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

public class SimpleFrameTest {
	public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable(){   //  开一个线程
			public void run() {
				SimpleFrame frame = new SimpleFrame();
				frame.setTitle("记事本");    // 设置title
				frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
				frame.setVisible(true);
			}
		});
	}
}

class SimpleFrame extends JFrame{

	/*
     *通过调用Toolkit类静态方法getDefaultToolkit得到一个Toolkit对象 ,
     *调用getScreenSize方法,返回屏幕大小
	 */
	Toolkit kit = Toolkit.getDefaultToolkit();  
	Dimension screenSize = kit.getScreenSize();
	int screenHeight = screenSize.height;   // 屏幕实际高
	int screenWidth = screenSize.width;     //屏幕实际宽

    
    public SimpleFrame () {
    	setSize(screenWidth/2,screenHeight/2);  // 设置显示的窗口的大小
    }
}

结果：