Question

I have heard that there are 2 ways that we can set the PLAF in Swing. What is the real meaning of PLAF, and what are those 2 ways?

Answer 1

PLAF = Pluggable Look and Feel. Basically, the same UI can have different skins with very little work.

Check out this site for a detailed description of L&F. In short, there are actually 3 ways to set up your PLAF.

1) Via command line:
java -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel MyApp
or
java -Dswing.defaultlaf=com.sun.java.swing.plaf.windows.WindowsLookAndFeel MyApp

2) Via swing.properties file in your java lib.
swing.defaultlaf=com.sun.java.swing.plaf.windows.WindowsLookAndFeel

3) Programatically with either:

//At the beginning of your program
UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
//Another option
UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel");
//And another
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

//Or during your program - 
UIManager.setLookAndFeel(lnfName);
SwingUtilities.updateComponentTreeUI(frame);
frame.pack();

The above link also has code examples to demonstrate how to change your look and feel.

Hope that helps!