Swing-JFrame窗体居中

Question

Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        Dimension frameSize = this.getSize();
        if(frameSize.height > screenSize.height) {
            frameSize.height = screenSize.height;
        }
        if(frameSize.width > screenSize.width) {
            frameSize.width = screenSize.width;
        }
        if(frameSize.height <= 0 || frameSize.width <= 0) {
            this.setLocation(100, 100);            // 有些窗口的大小在显示前是不可知的，
            //     只能固定它的位置
        }else {
            this.setLocation((screenSize.width - frameSize.width) / 2,
                          (screenSize.height - frameSize.height) / 2);
        }
    }