android 内部类的实例化

如何实现内部类的实例化呢，我的代码错误Unable to instantiate application android.app.Application:

java.lang.NullPointerException
 dev_item.class
 public class dev_item {
    static public final byte DEV_MAX_NUM = 32;
    public byte dev_type;
    public String dev_name;
    public String dev_no;
    public String dev_ip;
    public byte  send_stat;
    public int radio_flag;

    public dev_item(){

    }
}
`



dev_grp.class
public class dev_grp {
    public dev_grp()
    {
        dev_list = new dev_item[dev_item.DEV_MAX_NUM];
        for(int i=0; i<dev_item.DEV_MAX_NUM; i++)
        {
            dev_list[i] = new  dev_item();
            dev_num = 0;
        }
    }
    static public byte dev_num;
    static public byte dev_send_ctrl_index;
    static public dev_item dev_list[];
}
``