Kod İle JFrame Ekranı Kaplama

import java.awt.*; 
import javax.swing.*; 

public class Test extends JFrame {
    public Test() {
        GraphicsEnvironment env =
        GraphicsEnvironment.getLocalGraphicsEnvironment();
        this.setMaximizedBounds(env.getMaximumWindowBounds());
        this.setExtendedState(this.getExtendedState() | this.MAXIMIZED_BOTH);
    }

    public static void main(String[] args) {
        JFrame.setDefaultLookAndFeelDecorated(true);
        Test t = new Test();
        t.setVisible(true);
    }
}