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Question

它發射到我的設備我的應用程序被終止我創建了一個簡單的應用程序，該做的唯一的事情就是打印的「Hello world」。 當我嘗試啓動我的裝置可以在它停止並說：「不幸的是，（我的應用程序的名稱）已停止。」我到處找一些點，但我無法找到一個解決這個問題。

這裏是我的activity_main.xml中：

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout 
xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
tools:context=".MainActivity"> 

<TextView 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="@string/hello_world" 
    android:textSize="36sp" 
    android:textColor="@android:color/holo_blue_dark" 
    android:textStyle="bold" 
    android:layout_marginLeft="12dp" 
    android:layout_marginTop="12dp" 
    /> 


</RelativeLayout> 

AndroidManifest.xml中（如果這是有幫助的。）：

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

<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> 

的logcat：

Process: com.example.android.test, PID: 6266 
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.test/com.example.android.test.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference 
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3319) 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415) 
at android.app.ActivityThread.access$1100(ActivityThread.java:229) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:148) 
at android.app.ActivityThread.main(ActivityThread.java:7325) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference 
at com.example.android.test.MainActivity.onCreate(MainActivity.java:21) 
at android.app.Activity.performCreate(Activity.java:6904) 
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1136) 
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3266) 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415) 
at android.app.ActivityThread.access$1100(ActivityThread.java:229) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:148) 
at android.app.ActivityThread.main(ActivityThread.java:7325) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 

我希望這是什麼需要幫助我解決這個問題。如果你需要更多的消息，請讓我知道。

編輯

這裏是我的Main.Activity.java代碼：

package com.example.android.test; 

import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.widget.TextView; 

public class MainActivity extends AppCompatActivity { 

// Used to load the 'native-lib' library on application startup. 
static { 
    System.loadLibrary("native-lib"); 
} 

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

    // Example of a call to a native method 
    //TextView tv = (TextView) findViewById(R.id.sample_text); 
    //tv.setText(stringFromJNI()); 
} 

/** 
* A native method that is implemented by the 'native-lib' native library, 
* which is packaged with this application. 
*/ 
public native String stringFromJNI(); 

public class SampleActivity extends AppCompatActivity { 

    // Create the variable 
    TextView mTextView; 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     // Other view stuff 

     // Get the reference 
     mTextView = (TextView) findViewById(R.id.text_view); 

     // Now you can do things to the Textview like change its text 
     mTextView.setText("Hello world!"); 

    } 

} 

}

Answer 1

按照logcat的輸出，它看起來像你正在嘗試前將TextView的文本讓你參考它，試試這個。 首先確保您將ID字段添加到您的xml中。

<TextView 
android:layout_width="wrap_content" 
android:layout_height="wrap_content" 
android:text="@string/hello_world" 
android:textSize="36sp" 
android:textColor="@android:color/holo_blue_dark" 
android:textStyle="bold" 
android:layout_marginLeft="12dp" 
android:layout_marginTop="12dp" 

android:"@+id/text_view" 
/> 

然後打開MainActivity類別，並添加以下到您的類

public class SampleActivity extends AppCompatActivity { 
 

 
    // Create the variable 
 
    TextView mTextView; 
 

 
    @Override 
 
    public void onCreate(Bundle savedInstanceState) { 
 
     super.onCreate(savedInstanceState); 
 
     // Other view stuff 
 

 
     // Get the reference 
 
     mTextView = (TextView) findViewById(R.id.text_view); 
 

 
     // Now you can do things to the Textview like change its text 
 
     mTextView.setText("Hello World!"); 
 
     
 
    } 
 

 
}