使用Kotlin使用OkHTTTP运行Android studio应用程序时出现实例化错误

Question

使用OkHTTP，我试图检索一个文本文件，并将帽子放入主活动的textView中，Gradle会说构建成功，但模拟器应用程序在启动时会崩溃并提供此错误。

我在AndroidManifest.xml中添加了正确的权限，并在渐变构建中添加了错误的实现

package com.example.vectortestapi
import android.os.Bundle
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity
import okhttp3.*
import java.io.IOException

abstract class MainActivity : AppCompatActivity() {
private val mTextView: TextView =   findViewById<TextView>(R.id.resultText);
private val client = OkHttpClient()
private val url = "https://publicobject.com/helloworld.txt"
private val request: Request = Request.Builder()
.url(url)
.build();
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
getAPI();
}
private fun getAPI(){
client.newCall(request).enqueue(object: Callback {
override fun onFailure(call: Call, e: IOException) {
e.printStackTrace()
}
override fun onResponse(call: Call, response: Response) {
val inputStream = response.body?.string()
runOnUiThread{
mTextView.text = inputStream
}
}
})
}
}

产生以下错误：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.vectortestapi, PID: 9900
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.vectortestapi/com.example.vectortestapi.MainActivity}: java.lang.InstantiationException: java.lang.Class<com.example.vectortestapi.MainActivity> cannot be instantiated
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3365)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
Caused by: java.lang.InstantiationException: java.lang.Class<com.example.vectortestapi.MainActivity> cannot be instantiated
at java.lang.Class.newInstance(Native Method)
at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:45)
at android.app.Instrumentation.newActivity(Instrumentation.java:1253)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3353)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
D/libEGL: loaded /vendor/lib/egl/libGLESv2_emulation.so
I/Process: Sending signal. PID: 9900 SIG: 9

Answer 1

抽象类不能实例化，但它们可以被子类化