Void类是一个不可实例化的占位符类，用于保存对表示Java关键字void的Class对象的引用。 应用示例 ``` package com.amztools; import java.awt.Toolkit; import java.util.concurrent.TimeUnit; import javax.swing.SwingWorker; import com.amztools.dao.KeyDao; public class Task extends SwingWorker<Void, Void>{ private KWResult ui; public Task(KWResult ui) { this.ui = ui; } @Override protected Void doInBackground() throws Exception { int all = KeyDao.me.count(); ui.getProgressBar().setMaximum(all); int done = KeyDao.me.countDone(); while (done != all) { try { // System.err.println(done); // done = KeyDao.me.countDone(); setProgress(done); TimeUnit.SECONDS.sleep(5); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } } return null; } @Override protected void done() { Toolkit.getDefaultToolkit().beep(); ui.getBtnImport().setEnabled(true); // startButton.setEnabled(true); } } ```