java.lang.ClassCastException:android.support.constraint.ConstraintLayout无法转换为android.widget.TextView

时间:2018-12-07 06:40:30

标签: java android android-studio

因此,当我尝试运行它时,我的应用程序(QR码扫描仪)当前崩溃。调试产生的错误消息是:

  

E / Android运行时:致命异常:主要       流程:com.example.a16007637.qrcodereader,PID:4032       java.lang.RuntimeException:无法启动活动ComponentInfo {com.example.a16007637.qrcodereader / com.example.a16007637.qrcodereader.MainActivity}:java.lang.ClassCastException:   android.support.constraint.ConstraintLayout无法转换为   android.widget.TextView           在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)           在android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)           在android.app.ActivityThread.-wrap11(未知来源:0)           在android.app.ActivityThread $ H.handleMessage(ActivityThread.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)           在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)        造成原因:java.lang.ClassCastException:android.support.constraint.ConstraintLayout无法强制转换为   android.widget.TextView           在com.example.a16007637.qrcodereader.MainActivity.onCreate(MainActivity.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(未知来源:0)           在android.app.ActivityThread $ H.handleMessage(ActivityThread.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)           在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)

引起我注意的主要内容是此行:

  

ConstraintLayout无法转换为android.widget.TextView

我尝试对此进行更多的研究,但不幸的是,我在网上发现的所有答案对我来说都行不通。

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>

任何解决此问题的帮助将不胜感激。

2 个答案:

答案 0 :(得分:0)

您已将ID output用于根目录布局。即ConstraintLayout,它将为您提供ConstraintLayout的参考。显然,对ConstraintLayout的引用不能存储在类型为TextView的变量中,如果完成,则会引发异常。

使用以下代码引用您的TextView

QR_output = findViewById(R.id.QROutput);

答案 1 :(得分:0)

您为TextView输入了错误的ID。您编写的ID指向XML上的ConstraintLayout对象。

代替(第31行):

QR_output = findViewById(R.id.output);

写下:

QR_output = findViewById(R.id.QROutput);