如何防止 android 应用程序因 recyclerview 中的空列表而崩溃？

Question

这是我的适配器 class。当我打开回收器视图时，如果列表为空应用程序崩溃，否则应用程序运行顺利。我想我必须对清单做点什么。我的意思是，如果它为空，请做一些可以防止应用程序崩溃的事情。有人可以帮我吗？

class adsAdapter(
    private val adsList: List<adsModel>,
    private val listener: ManageAdsFragment
    ): RecyclerView.Adapter<adsAdapter.adsViewHolder>(){


    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): adsViewHolder {
        val itemView = LayoutInflater.from(parent.context).inflate(R.layout.ads_item,
            parent, false)
        return adsViewHolder(itemView)
    }

    override fun onBindViewHolder(holder: adsViewHolder, position: Int) {
        holder.title.text = adsList[position].title
        val quantity =  adsList[position].quantity
        val watched = adsList[position].watched
        val adData =  "$watched/$quantity"
        holder.adsData.text = adData
        val status: String = adsList[position].status.toString()
        when{
            status == "1" -> {
                holder.onlineImageView.visibility = View.VISIBLE
                holder.completeImageView.visibility = View.INVISIBLE
                holder.cancelImageView.visibility = View.INVISIBLE
            }
            status == "2" -> {
                holder.onlineImageView.visibility = View.INVISIBLE
                holder.completeImageView.visibility = View.VISIBLE
                holder.cancelImageView.visibility = View.INVISIBLE
            }
            status == "3" ->{
                holder.onlineImageView.visibility = View.INVISIBLE
                holder.completeImageView.visibility = View.INVISIBLE
                holder.cancelImageView.visibility = View.VISIBLE
            }
        }
    }

    override fun getItemCount() = adsList.size

    inner class adsViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView)
    ,View.OnClickListener{
        val onlineImageView: ImageView = itemView.findViewById(R.id.onlineImageView)
        val completeImageView: ImageView = itemView.findViewById(R.id.completeImageView)
        val cancelImageView: ImageView = itemView.findViewById(R.id.cancelImageView)
        var adsData: TextView = itemView.findViewById<TextView>(R.id.adsDataTextView)
        var title: TextView = itemView.findViewById<TextView>(R.id.titleTextView)
        init {
            itemView.setOnClickListener(this)
        }

        override fun onClick(v: View?) {
            val position: Int = adapterPosition
            if (position != RecyclerView.NO_POSITION) {
                listener.onItemClick(position)
            }
        }
    }

    interface OnItemClickListener{
        fun onItemClick(position: Int)
    }


}

这是logcat

2021-08-27 06:13:42.176 2355-2411/com.crazybot.rewardoomadvertiser D/EGL_emulation: eglMakeCurrent: 0x98714a40: ver 2 0 (tinfo 0x853620a0)
2021-08-27 06:13:42.212 2355-2360/com.crazybot.rewardoomadvertiser I/art: Do partial code cache collection, code=61KB, data=62KB
2021-08-27 06:13:42.215 2355-2360/com.crazybot.rewardoomadvertiser I/art: After code cache collection, code=60KB, data=62KB
2021-08-27 06:13:42.215 2355-2360/com.crazybot.rewardoomadvertiser I/art: Increasing code cache capacity to 256KB
2021-08-27 06:13:42.528 2355-2396/com.crazybot.rewardoomadvertiser D/FA: Connected to remote service
2021-08-27 06:13:42.529 2355-2396/com.crazybot.rewardoomadvertiser V/FA: Processing queued up service tasks: 5
2021-08-27 06:13:44.068 2355-2355/com.crazybot.rewardoomadvertiser E/RecyclerView: No adapter attached; skipping layout
2021-08-27 06:13:44.081 2355-2355/com.crazybot.rewardoomadvertiser D/AndroidRuntime: Shutting down VM
2021-08-27 06:13:44.083 2355-2355/com.crazybot.rewardoomadvertiser E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.crazybot.rewardoomadvertiser, PID: 2355
    java.lang.NullPointerException: null cannot be cast to non-null type kotlin.collections.List<com.crazybot.rewardoomadvertiser.model.adsModel>
        at com.crazybot.rewardoomadvertiser.ui.fragment.ManageAdsFragment$getData.onResponse(ManageAdsFragment.kt:58)
        at retrofit2.DefaultCallAdapterFactory$ExecutorCallbackCall.lambda$onResponse[=12=]$DefaultCallAdapterFactory$ExecutorCallbackCall(DefaultCallAdapterFactory.java:89)
        at retrofit2.DefaultCallAdapterFactory$ExecutorCallbackCall$$ExternalSyntheticLambda1.run(Unknown Source)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)

