Customize single cells in JTable

Categories Java, Languages

JTables are very useful for develop desktop applications but when it comes to customize properties at cell level it can be a little tricky. The key concept here is that we have to deal with Cell Renderers for each column. TableCellRenderer is an interface that forces to implement the following method:

Component getTableCellRendererComponent(JTable table, Object value,
boolean isSelected, boolean hasFocus, int row, int column)

The most common way to customize Cell Rendering is to create a class that extends a Component and implements TableCellRenderer. Most people use a simple JLabel. For example,  the following class will allow us to change the background of a single cell:

public class JLabelCellRenderer extends JLabel implements TableCellRenderer
  public Component getTableCellRendererComponent(JTable table,Object value,
  boolean isSelected,boolean hasFocus, int row,int col)
    JLabel label = (JLabel) value;
    setOpaque(true); /* Important to see the background color */
    return this;

Now lets create our table and use our custom cell renderer:

JTable table = new JTable();
String[] columnNames = {"Red","Blue"}; /* Two columns */
JLabel[][] data = new JLabel[1][2]; /* One row, two columns */
data[0][0] = new JLabel("");
data[0][1] = new JLabel("");
table.setModel(new DefaultTableModel(data, columnNames));
/* Now we use our new class for rendering cells */
table.getColumnModel().getColumn(0).setCellRenderer(new JLabelCellRenderer());
table.getColumnModel().getColumn(1).setCellRenderer(new JLabelCellRenderer());

Now we put it in a JFrame and the result is the following:

JTable 2 cell colors example
So it worked! Now you can improve the cell renderer and get more properties from the JLabel such as Font, Foreground, Border… etc.
I’ve developed a more complex but funnier example in which I’ve used most common methods and properties to customize the aspect of a JTable. A couple of screenshots are shown below so you can get the idea.

JTable example serious JTable example smiling


You can get the source code -> (363 downloads)

and the executable jar file ->  Smiley.jar (275 downloads)


Unsupported major.minor version 51.0

Categories Java, Languages

The other day I was trying to compile and run some swing example code using the command line and when I tried to execute I got the following error:

Exception in thread “main” java.lang.UnsupportedClassVersionError: Main : Unsupported major.minor version 51.0

I found on the Internet that version 51.0 corresponds to java 1.7 and then I realised that I have jdk 1.7 installed while my jre is 1.6.

I usually don’t care about this compatibilty issues because NetBeans takes care of this.

So the solution is to tell javac to compile it for 1.6:

javac -source 1.6 -target 1.6

And then it worked like a charm.


This kind of error is due to version differences between the binaries and the virtual machine and can be solved like I wrote above. Having said that remember that if you have some incompatible source code between versions javac compiler will complain.


Working with Java in the console (terminal)

Categories Backtrack, Debian, Java, Languages, Linux

I think this is very useful. I know there are powerful script languages like python or ruby which, by the way, I strongly recommend you to learn. But maybe you feel comfortable (like me) using Java for simple and not so simple programs or scripts. I’m writing this post because the other day I wanted to create a word list file to use in Backtrack with specific restrictions that I had in mind. It had been long time without using Ruby and I had to refresh my knowledge about it to make it. But then I thought that I could have made the word list faster just with a few lines of Java. But of course we don’t want to use an IDE like Eclipse o NetBeans for this kind of programs, we just want our lovely console :).

Enough chatter let’s go to practice. We’re going to create a and run it all from the console.

First of all we need to install the Java Development Kit (JDK) in order to run the Java compiler (javac). We can use the one from Sun, but I’ll be using openjdk which is installed in my Backtrack.

If you don’t have it installed yet, just type:

apt-get install openjdk-6-jdk


apt-get install default-jdk

We are going to use the Java Compiler (javac) to create the binary file HelloWorld.class, the one that can be executed by the Java Virtual Machine (JVM).

Let’s write the source code. Use your favorite editor, I’ll use nano:


And now the code:

/* Java Hello World from mendrugox */
public class HelloWorld
	public static void main(String[] args)
		System.out.println("Hello World!");

Save and close (Ctrl +x in nano).

Now we have our code in the file, let’s compile it:


The file HelloWorld.class is created and now we can execute our program:

java HelloWorld

*Notice that I put HelloWorld and not HelloWorld.class.


We’re done.


Using resources from inside .jar package.

Categories Java, Languages

When we pack all our java classes, libraries and resources in a single .jar in order to share our application, if our code has references to resources using absolute or relative paths to the files in our working directory like for example:

ImageIcon image = new ImageIcon('/gui/images/logo.png');

We won’t be able to get this resource when executing our standalone .jar file. We’ll have to use the getResource() method from the class. Imagine that we are using Swing for creating a cool GUI for our application. Now we want to view our custom logo in our JFrame class instead the default one. And, of course, we want this to be working even when we distribute our app in a single .jar package. If our logo were located at /src/gui/images/logo.png,  this is the code that we’ll be using for our purpose inside the JFrame class:

private void customize()
            URL res = getClass().getResource("/gui/images/logo.png");
            ImageIcon image = new ImageIcon(res);
        catch (Exception ex)

The same way for any other resources like files or music :).