java - 我的第一个应用程序不工作，它崩溃了 : UnsupportedOperationException

Question

我是 Android 的新手，我开始按照教程等制作我的第一个应用程序。但是，当我单击运行按钮时，它会在 logcat 上显示以下错误，我无法从中识别出错误所在。因此，这是我的代码，希望得到一些建议。提前致谢。

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button btn = (Button) findViewById(R.id.btn);
        btn.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                EditText firstnum = (EditText) findViewById(R.id.numberinput);
                TextView resultTextView = (TextView) findViewById(R.id.resulttextview);
                int num1 = Integer.parseInt(firstnum.getText().toString()) ;
                int result = num1 * num1;
                resultTextView.setText(result + "");
            }
        });
    }
}

修复渲染问题后的 LogCat:

7-16 01:27:14.359 10870-10870/com.example.abarimess.myfirstapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.abarimess.myfirstapp, PID: 10870
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.abarimess.myfirstapp/com.example.abarimess.myfirstapp.MainActivity}: java.lang.UnsupportedOperationException: Can't convert to dimension: type=0x4
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3184)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3294)
    at android.app.ActivityThread.access$1000(ActivityThread.java:210)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1704)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:145)
    at android.app.ActivityThread.main(ActivityThread.java:6938)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
 Caused by: java.lang.UnsupportedOperationException: Can't convert to dimension: type=0x4
    at android.content.res.TypedArray.getDimensionPixelOffset(TypedArray.java:546)
    at android.support.constraint.ConstraintLayout$LayoutParams.<init>(ConstraintLayout.java:2685)
    at android.support.constraint.ConstraintLayout.generateLayoutParams(ConstraintLayout.java:1915)
    at android.support.constraint.ConstraintLayout.generateLayoutParams(ConstraintLayout.java:476)
    at android.view.LayoutInflater.rInflate(LayoutInflater.java:820)
    at android.view.LayoutInflater.inflate(LayoutInflater.java:511)
    at android.view.LayoutInflater.inflate(LayoutInflater.java:415)
    at android.view.LayoutInflater.inflate(LayoutInflater.java:366)
    at android.support.v7.app.AppCompatDelegateImpl.setContentView(AppCompatDelegateImpl.java:467)
    at android.support.v7.app.AppCompatActivity.setContentView(AppCompatActivity.java:140)
    at com.example.abarimess.myfirstapp.MainActivity.onCreate(MainActivity.java:11)
    at android.app.Activity.performCreate(Activity.java:6575)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1134)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3137)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3294) 
    at android.app.ActivityThread.access$1000(ActivityThread.java:210) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1704) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:145) 
    at android.app.ActivityThread.main(ActivityThread.java:6938) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199) 

以及布局 XML:

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    tools:layout_editor_absoluteY="73dp">

    <TextView
        android:id="@+id/textView3"
        android:layout_width="360dp"
        android:layout_height="119dp"
        android:layout_marginTop="50dp"
        android:text="@string/square_of_the_number_is"
        android:textAppearance="@style/TextAppearance.AppCompat.Body2"
        android:textColor="@android:color/holo_purple"
        android:textSize="40sp"
        android:visibility="visible"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.49"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/numberinput" />

    <TextView
        android:id="@+id/Resulttextview"
        android:layout_width="390dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="25dp"
        android:background="@android:color/holo_red_dark"
        android:text="@string/the_result_is"
        android:textAlignment="center"
        android:textAppearance="@style/TextAppearance.AppCompat.Body2"
        android:textColor="#ffffff"
        android:textSize="50sp"
        android:visibility="visible"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/resulttextview"
        android:layout_width="30dp"
        android:layout_height="wrap_content"
        android:layout_marginEnd="8dp"
        android:layout_marginTop="30dp"
        android:background="@android:color/darker_gray"
        android:text="@string/_0"
        android:textAlignment="center"
        android:textAppearance="@style/TextAppearance.AppCompat.Body2"
        android:textColor="@color/Optional"
        android:textSize="40sp"
        android:visibility="visible"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.51"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/Resulttextview" />

