应用投影的最佳方法是什么？(What's the best way to apply a drop shadow?)

 在将Panel添加到FinestradiRicerca之前，您需要将新元素添加到riquadroRicerca ，我建议您不要使用null布局，而是使用布局管理器或它们的组合。 如果您坚持保留null布局，请参阅下面的示例。 但对于这种应用程序，我建议使用CardLayout 。 
 
 我还建议不要使用多个JFrames因为它们会在用户不友好的任务栏上打开多个窗口。 请参阅： 使用多个JFrame，好/坏练习 
 
 另外，请遵循Java命名约定 。 例如，您将JFrame称为FinestradiRicerca而是将其重命名为： finestradiRicerca （ 变量的第一个字母为小写）。 
 
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class MainWindow {
    // Seconda Finestra
    public static void NuovaFinestra (JPanel panel) {
        panel.setLayout(null);
        JButton Ricerca = new JButton("Ricerca");
        Ricerca.setBounds(100, 100, 200, 50);
        panel.add(Ricerca);
        Ricerca.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                JFrame FinestradiRicerca = new JFrame("Finestra di Ricerca");
                FinestradiRicerca.setBounds(300, 300, 500, 500);
                //If you don't want to close whole app when closing this windo change it to: JFrame.DISPOSE_ON_CLOSE
                FinestradiRicerca.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                JPanel riquadroRicerca = new JPanel();
                JTextField ciao;
                JLabel myLabel = new JLabel("Here goes your label text");
                ciao = new JTextField ();
                ciao.setColumns(20);
                riquadroRicerca.add(myLabel);
                riquadroRicerca.add(ciao);
                FinestradiRicerca.add(riquadroRicerca);
                FinestradiRicerca.setVisible(true);
            }
        });
    }

    //Main  
    public static void main(String[] args) {
        //Finestra Principale
        JFrame finestra = new JFrame("Finestra principale");
        finestra.setSize(500, 500);
        finestra.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //JPanel della finestra principale
        JPanel riquadro = new JPanel();
        finestra.add(riquadro);
        finestra.setVisible(true);
        NuovaFinestra(riquadro);
    }
}
 
 因此，经过一些修改后，为了使JLabel和JTextField可见，您的代码将提供以下输出： 
 
  
 
 但请遵循我的上述建议。 

You needed to add your new elements to riquadroRicerca BEFORE adding the Panel to FinestradiRicerca, I recommend you NOT to use null layout but a Layout Manager or combinations of them. If you insist on keeping null layout then see below example. But for this kind of app I'd suggest a CardLayout.
 
I also suggest not using multiple JFrames since they will open multiple windows on task bar which is user unfriendly. See: Use of multiple JFrames, Good / Bad Practice
 
As an aside note, follow Java naming conventions. For example you called a JFrame as FinestradiRicerca instead rename it to: finestradiRicerca (1st letter of a variable in lowercase).
 
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class MainWindow {
    // Seconda Finestra
    public static void NuovaFinestra (JPanel panel) {
        panel.setLayout(null);
        JButton Ricerca = new JButton("Ricerca");
        Ricerca.setBounds(100, 100, 200, 50);
        panel.add(Ricerca);
        Ricerca.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                JFrame FinestradiRicerca = new JFrame("Finestra di Ricerca");
                FinestradiRicerca.setBounds(300, 300, 500, 500);
                //If you don't want to close whole app when closing this windo change it to: JFrame.DISPOSE_ON_CLOSE
                FinestradiRicerca.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                JPanel riquadroRicerca = new JPanel();
                JTextField ciao;
                JLabel myLabel = new JLabel("Here goes your label text");
                ciao = new JTextField ();
                ciao.setColumns(20);
                riquadroRicerca.add(myLabel);
                riquadroRicerca.add(ciao);
                FinestradiRicerca.add(riquadroRicerca);
                FinestradiRicerca.setVisible(true);
            }
        });
    }

    //Main  
    public static void main(String[] args) {
        //Finestra Principale
        JFrame finestra = new JFrame("Finestra principale");
        finestra.setSize(500, 500);
        finestra.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //JPanel della finestra principale
        JPanel riquadro = new JPanel();
        finestra.add(riquadro);
        finestra.setVisible(true);
        NuovaFinestra(riquadro);
    }
}
 
So, your code after a few modifications, to make JLabel and JTextField visible gives the following output:
 
 
However, please follow my above recomendations.