To obtain the current screen size requires interfacing with the native window system. The Java AWT library allows this
through the Toolkit class. You will be using:

Toolkit.getDefaultToolkit()
getScreenSize() 

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import java.awt.*;

public final class ScreenSize {
public static final void main(String[] args) {
Dimension size;
size = Toolkit.getDefaultToolkit().getScreenSize();
System.out.println("Width: " + size.width + " Height: " + size.height);
System.exit(0);
}
}