JAVAClassCastException:安卓。支持限制ConstraintLayout无法强制转换为安卓。小装置。文本框
所以现在我的应用程序（二维码扫描器）在我尝试运行时崩溃了。调试产生的错误消息如下：
E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.a16007637.qrcodereader, PID: 4032 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.a16007637.qrcodereader/com.example.a16007637.qrcodereader.MainActivity}: java.lang.ClassCastException: 安卓.support.constraint.ConstraintLayout cannot be cast to 安卓.widget.TextView at 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778) at 安卓.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) at 安卓.app.ActivityThread.-wrap11(Unknown Source:0) at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) at 安卓.os.Handler.dispatchMessage(Handler.java:106) at 安卓.os.Looper.loop(Looper.java:164) at 安卓.app.ActivityThread.main(ActivityThread.java:6494) at java.lang.reflect.Method.invoke(Native Method) at com.安卓.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:807) Caused by: java.lang.ClassCastException: 安卓.support.constraint.ConstraintLayout cannot be cast to 安卓.widget.TextView at com.example.a16007637.qrcodereader.MainActivity.onCreate(MainActivity.java:33) at 安卓.app.Activity.performCreate(Activity.java:7009) at 安卓.app.Activity.performCreate(Activity.java:7000) at 安卓.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214) at 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731) at 安卓.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) at 安卓.app.ActivityThread.-wrap11(Unknown Source:0) at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) at 安卓.os.Handler.dispatchMessage(Handler.java:106) at 安卓.os.Looper.loop(Looper.java:164) at 安卓.app.ActivityThread.main(ActivityThread.java:6494) at java.lang.reflect.Method.invoke(Native Method) at com.安卓.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:807)
吸引我眼球的主要是这句话：
ConstraintLayout cannot be cast to 安卓.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
我的主要活动。java代码是：
import 安卓.content.Intent;
import 安卓.os.Bundle;
import 安卓.support.v7.app.AppCompatActivity;
import 安卓.widget.Button;
import 安卓.widget.TextView;
import 安卓.widget.Toast;
import 安卓.view.View;
import com.google.zxing.integration.安卓.IntentIntegrator;
import com.google.zxing.integration.安卓.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"?>
<安卓.support.constraint.ConstraintLayout xmlns:安卓="http://schemas.安卓.com/apk/res/安卓"
xmlns:app="http://schemas.安卓.com/apk/res-auto"
xmlns:tools="http://schemas.安卓.com/tools"
安卓:id="@+id/output"
安卓:layout_width="match_parent"
安卓:layout_height="match_parent"
tools:context=".MainActivity">
<Button
安卓:id="@+id/Scan"
安卓:layout_width="wrap_content"
安卓:layout_height="wrap_content"
安卓:layout_marginStart="148dp"
安卓:layout_marginTop="514dp"
安卓:layout_marginEnd="148dp"
安卓:layout_marginBottom="5dp"
安卓: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
安卓:id="@+id/QROutput"
安卓:layout_width="wrap_content"
安卓:layout_height="wrap_content"
安卓:layout_marginStart="172dp"
安卓:layout_marginTop="358dp"
安卓:layout_marginEnd="166dp"
安卓:layout_marginBottom="137dp"
安卓:text="output"
app:layout_constraintBottom_toTopOf="@+id/Scan"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</安卓.support.constraint.ConstraintLayout>
如能为解决此问题提供任何帮助，我们将不胜感激
# 1 楼答案
您为TextView写入了错误的id。您编写的id指向XML上的ConstraintLayout对象
而不是（第31行）：
写下：
# 2 楼答案
您已经将id
output用于根布局。i、 e.ConstraintLayout，它将为您提供ConstraintLayout的参考。显然，ConstraintLayout的引用不能存储在类型为TextView的变量中，如果这样做，就会引发异常使用以下代码引用
TextView