Java构造函数NullPointerException问题

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 4 年前。

我在理解如何创建子类的构造函数(它扩展了父类)时遇到了一些问题。这是我正在使用的代码:

父类构造函数:

public RPCPacket(   int apacketId,
                        RPCPacketType apacketType,
                        RPCOperationType aoperationType,
                        String tmpPacket_objectOid,
                        int aobjectId,
                        int adataSize,
                        String tmpPacket_dataHash,
                        RPCPacketDataType adataType,
                        byte[] apacketData){


        RPCPacket packet=null;

        switch(apacketType){
            case ST_OBJECT_TYPE_INFO_START:
            {
                packet = new InfoStartRPCPacket();
                break;
            }
            case ST_OBJECT_TYPE_INFO_END:
            {
                packet = new InfoEndRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_INFO_ERROR:
            {
                packet = new InfoErrorRPCPacket();

                break;
            }

                // Basic packets
            case ST_OBJECT_TYPE_COLLECTION:
            {
                packet = new CollectionRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_CATEGORY:
            {
                packet = new CategoryRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_CARD:
            {
                packet = new CardRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_MESSAGE:
            {
                packet = new MessageRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_GENRE:
            {
                packet = new GenreRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_TAG:
            {
                packet = new TagRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_USER:
            {
                packet = new UserRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_CARDSTATS_CATEGORY:
            {
                packet = new CardStatsCategoryRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_CONTENT:
            {
                packet = new ContentRPCPacket();

                break;
            }

                // Media packets
            case ST_OBJECT_TYPE_MEDIA_COLLECTION:
            {
                packet = new MediaCollectionRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_MEDIA_CATEGORY:
            {
                packet = new MediaCategoryRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_MEDIA_CARD:
            {
                packet = new MediaCardRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_MEDIA_TAG:
            {
                packet = new MediaTagRPCPacket();

                break;
            }
            case ST_OBJECT_TYPE_MEDIA_COLLECTION_BUTTON:
            {
                packet = new MediaCollectionButtonRPCPacket();

                break;
            }

                // unknown packet
            default: 
            {
                packet=null;

                break;
            }
        }

    }

子类构造函数:

static int apacketId;
static RPCPacketType apacketType;
static RPCOperationType aoperationType;
static String tmpPacket_objectOid;
static int aobjectId;
static int adataSize;
static String tmpPacket_dataHash;
static RPCPacketDataType adataType;
static byte[] apacketData;

public InfoStartRPCPacket() {
        super(apacketId, apacketType, aoperationType, tmpPacket_objectOid, aobjectId,
                adataSize, tmpPacket_dataHash, adataType, apacketData);//line 46 from LogCat error
    }

以及 LogCat 异常:

09-08 11:18:05.614: WARN/System.err(3376): java.lang.NullPointerException
09-08 11:18:05.614: WARN/System.err(3376):     at com.stampii.stampii.comm.rpc.RPCPacket.<init>(RPCPacket.java:68)
09-08 11:18:05.614: WARN/System.err(3376):     at com.stampii.stampii.comm.rpc.InfoStartRPCPacket.<init>(InfoStartRPCPacket.java:46)
09-08 11:18:05.614: WARN/System.err(3376):     at com.stampii.stampii.comm.rpc.RPCPacket.<init>(RPCPacket.java:71)
09-08 11:18:05.614: WARN/System.err(3376):     at com.stampii.stampii.user.UserLogin$2.onClick(UserLogin.java:122)
09-08 11:18:05.614: WARN/System.err(3376):     at android.view.View.performClick(View.java:2408)
09-08 11:18:05.614: WARN/System.err(3376):     at android.view.View$PerformClick.run(View.java:8817)
09-08 11:18:05.614: WARN/System.err(3376):     at android.os.Handler.handleCallback(Handler.java:587)
09-08 11:18:05.614: WARN/System.err(3376):     at android.os.Handler.dispatchMessage(Handler.java:92)
09-08 11:18:05.614: WARN/System.err(3376):     at android.os.Looper.loop(Looper.java:144)
09-08 11:18:05.614: WARN/System.err(3376):     at android.app.ActivityThread.main(ActivityThread.java:4937)
09-08 11:18:05.614: WARN/System.err(3376):     at java.lang.reflect.Method.invokeNative(Native Method)
09-08 11:18:05.614: WARN/System.err(3376):     at java.lang.reflect.Method.invoke(Method.java:521)
09-08 11:18:05.614: WARN/System.err(3376):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
09-08 11:18:05.614: WARN/System.err(3376):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
09-08 11:18:05.624: WARN/System.err(3376):     at dalvik.system.NativeStart.main(Native Method)

有什么建议可以解决我的问题吗？

Answer 1

您收到 NullPointerException 是因为(我假设)您没有实例化您在构造函数中使用的任何静态变量。您需要在调用 InfoStartRPCPacket() 之前传入这些参数的值。

但是，这对我来说看起来像是一个糟糕的面向对象的情况，除非您非常具体地需要在子类中使用静态变量，否则我建议您的子构造函数应该采用参数父级的构造函数，这样您就可以在子级中优雅地使用 super(...) 构造函数。就像这样:

InfoStartRPCPacket(
    int apacketId,
    RPCPacketType apacketType,
    RPCOperationType aoperationType,
    String tmpPacket_objectOid,
    int aobjectId,
    int adataSize,
    String tmpPacket_dataHash,
    RPCPacketDataType adataType,
    byte[] apacketData) {
    super(apacketId, apacketType, aoperationType, tmpPacket_objectOid, aobjectId,
            adataSize, tmpPacket_dataHash, adataType, apacketData);
    //your custom constructor code here
}

这样，在调用构造函数之前，没有人需要知道实例化所有静态类型变量(一种称为顺序耦合的反模式)。

更好的是，如果您在 InfoStartRPCPacket 中没有其他工作需要完成，则完全省略子构造函数。它将简单地继承父级的构造函数。