:

How to Set JFrame in Center of Screen

The LocalGraphicsEnvironment in Java will give you the center point of the display or screen. There is a static method getLocalGraphicsEnvironment() in GraphicsEnvironment class. To get the center point of the screen, you may use the following java code:

Point centre=GraphicsEnvironment.getLocalGraphicsEnvironment().getCenterPoint();

You should include these import statements in the file.

import java.awt.Point; import java.awt.GraphicsEnvironment;

How to Set JFrame in Center of Screen

By default, a java application appears at top left corner of the screen. You might have seen that most of the applications appear in the middle of the screen. I will tell you how to center your java application on screen.

To set JFrame in center of Just call this.setLocationRelativeTo(null); in constructor of the JFrame. If you are using Netbeans IDE, it should be after calling initComponents() method. But if you call setLocation() method of JFrame after this, your JFrame will get relocated.

In most cases this works and this is enough. if you want to get the center point of the screen, just refer to the following post:

How to Get Center Point of Screen in Java

How to Set Background Color of a JFrame in Java

It is easy to set Background color of a JFrame in java. But still many people cannot do it (me too was once like this). Most such people may have tried this java code to set java JFrame background color: setBackground(Color.BLACK);. But this won't work. Instead, you should set the background color of the ContentPane of the JFrame. So, use the following code to set Background color of Frame in your Java Application.

getContentPane().setBackground(Color.BLACK);

or

getContentPane().setBackground(new Color(34,172,176));

Keep in mind that you have to import java.awt.Color; You can use the first one if you don't know RGB values of the desired color. Most common colors are defined as constants in Font class. If you know the RGB color values for your desired color, you can use the second example.

How to Change Look and Feel of a Java Application

The following code fragment which can be used within main method or in constructor of the class tells you how to change the look and feel of a java program (java application). This sample code sets the LookAndFeel of the java application into Windows Look and Feel.

Java Application Source Code to Scroll Text Along JFrame

This is a simple java program for beginners to show how to scroll text in the JFrame. The program uses an anonymous (unnamed) thread object to move the scroll or move the text. The Thread moves two JLabel objects (containing text) by repeatedly calling setLocation(int x, int y) method of JLabel. The speed of the motion of text in this program can be changed by changing value of i or changing sleep period of the thread.

Free, Simple and Light weight Download Manager Java Application - Application and Source Code

This is a free Internet Download Manager Application for PC developed in java and free to use. It is

very light weight with size only below 50KB. This download manager allows pause and resume for downloads and requires java jre 1.8 to run. The program source code is given below. This will help you to understand how to download files from internet using java. The program may help you to understand how to pause and resume downloads in a java program. The application also provides drag and drop support. You can drag a link to file to be downloaded from browser in to the drag and drop area (drop target) in the application and it will automatically start download or add to the download queue. This will also help you study how to use Drag and Drop and how to set a drop target.

Download free Download Manager Application for Computer

You may download the  lightweight java Download Manager Software from following link:

https://drive.google.com/file/d/0B5WYaNmq5rbueUl5bW03VllIc2M/view?usp=sharing

View Source code

Free Two Person Text Chatting Application Software for Local Networks using Java With Source Code

Java text Chatting program

Free two person chat program for local networks. This java text chatting application can be used for two person chatting within private or local networks. This java application uses TCP connection.

How to use

Both users should open this java application in their computer. One of them should click Make a Connection  while the other should enter the first person's ip address in the textfield provided and click Connect to this IP.

YouTube Video Locator For YouTube Partners (Uploaders) to Check Keyword Optimisation in YouTube

In this post, i am sharing an application which will help you to check the search engine optimization of your youtube video. The inputs for this java application are your channel address and the keywords to search for. The application will tell you in which page does your video come for that combination of keywords. This app was coded by myself when i was a youtube uploader. This can be useful for YouTube partners (uploaders). You require JRE 1.8 or later installed in your computer to run this lightweight application software. You can locate your video in youtube search results and check how much search optimised is your video for the given keyword. You can download the java application from following link.

Download YTViewer Youtube Video Locator for YouTube Partners (YouTube upoaders)

Bank Application in Java - Simulation of Banking Services in Java

Java Banking services simulation application

This is a simple bank simulation application in java. The application first asks username and password. The default username and password for the application :

username: operator1
password: password

You can change this username and password in the application. This java bank application software supports following banking services.

• Opening a new account
• Money Withdrawal from the bank account
• Making Deposits in the accout
• Transfer of funds

TCP Server Client Two Person Chat Program in Java with GUI

Server-Client chat program or two way chat using TCP connection is a common problem for java practical labs or networking labs. Here this post introduces a simple lightweight Server client two person chat program using java. The program uses ServerSocket and Socket classes available in java.net package. This program is written so as to use the same as server or as client. Both the persons who chat use the same program. One should select hosting (acting as server) while the other could connect to him by entering the server ip address. I have added screenshots of the program also.
See Screenshots

Simple Pause and Resume Supported Download Manager using Java

In this post i am adding a simple download manager made using java programming language. The file is light weight. This java application supports pause and resume. You can resume a paused download at any time if the url is resume supported. This java desktop application maintains a simple download queue. You can directly drag and drop download links into this java download manager app or add paste links. When the app exits, it saves it current progress and download queue so that you can easily resume the download in this java download manager desktop app when you start the application later.

Free Java Download Manager for 

Features:

• Light weight
• Pause and resume supported
• Drag and drop download links
• Download queuing supported.
• Source code available

Download DownloadManager.jar (app) here.

Download java Download Manager Source code here.