I tried something really stupid but it worked
Code:
import java.awt.Toolkit;
import java.awt.event.FocusEvent;
import java.awt.event.FocusListener;
import javax.swing.JFrame;

public class Util {

    public static void main(String[] args) throws Exception {
	int screenHeight = Toolkit.getDefaultToolkit().getScreenSize().height;
	final JFrame tempFrame = new JFrame();
	tempFrame.addFocusListener(new FocusListener() {

	    public void focusGained(FocusEvent e) {
		tempFrame.dispose();
	    }

	    public void focusLost(FocusEvent e) {
	    }
	});
	tempFrame.setExtendedState(JFrame.MAXIMIZED_BOTH);
	tempFrame.setVisible(true);
	int frameHeight = tempFrame.getHeight();
	int taskbarHeight = screenHeight - frameHeight;
	System.out.println(taskbarHeight);
    }
}