Android 为什么每当我尝试向RecyclerView添加数据类元素时，应用程序就会崩溃

我正在学习如何在大学作业中使用kotlin，我们需要使用GSON保存数据类，并将数据类转换为字符串，使用SharedReferences保存字符串。调用保存的对象后，当我尝试将数据传递给recyclerview时，应用程序崩溃

这是保存对象的位置

package com.example.juliaojonah_hci_outcome2_v1

import android.content.Context
import android.content.Intent
import android.os.Bundle
import android.view.View
import android.widget.Button
import android.widget.EditText
import android.widget.TextView
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import com.google.gson.Gson
import kotlinx.android.synthetic.main.activity_main.*
import java.security.spec.PSSParameterSpec
import java.text.DateFormat
import java.util.*
import kotlin.random.Random


class MainActivity : AppCompatActivity() {

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

        val dateTime = Calendar.getInstance().time

        val dateFormatted = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM).format(dateTime)

        val dateTextView : TextView = findViewById(R.id.textDate) as TextView
        dateTextView.setText(dateFormatted)
        }


    fun secondActivity(view: View){
        val intent = Intent(this, passwords::class.java)
        startActivity(intent)
    }

    fun randomise(view: View){


        val passwordSet1 = Random.nextInt(0,99)
        val passwordSet2 = Random.nextInt(0,99)
        val passwordSet3 = Random.nextInt(0,99)
        val passwordSet4 = Random.nextInt(0,99)

        val editText1 = findViewById<EditText>(R.id.password1)
        editText1.setText(passwordSet1.toString())

        val editText2 = findViewById<EditText>(R.id.password2)
        editText2.setText(passwordSet2.toString())

        val editText3 = findViewById<EditText>(R.id.password3)
        editText3.setText(passwordSet3.toString())

        val editText4 = findViewById<EditText>(R.id.password4)
        editText4.setText(passwordSet4.toString())

    }



    fun save(view: View){

        val sharedPrefsCounter = getSharedPreferences("passwordAppCounter", Context.MODE_PRIVATE)
        var i = sharedPrefsCounter.getInt("counter", 0)

        if (i == 0){
            val editorCounter = sharedPrefsCounter.edit()

            editorCounter.putInt("counter", 1)
            editorCounter.apply()

            i = 1
        }


        val dateTime = Calendar.getInstance().time

        val dateFormatted = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM).format(dateTime)


        val description = descriptionName.text.toString().trim()
        val password = password1.text.toString().trim() + "-" + password2.text.toString().trim() + "-" + password3.text.toString().trim() + "-" + password4.text.toString().trim()
        if (description == "" || password == "---"){
            Toast.makeText(this, "Please Enter Proper Value", Toast.LENGTH_LONG).show()
        } else {
            val date = dateTime.toString()

            val data = PasswordCluster(description, password, date)
            val jsonPassword = Gson().toJson(data)


            val sharedPrefsPasswords =
                getSharedPreferences("passwordAppPasswords", Context.MODE_PRIVATE)
            val editorPassword = sharedPrefsPasswords.edit()

            editorPassword.putString("passwordObject" + i, jsonPassword)

            editorPassword.apply()

            Toast.makeText(this, "Data Saved", Toast.LENGTH_LONG).show()

            val editorCounter = sharedPrefsCounter.edit()

            editorCounter.putInt("counter", i + 1)
            editorCounter.apply()


        }


    }

}

这就是我尝试运行粘贴对象数据的活动时出现的错误

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.juliaojonah_hci_outcome2_v1, PID: 24773
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.juliaojonah_hci_outcome2_v1/com.example.juliaojonah_hci_outcome2_v1.passwords}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.juliaojonah_hci_outcome2_v1.PasswordCluster.getDescription()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.juliaojonah_hci_outcome2_v1.PasswordCluster.getDescription()' on a null object reference
        at com.example.juliaojonah_hci_outcome2_v1.passwords.onCreate(passwords.kt:52)
        at android.app.Activity.performCreate(Activity.java:6662)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6077) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756) 

---

感谢您的帮助

您从SharedReference获得的

objectString

的默认值为空字符串。因此，如果您尝试检索的共享首选项还不存在，则将

“

传递给

Gson.fromJson（）

。然后它返回null，因为它无法将其解析为任何内容

---

因为Gson是一个Java库，所以Kotlin不会对返回值强制null安全性

那么我如何确保它不是空的呢？从共享首选项中提取对象是否工作不正常？如果为空，您可以将其更改为

“”

。我似乎记得，如果您存储空字符串首选项，当您稍后检索它时，它是空的。因为它已经存在，所以得到的是null，而不是传递给

getString（）

的默认值。它起作用了。谢谢我没有意识到修复它只会修复一切。我将数据类的测试版本转换为json字符串，并将其作为默认值传递。我检查了SharedReferences，文件确实存在，所以我不明白为什么它说它是空的

package com.example.juliaojonah_hci_outcome2_v1

data class PasswordCluster(var description: String, var passwordCluster: String, var dateTime: String)

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.juliaojonah_hci_outcome2_v1, PID: 24773
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.juliaojonah_hci_outcome2_v1/com.example.juliaojonah_hci_outcome2_v1.passwords}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.juliaojonah_hci_outcome2_v1.PasswordCluster.getDescription()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.juliaojonah_hci_outcome2_v1.PasswordCluster.getDescription()' on a null object reference
        at com.example.juliaojonah_hci_outcome2_v1.passwords.onCreate(passwords.kt:52)
        at android.app.Activity.performCreate(Activity.java:6662)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6077) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)