如果在类初始化期间调用重写的基方法，则java实例字段不会初始化

Question

有一个基类ServerAdapter：

public abstract class ServerAdapter {
    public ServerAdapter() {
        initGUI();
    }

    protected abstract void initGUI();
}

以及继承ServerAdapter的子类：

public abstract class LinuxServerAdapter extends ServerAdapter {
    protected CheckBox key = new CheckBox();

    public LinuxServerAdapter() {
        super();
    }

    @Override
    public void initGUI() {
        //NPE is thrown here, because key = null
        key.addClickHandler(new ClickHandler() {
            @Override
            public void onClick(ClickEvent event) {
                //Something happens here
            }
        });
    }
}

继承LinuxServerAdapter的最终类：

public class MyLinuxServerAdapter extends LinuxServerAdapter {

    public MyLinuxServerAdapter() {
        super();
    }

    public static void main(String args[]) {
        ServerAdapter server = new MyLinuxServerAdapter();
    }
}

当我尝试在键上添加clickHandler时，会抛出NPE

为什么key没有初始化？在这种情况下，初始化顺序是否以特定方式工作