textView setText() NullPointerException

我有一个几乎完全香草的应用程序。我想做的就是更改 textView 的文本，但它给了我一个 NullPointerException。我没有 XML 配置，没有添加方法，没有什么特别的。我检查了我的类，它指向正确的 XML，并且 XML 中确实有一个具有正确 id 的 textView 元素。

我还尝试在类级别的 onCreate() 之外创建 textView 实例，但无论如何应用程序都无法启动。我确信我忽略了简单，但我找不到答案。谢谢您的帮助。 编辑： 我意识到我需要移动此代码以便它访问片段资源，但是当我这样做时，它会给我一个错误：

无法从 Activity 类型对非静态方法 findViewById(int) 进行静态引用

如果我的所有视图都在我的片段中，而不是在我的 Activity XML 中，我应该将代码放在哪里？或者我应该让活动首先加载片段？ 包com.example.testapp；

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.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;
import android.os.Build;

public class MainActivity extends ActionBarActivity {

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

        TextView tv = (TextView) findViewById(R.id.textView1);
        tv.setText("Teststring");

    if (savedInstanceState == null) {
        getSupportFragmentManager().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);
    }


    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

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

    }


}

错误日志：

06-14 17:02:27.405: E/AndroidRuntime(1420): FATAL EXCEPTION: main
06-14 17:02:27.405: E/AndroidRuntime(1420): Process: com.example.testapp, PID: 1420
06-14 17:02:27.405: E/AndroidRuntime(1420): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.testapp/com.example.testapp.MainActivity}: java.lang.NullPointerException
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.os.Handler.dispatchMessage(Handler.java:102)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.os.Looper.loop(Looper.java:136)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.main(ActivityThread.java:5017)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at java.lang.reflect.Method.invokeNative(Native Method)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at java.lang.reflect.Method.invoke(Method.java:515)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at dalvik.system.NativeStart.main(Native Method)
06-14 17:02:27.405: E/AndroidRuntime(1420): Caused by: java.lang.NullPointerException
06-14 17:02:27.405: E/AndroidRuntime(1420):     at com.example.testapp.MainActivity.onCreate(MainActivity.java:23)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.Activity.performCreate(Activity.java:5231)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
06-14 17:02:27.405: E/AndroidRuntime(1420):     ... 11 more

XML 片段_主

<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"
    tools:context="com.example.testapp.MainActivity$PlaceholderFragment" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

</RelativeLayout>

你能发布xml吗？你的java假设的id“R.id.textView1”很可能是错误的。也许是 R.id.textview1？

如果您的文本视图位于片段内，只需执行以下操作：

@Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            TextView tv = (TextView) rootView.findViewById(R.id.textView1);
        tv.setText("Teststring");
            return rootView;
        }