所以现在我的应用程序（二维码扫描器）在运行时崩溃了。调试产生的错误消息如下：
e/androidruntime:致命异常：主进程：com.example.a16007637.qrcodereader，pid:4032 java.lang.runtimeexception:无法启动activity componentinfo{com.example.a16007637.qrcodereader/com.example.a16007637.qrcodereader.mainactivity}:java.lang.classcastexception:android.support.constraint.constraintlayout无法强制转换为android.widget.textview atandroid.app.activitythread.performlaunchactivity（活动线程。java:2778)在android.app.activitythread.handlelaunchactivity（activitythread。java:2856)在android.app.activitythread.-wrap11（未知）source:0)在android.app.activitythread$h.handlemessage上。java:1589)在android.os.handler.dispatchmessage（handler。java:106)在android.os.looper.loop（looper。java:164)在android.app.activitythread.main（activitythread。java:6494)位于com.android.internal.os.runtimeinit$methodAndArgscaler.run（runtimeinit）的java.lang.reflect.method.invoke（本机方法）。java:438)在com.android.internal.os.zygoteinit.main（zygoteinit。java:807)原因：java.lang.classcastexception：android.support.constraint.constraintlayout不能强制转换为com.example.a16007637.qrcodereader.mainactivity.oncreate（mainactivity）上的android.widget.textview。java:33)在android.app.activity.performcreate（activity。java:7009)在android.app.activity.performcreate（activity。java:7000)在android.app.instrumentation.callactivityoncreate（instrumentation。java:1214)在android.app.activitythread.performlaunchactivity（activitythread。java:2731)在android.app.activitythread.handlelaunchactivity（activitythread。java:2856)  在android.app.activitythread.-wrap11（未知）source:0)  在android.app.activitythread$h.handlemessage上。java:1589)  在android.os.handler.dispatchmessage（handler。java:106)  在android.os.looper.loop（looper。java:164)  在android.app.activitythread.main（activitythread。java:6494)  位于java.lang.reflect.method.invoke（本机方法）  在com.android.internal.os.runtimeinit$methodandargscaller.run（runtimeinit。java:438)  在com.android.internal.os.zygoteinit.main（zygoteinit。java:807)
最吸引我眼球的是这句话：
constraintlayout不能强制转换为android.widget.textview
我试图对此进行更多的研究，但不幸的是，我在网上找到的所有答案都不适用于我。
我的mainactivity.java代码是：

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
import android.view.View;
import com.google.zxing.integration.android.IntentIntegrator;
import com.google.zxing.integration.android.IntentResult;
import org.json.JSONException;
import org.json.JSONObject;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private Button Scan;
    private TextView QR_output;
    private IntentIntegrator ScanCode;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        // Defines the Scan button
        Scan = findViewById(R.id.Scan);
        // defines the output for text
        QR_output = findViewById(R.id.output);

        // looks for the user clicking "Scan"
        Scan.setOnClickListener(this);

        ScanCode = new IntentIntegrator(this);
        // Means the scan button will actually do something
        Scan.setOnClickListener(this);
    }

    // will scan the qr code and reveal its secrets
    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        IntentResult result = IntentIntegrator.parseActivityResult(requestCode, resultCode, data);
        if (result != null) {
            // if an empty QR code gets scanned it returns a message to the user
            //if qr contains data
            if (result.getContents() == null) {
                Toast.makeText(this, "This QR code is empty.", Toast.LENGTH_LONG).show();
            } else try {
                //converting the data to json
                JSONObject obj = new JSONObject(result.getContents());
                //setting values to textviews
                QR_output.setText(obj.getString("name"));
            } catch (JSONException e) {
                e.printStackTrace();
                //if control comes here
                //that means the encoded format not matches
                //in this case you can display whatever data is available on the qrcode
                //to a toast
                Toast.makeText(this, result.getContents(), Toast.LENGTH_LONG).show();
            }
        } else {
            super.onActivityResult(requestCode, resultCode, data);
        }
    }
    @Override
    public void onClick(View view) {
        //initiating the qr code scan
        ScanCode.initiateScan();
    }
}

我的xml文件在这里：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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/output"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/Scan"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="148dp"
        android:layout_marginTop="514dp"
        android:layout_marginEnd="148dp"
        android:layout_marginBottom="5dp"
        android:text="@string/Scan"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0" />

    <TextView
        android:id="@+id/QROutput"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="172dp"
        android:layout_marginTop="358dp"
        android:layout_marginEnd="166dp"
        android:layout_marginBottom="137dp"
        android:text="output"
        app:layout_constraintBottom_toTopOf="@+id/Scan"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>

如能帮助解决这一问题，我们将不胜感激。