NullPointerException：尝试在空对象引用上调用接口方法“boolean java.util.Collection.isEmpty()”

Question

我正在尝试从 API 获取一些数据，并且由于我有这个回调指向一个空响应，所以我收到以下错误：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.newsapp.develop, PID: 19479
    java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.Collection.isEmpty()' on a null object reference
        at com.example.newsapp.API.NewsRemoteDataSource$getNewsList$1.onResponse(NewsRemoteDataSource.kt:21)
        at retrofit2.DefaultCallAdapterFactory$ExecutorCallbackCall$1.lambda$onResponse$0$DefaultCallAdapterFactory$ExecutorCallbackCall$1(DefaultCallAdapterFactory.java:89)
        at retrofit2.-$$Lambda$DefaultCallAdapterFactory$ExecutorCallbackCall$1$hVGjmafRi6VitDIrPNdoFizVAdk.run(Unknown Source:6)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7094)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:975)

如何在保留功能的同时修复它？这意味着，保持回调并获得 OnSuccess 的结果。

这是带有回调的适配器：

package com.example.newsapp.Adapters

import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.ImageView
import android.widget.TextView
import androidx.recyclerview.widget.RecyclerView
import com.example.newsapp.ArticlesData
import com.example.newsapp.R
import com.example.newsapp.databinding.NewsItemBinding
import com.squareup.picasso.Picasso

class RecyclerAdapter (private var Articles: List<ArticlesData>) : RecyclerView.Adapter<RecyclerAdapter.ViewHolder>(){

    inner class ViewHolder(
        view: View
    ): RecyclerView.ViewHolder(view){
        private val binding = NewsItemBinding.bind(view)

        val itemTitle: TextView = binding.tvTitle
        val itemImage: ImageView = binding.ivNewsImage

        fun bind(articles: ArticlesData){
            Picasso.get().load(articles.urlToImage).into(binding.ivNewsImage)
            itemTitle.text = articles.Title
        }
    }

    interface Callback {
        fun onArticleClicked(articleClicked: ArticlesData)
    }

    var callback: Callback? = null

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

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.bind(Articles[position])

        holder.itemImage.setOnClickListener {
            callback?.onArticleClicked(Articles[position])
        }

    }

    override fun getItemCount(): Int {
        return Articles.size
    }

    fun setNewsList(newList: List<ArticlesData>) {
        this.Articles = newList
        notifyDataSetChanged()
    }


}

这是问题所在的远程数据源：

package com.example.newsapp.API

import com.example.newsapp.ArticlesData
import com.example.newsapp.NewsListResponse
import retrofit2.Call
import retrofit2.Callback
import retrofit2.Response

class NewsRemoteDataSource {
    fun getNewsList(networkResponse: NetworkResponse<List<ArticlesData>>) {
        val service = RetrofitServices.instance
            .create(APIService::class.java)
            .getNewsList()

        service.enqueue(object : Callback<NewsListResponse> {
            override fun onResponse(
                call: Call<NewsListResponse>,
                response: Response<NewsListResponse>
            ) {
                val resource = response.body()?.run {
                    if (results.isNotEmpty())
                        Resource(NetworkStatus.SUCCESS, results)
                    else
                        Resource(NetworkStatus.ERROR)
                } ?: run {
                    Resource(NetworkStatus.ERROR)
                }
                networkResponse.onResponse(resource)
            }

            override fun onFailure(call: Call<NewsListResponse>, t: Throwable) {
                networkResponse.onResponse(Resource(NetworkStatus.ERROR, message = t.message))
            }
        })
    }


}

interface NetworkResponse<T> {
    fun onResponse(value: Resource<T>)
}

API 响应：

data class NewsResponse (
    @SerializedName("status") var Status: String,
    @SerializedName("totalResults") var totalResults: Int,
    @SerializedName("articles") var Articles: List<ArticlesData>
    )

@Parcelize
data class ArticlesData (
    @SerializedName("source") var Source: @RawValue SourceData,
    @SerializedName("author") var Author: String? = " ",
    @SerializedName("title") var Title: String,
    @SerializedName("description") var Description: String,
    @SerializedName("url") var Url: String,
    @SerializedName("urlToImage") var urlToImage: String? =" ",
    @SerializedName("publishedAt") var publishedAt: String,
    @SerializedName("content") var Content: String? = " "
) : Parcelable

data class SourceData (
    @SerializedName("name") var Name: String,
)

data class NewsListResponse(
    val results: List<ArticlesData>
)

Answer 1

set results?.isNotEmpty() == true 这会导致异常，因为结果可能为空

Answer 2

由于某种原因result 似乎是null，您需要将您的 if 条件修改为

if (results.isNullOrEmpty())
    Resource(NetworkStatus.ERROR)
else
    Resource(NetworkStatus.SUCCESS, results)