ManageAdsFragment 中的功能（我在其中获取回收站视图的数据），最后我使用改造从 rest API 获取数据。当我在回收站视图中使用 firebase 时，我没有收到此类错误

 private fun getData() {
        val uid = auth.uid.toString()
        val call: Call<List<adsModel?>?>? = apiController
            .instance?.api?.getAds(uid)
        call?.enqueue(object : Callback<List<adsModel?>?>{
            override fun onResponse(
                call: Call<List<adsModel?>?>,
                response: Response<List<adsModel?>?>
            ) {
                obj = response.body()
                val adapter: adsAdapter = adsAdapter(obj as List<adsModel>, this@ManageAdsFragment)
                binding.recyclerview.layoutManager = LinearLayoutManager(requireContext())
                binding.recyclerview.adapter = adapter
                binding.recyclerview.setHasFixedSize(true)
            }

            override fun onFailure(call: Call<List<adsModel?>?>, t: Throwable) {
                Toast.makeText(requireContext(), t.message.toString(), Toast.LENGTH_SHORT).show()
            }

        })
    }

Answer 1

确保在行 obj = response.body() 中您没有得到空值

Answer 2

您的 obj 在这一行为空：

val adapter: adsAdapter = adsAdapter(obj as List<adsModel>, this@ManageAdsFragment)

最好这样做：

val adapter = adsAdapter(
    obj ? as List<adsModel> : emptyList(), 
    this@ManageAdsFragment
)

Answer 3

java.lang.NullPointerException: null cannot be cast to non-null type kotlin.collections.List<com.crazybot.rewardoomadvertiser.model.adsModel>

您正在尝试将 null 转换为在以下转换中引发的非 null 类型：

val adapter: adsAdapter = adsAdapter(obj as List<adsModel>, this@ManageAdsFragment)

obj 是 null，这是真的，因为 API 响应在某些情况下可以为 null。

并且非 null 类型是 adsAdapter 构造函数的第一个参数，您没有使用 Optional ?:

指定它可以为空

因此，要解决此问题：

使 adsAdapter 的 List<adsModel> 参数可为空，这需要您将 adsList 的每个实例替换为 adsList?:

class adsAdapter(
    private val adsList: List<adsModel>?,

在实例化适配器之前检查 obj 是否为空：

call?.enqueue(object : Callback<List<adsModel?>?>{
    override fun onResponse(
        call: Call<List<adsModel?>?>,
        response: Response<List<adsModel?>?>
    ) {
        obj = response.body()
        if (obj != null) {
            val adapter: adsAdapter = adsAdapter(obj as List<adsModel>, this@ManageAdsFragment)
            binding.recyclerview.layoutManager = LinearLayoutManager(requireContext())
            binding.recyclerview.adapter = adapter
            binding.recyclerview.setHasFixedSize(true)
        }
    }

    override fun onFailure(call: Call<List<adsModel?>?>, t: Throwable) {
        Toast.makeText(requireContext(), t.message.toString(), Toast.LENGTH_SHORT).show()
    }

})

Answer 4

@zain 答案是正确的，但我认为您必须更改适配器中的一行代码还不够 class

更改此行

override fun getItemCount() = adsList.size

对此

 val adapter: adsAdapter = adsAdapter(obj as List<adsModel>?, this@ManageAdsFragment)

如何防止 android 应用程序因 recyclerview 中的空列表而崩溃？

how to prevent android app crashing because of empty list on recycler view?

android

android-adapter

kotlin

android-recyclerview