我在 kotlin(Android) 的 findviewbyid 函数中面临空指针异常

Question

Android Studio 的 Logcat 声明“Caused by: java.lang.NullPointerException: findViewById(R.id.rvMaps) 不能为空“

由于空指针，安装在 Android 模拟器或物理设备上的应用无法打开 例外。

几天来一直试图找到解决方案，但一直找不到。

帮我解决这个问题。 谢谢！

MainActivity.kt 中的代码是：

package eu.tutorials.mymaps

import androidx.appcompat.app.AppCompatActivity 

import android.os.Bundle

import androidx.recyclerview.widget.LinearLayoutManager

import androidx.recyclerview.widget.RecyclerView

import eu.tutorials.mymaps.models.Place

import eu.tutorials.mymaps.models.UserMap

class MainActivity : AppCompatActivity() {

    private lateinit var rvMaps: RecyclerView
    override fun onCreate(savedInstanceState: Bundle?) {
    
        rvMaps = findViewById(R.id.rvMaps)
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    
        val userMaps = generateSampleData()
        // Set layout manager on the recycler view
        rvMaps.layoutManager = LinearLayoutManager(this)
        // Set adapter on the recycler view
        rvMaps.adapter = MapsAdapter(this, userMaps)
    }
    
    private fun generateSampleData(): List<UserMap> {
        return listOf(
            UserMap(
                "Memories from University",
                listOf(
                    Place("Branner Hall", "Best dorm at Stanford", 37.426, -122.163),
                    Place("Gates CS building", "Many long nights in this basement", 37.430, -122.173),
                    Place("Pinkberry", "First date with my wife", 37.444, -122.170)
                )
            ),
            UserMap("January vacation planning!",
                listOf(
                    Place("Tokyo", "Overnight layover", 35.67, 139.65),
                    Place("Ranchi", "Family visit + wedding!", 23.34, 85.31),
                    Place("Singapore", "Inspired by \"Crazy Rich Asians\"", 1.35, 103.82)
                )),
            UserMap("Singapore travel itinerary",
                listOf(
                    Place("Gardens by the Bay", "Amazing urban nature park", 1.282, 103.864),
                    Place("Jurong Bird Park", "Family-friendly park with many varieties of birds", 1.319, 103.706),
                    Place("Sentosa", "Island resort with panoramic views", 1.249, 103.830),
                    Place("Botanic Gardens", "One of the world's greatest tropical gardens", 1.3138, 103.8159)
                )
            ),
            UserMap("My favorite places in the Midwest",
                listOf(
                    Place("Chicago", "Urban center of the midwest, the \"Windy City\"", 41.878, -87.630),
                    Place("Rochester, Michigan", "The best of Detroit suburbia", 42.681, -83.134),
                    Place("Mackinaw City", "The entrance into the Upper Peninsula", 45.777, -84.727),
                    Place("Michigan State University", "Home to the Spartans", 42.701, -84.482),
                    Place("University of Michigan", "Home to the Wolverines", 42.278, -83.738)
                )
            ),
            UserMap("Restaurants to try",
                listOf(
                    Place("Champ's Diner", "Retro diner in Brooklyn", 40.709, -73.941),
                    Place("Althea", "Chicago upscale dining with an amazing view", 41.895, -87.625),
                    Place("Shizen", "Elegant sushi in San Francisco", 37.768, -122.422),
                    Place("Citizen Eatery", "Bright cafe in Austin with a pink rabbit", 30.322, -97.739),
                    Place("Kati Thai", "Authentic Portland Thai food, served with love", 45.505, -122.635)
                )
            )
        )
    }

}

我不知道这个问题的确切原因，但我尝试访问 rvMaps 回收器视图 通过它在 MainActivity.kt 中的 id，抛出一个 nullpointerexception 使应用程序崩溃。

Answer 1

rvMaps = findViewById(R.id.rvMaps)

在下面写这段代码

setContentView(R.layout.activity_main)

代码

override fun onCreate(savedInstanceState: Bundle?) {

    //rvMaps = findViewById(R.id.rvMaps) //you can't write here
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    rvMaps = findViewById(R.id.rvMaps) //it should be below setcontentview

    val userMaps = generateSampleData()
    // Set layout manager on the recycler view
    rvMaps.layoutManager = LinearLayoutManager(this)
    // Set adapter on the recycler view
    rvMaps.adapter = MapsAdapter(this, userMaps)
}