    <Button
        android:id="@+id/btn"
        android:layout_width="370dp"
        android:layout_height="80dp"
        android:layout_marginTop="50dp"
        android:background="?android:attr/colorActivatedHighlight"
        android:text="@string/butan"
        android:textColor="@android:color/holo_blue_bright"
        android:textSize="20sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView3" />

    <EditText
        android:id="@+id/numberinput"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="50dp"
        android:ems="10"
        android:hint="@string/number_in_here"
        android:inputType="numberDecimal"
        android:singleLine="false"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/resulttextview" />

</android.support.constraint.ConstraintLayout>

评论中要求的 manifest.Xml :

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.abarimess.myfirstapp">

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

</manifest>

Answer 1

在您的布局 xml 文件中，您有两个单独的 TextView，如下所示。

<TextView
    android:id="@+id/Resulttextview"
    android:layout_width="390dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="25dp"
    android:background="@android:color/holo_red_dark"
    android:text="@string/the_result_is"
    android:textAlignment="center"
    android:textAppearance="@style/TextAppearance.AppCompat.Body2"
    android:textColor="#ffffff"
    android:textSize="50sp"
    android:visibility="visible"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<TextView
    android:id="@+id/resulttextview"
    android:layout_width="30dp"
    android:layout_height="wrap_content"
    android:layout_marginEnd="8dp"
    android:layout_marginTop="30dp"
    android:background="@android:color/darker_gray"
    android:text="@string/_0"
    android:textAlignment="center"
    android:textAppearance="@style/TextAppearance.AppCompat.Body2"
    android:textColor="@color/Optional"
    android:textSize="40sp"
    android:visibility="visible"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.51"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/Resulttextview" />

除了第一个以大写 R 开头外，两个 TextView 的名称完全相同。您可能会将第一个 TextView 的名称考虑为其他名称，例如 resulttextview1，如下所示。

<TextView
    android:id="@+id/resulttextview1"
    android:layout_width="390dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="25dp"
    android:background="@android:color/holo_red_dark"
    android:text="@string/the_result_is"
    android:textAlignment="center"
    android:textAppearance="@style/TextAppearance.AppCompat.Body2"
    android:textColor="#ffffff"
    android:textSize="50sp"
    android:visibility="visible"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<TextView
    android:id="@+id/resulttextview"
    android:layout_width="30dp"
    android:layout_height="wrap_content"
    android:layout_marginEnd="8dp"
    android:layout_marginTop="30dp"
    android:background="@android:color/darker_gray"
    android:text="@string/_0"
    android:textAlignment="center"
    android:textAppearance="@style/TextAppearance.AppCompat.Body2"
    android:textColor="@color/Optional"
    android:textSize="40sp"
    android:visibility="visible"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.51"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/resulttextview1" />

请避免使用大写字母命名布局标识符。请改用下划线(例如 result_text_view)。

您的代码中的另一个潜在问题是，如果您的 EditText 中没有输入任何内容，应用程序将再次崩溃，并返回一个 NullPointerException 作为返回值 firstnum.getText().toString() 将具有空值。

在这种情况下，您可以考虑在此处添加空检查。

@Override
public void onClick(View v) {
    EditText firstnum = (EditText) findViewById(R.id.numberinput);
    TextView resultTextView = (TextView) findViewById(R.id.resulttextview);

    // Add a null check here for safety
    if(firstnum.getText().toString() == null) return;

    int num1 = Integer.parseInt(firstnum.getText().toString()) ;
    int result = num1 * num1;
    resultTextView.setText(result + "");
}

如果在 EditText 中输入数字以外的字符串，您将得到 NumberFormatException。因此，您可能还需要处理这种情况。但是，如果您只处理 EditText 中的数字，则可以忽略这一点。