ResetText

C#:
using System.Linq;
using System.Windows.Forms;

public static void ClearText(this GroupBox gb)
{
    foreach (ListControl list in gb.Controls.OfType<ComboBox>())
    {
        list.ResetText();
        list.SelectedIndex = -1;
    }
    foreach (TextBoxBase text in gb.Controls.OfType<TextBoxBase>())
    {
        text.Clear();
    }
    foreach (DateTimePicker dtp in gb.Controls.OfType<DateTimePicker>())
    {
        dtp.ResetText();
    }
    foreach (Control ctrl in GroupBox.Controls)
    {
        if (ctrl is TextBoxBase || ctrl is ListControl || ctrl is DateTimePicker)
            ctrl.ResetText();
        ListControl list = ctrl as ListControl;
        if (list != null)
            list.SelectedIndex = -1;
    }
}

JAVA:
import javax.swing.JPanel;
import javax.swing.JComboBox;
import javax.swing.JTextArea;
import javax.swing.JTextField;

for(Component cpt : JPanel.getComponents())
{
    if (cpt instanceof JComboBox)
    {
        ((JComboBox)cpt).setSelectedItem(null);
        ((JComboBox)cpt).setSelectedIndex(-1);
        continue;
    }
    if (cpt instanceof JTextComponent)
        ((JTextComponent)cpt).setText(null);
}