我的MainActivity上的textView中的setText

Question

可能是我要問一個非常愚蠢的問題。 實際上，我是.net開發人員，現在改用Android開發 。 我有一個文本視圖，我想使用setText()將文本放入其中。

我的MainActivity.java：

public class MainActivity extends ActionBarActivity {

    @Override

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

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }

        TextView txView = (TextView) findViewById(R.id.lblStatus);
        txView.setText("Status Test");
    }

XML代碼是：

<TextView
            android:id="@+id/lblStatus"
            android:layout_width="300dp"
            android:layout_height="wrap_content"
            android:layout_toRightOf="@id/tvStatusTag"
            android:layout_below="@id/lblImei" />

當我運行此應用程序時，它會自動停止。

logcat的： -

04-29 08:27:02.560 1649-1649/com.example.app 
E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: com.example.app, PID: 1649 
java.lang.RuntimeException: Unable to start activity 
ComponentInfo{com.example.app/com.example.app.MainActivity}: 
java.lang.NullPointerException at 
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195) 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 
at android.app.ActivityThread.access$800(ActivityThread.java:135) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:136) 
at android.app.ActivityThread.main(ActivityThread.java:5017) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:515) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
at dalvik.system.NativeStart.main(Native Method) 
Caused by: java.lang.NullPointerException 
at com.example.app.MainActivity.onCreate(MainActivity.java:27) 
at android.app.Activity.performCreate(Activity.java:5231) 
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159) 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 
at android.app.ActivityThread.access$800(ActivityThread.java:135) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:136) at android.app.ActivityThread.main(ActivityThread.java:5017) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:515)

---

AndroidManifest.xml中

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

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.app.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

</manifest>

Answer 1

嘗試這個

1. 您應該在Fragment中聲明並初始化 TextView
2. 由於您的Activity實例狀態必須為null，因此您沒有使用onSaveInstanceState（）方法保存Activity實例。
3. 您應該在fragment_main.xml中添加TextView組件。

fragment_main.xml

<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:background="#e5e5e5"
tools:context="com.example.app.MainActivity$PlaceholderFragment" >

<TextView
    android:id="@+id/textView"
    android:textColor="#FF0000"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

activity_main.xml中

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.app.MainActivity"
tools:ignore="MergeRootFrame" />

AndroidManifest.xml中

<?xml version="1.0" encoding="utf-8"?>

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.example.app.MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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

MainActivity.java

package com.example.app;

import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class MainActivity extends Activity {

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

    if (savedInstanceState == null) {
        getFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {

    /**
     * Declare TextView.
     */
    private TextView myText; 
    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);

        /**
         * Inititlize.
         */
        myText = (TextView)rootView.findViewById(R.id.textView);
        return rootView;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        /**
         * Set Value for TextView.
         */
        myText.setText("I'm a TextView");
    }


}

}

結果是

Answer 2

碼

     public class MainActivity extends ActionBarActivity
{
    TextView txView;
      @Override

      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
         txView = (TextView) findViewById(R.id.lblStatus);
  check();
    }

     public void check()
     {
            if (savedInstanceState == null) {
              getSupportFragmentManager().beginTransaction()
                      .add(R.id.container, new PlaceholderFragment())
                      .commit();

               txView.setText("Status Test");
             }
            else
          {

          txView.setText("Status Test1");
         }
     }
      }

XML

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
      android:background="#FFE7DB"  >
<TextView
            android:id="@+id/lblStatus"
            android:layout_width="300dp"
            android:layout_height="wrap_content"
            android:layout_toRightOf="@id/tvStatusTag"
            android:layout_below="@id/lblImei" />

</RelativeLayout>

Answer 3

我只是從fragment_main.xml中刪除了TextView組件（lblStatus），並在MainActivity.java中進行了一些更改。 代碼如下

public class MainActivity extends ActionBarActivity {

    @Override

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

       /* if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        } */


        TextView txView = new TextView(this);
        txView.setText("Status Text");
        setContentView(txView);
    }

它比其他答案更有效..因為它非常簡單:)

Answer 4

拉維我認為您正在使用appcompat庫和4.4 api級別，因此，如果我是對的，則您在布局文件夾中有兩個布局文件，一個是簡單活動xml，另一個是fragment.xml。

而且我認為您是在activity.xml中聲明此TextView而不是在Fragment xml文件中聲明的，因此在運行時會收到null異常，因此請在Fragment.xml中聲明此內容。

Answer 5

我在下面試過了這個編輯代碼，它對於Set textview來說可以正常使用...（如果您要擴展ActionBarActivity，則必須正確導入app_compact_v7和android.support jar）。顯示錯誤...

MainActivity.java：

 public class MainActivity extends Activity {       
 @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    TextView txView = (TextView) findViewById(R.id.lblStatus);
    txView.setText("Status Test");
}

}

activity_main.xml：

    <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"
      android:gravity="center"
      android:orientation="vertical" >

<TextView
    android:id="@+id/lblStatus"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello" />

  </LinearLayout>