SwipeRefreshLayout 尝试在空对象引用上调用虚拟方法 [重复]

Question

我是安卓开发新手。我现在才做了大约一个月，所以如果这是一个简单的问题，我深表歉意。 尝试运行我的应用程序时，我不断收到以下错误

     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.swiperefreshlayout.widget.SwipeRefreshLayout.setOnRefreshListener(androidx.swiperefreshlayout.widget.SwipeRefreshLayout$OnRefreshListener)' on a null object reference
        at edu.cogswell.tn.prism.MainActivity.onCreate(MainActivity.kt:54)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

这是我的代码

package (package name)
import android.app.AlertDialog
import android.os.Bundle
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.navigation.findNavController
import androidx.navigation.ui.AppBarConfiguration
import androidx.navigation.ui.setupActionBarWithNavController
import androidx.navigation.ui.setupWithNavController
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.swiperefreshlayout.widget.SwipeRefreshLayout
import com.google.android.material.bottomnavigation.BottomNavigationView
import com.google.gson.Gson
import edu.cogswell.tn.prism.`interface`.NewsService
import edu.cogswell.tn.prism.adapter.viewHolder.ListSourceAdapter
import edu.cogswell.tn.prism.common.Common
import edu.cogswell.tn.prism.modle.WebSite
import io.paperdb.Paper
import kotlinx.android.synthetic.main.fragment_central.*
import retrofit2.Call
import retrofit2.Response
import javax.security.auth.callback.Callback

class MainActivity : AppCompatActivity() {

    lateinit var layoutManager: LinearLayoutManager
    lateinit var myService: NewsService
    lateinit var adapter: ListSourceAdapter
    lateinit var dialog: AlertDialog
    var mSwipeRefreshLayout: SwipeRefreshLayout? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val bottomNavigationView = findViewById<BottomNavigationView>(R.id.bottomNaigationView)
        val navController = findNavController(R.id.fragment)

        val appBarConfiguration = AppBarConfiguration(
            setOf(
                R.id.central,
                R.id.leftWing,
                R.id.rightWing
            )
        )
        setupActionBarWithNavController(navController, appBarConfiguration)
        bottomNavigationView.setupWithNavController(navController)

        Paper.init(this)

        myService = Common.newsService

        swipe_to_refresh_centreal.setOnRefreshListener {
            loadWebSiteSource(true)
        }

        recycler_view_source_news_central.setHasFixedSize(true)
        layoutManager = LinearLayoutManager(this)
        recycler_view_source_news_central.layoutManager = layoutManager

        //dialog = SpotsDialog.Builder().setContext(this).build()

        loadWebSiteSource(false)

    }

    private fun loadWebSiteSource(isRefresh: Boolean) {
        if(!isRefresh){
            val cache = Paper.book().read<String>("cache")
            if(cache != null && !cache.isBlank() && cache != "null"){
                val website = Gson().fromJson<WebSite>(cache, WebSite::class.java)
                adapter = ListSourceAdapter(baseContext, website)
                adapter.notifyDataSetChanged()
                recycler_view_source_news_central.adapter = adapter
            }
            else{
                dialog.show()
                myService.sources.enqueue(object:retrofit2.Callback<WebSite>{
                    override fun onResponse(call: Call<WebSite>, response: Response<WebSite>) {
                        adapter = ListSourceAdapter(baseContext,response!!.body()!!)
                        adapter.notifyDataSetChanged()
                        recycler_view_source_news_central.adapter = adapter

                        Paper.book().write("cache", Gson().toJson(response!!.body()!!))
                        dialog.dismiss()
                    }

                    override fun onFailure(call: Call<WebSite>, t: Throwable) {
                        Toast.makeText(baseContext,"Failed",Toast.LENGTH_SHORT).show()
                    }
                })
            }
        }
        else{
            swipe_to_refresh_centreal.isRefreshing = true

            myService.sources.enqueue(object:retrofit2.Callback<WebSite>{
                override fun onResponse(call: Call<WebSite>, response: Response<WebSite>) {
                    adapter = ListSourceAdapter(baseContext,response!!.body()!!)
                    adapter.notifyDataSetChanged()
                    recycler_view_source_news_central.adapter = adapter

                    Paper.book().write("cache", Gson().toJson(response!!.body()!!))
                    swipe_to_refresh_centreal.isRefreshing=false
                }

                override fun onFailure(call: Call<WebSite>, t: Throwable) {
                    Toast.makeText(baseContext,"Failed",Toast.LENGTH_SHORT).show()
                }
            })
        }
    }
}

这里是xml代码

<?xml version="1.0" encoding="utf-8"?>
<androidx.swiperefreshlayout.widget.SwipeRefreshLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/swipe_to_refresh_centreal"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".Central">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/recycler_view_source_news_central"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_margin="10dp" />

    </RelativeLayout>

</androidx.swiperefreshlayout.widget.SwipeRefreshLayout>

不太清楚为什么我不断收到此错误。我正在使用本教程https://www.youtube.com/watch?v=mvVcqzJYAbY 供参考。

提前谢谢你。

Answer 1

我看到你在这里使用了 kotlin 合成属性。它已被弃用。所以我建议改用任何其他乐器。在我看来，即使 findViewById 也更好。 那么这里有什么问题。这是与 kotlin 合成属性相关的常见问题 - 错误的导入。 看，你在一个活动中做了所有的动作，但是被导入了

import kotlinx.android.synthetic.main.fragment_central.*

所以这一行从名为fragment_central 的布局中导入视图。但是您反而夸大了activity_main。所以这里根本就不存在这种观点。

Answer 2

您正在导入错误的布局。你能试试这个。在导入部分传递您的主要活动布局，并从导入以及变量声明中删除 swiperefresh 布局。 import kotlinx.android.synthetic.main.your_layout.*