安卓 java。添加TextChangedListener时的lang.NullPointerException

Question

将addTextChangedListener添加到onCreate时出现以下错误 操作系统Android 4.4 API19：

错误：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.nes.安卓.emplacements/com.nes.安卓.emplacements.MainActivity}: java.lang.NullPointerException
Caused by: java.lang.NullPointerException
    at com.nes.安卓.emplacements.MainActivity.onCreate(MainActivity.java:32)
    at 安卓.app.Activity.performCreate(Activity.java:5231)
    at 安卓.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
    at 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
    at 安卓.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
    at 安卓.app.ActivityThread.access$800(ActivityThread.java:135)

这是我的密码：

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();
        }

        EditText valEmplacement = (EditText)findViewById(R.id.editText2);

        valEmplacement.addTextChangedListener(new TextWatcher(){
            public void afterTextChanged(Editable s) { }
            public void beforeTextChanged(CharSequence s, int start, int count, int after){}
            public void onTextChanged(CharSequence s, int start, int before, int count){}
        });

}

editText2位于fragment_main中，而不在Activy_main中

活动区。xml:

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

梅因。xml:

<RelativeLayout xmlns:安卓="http://schemas.安卓.com/apk/res/安卓"
    xmlns:tools="http://schemas.安卓.com/tools"
    安卓:layout_width="match_parent"
    安卓:layout_height="match_parent"
    安卓:paddingLeft="@dimen/activity_horizontal_margin"
    安卓:paddingRight="@dimen/activity_horizontal_margin"
    安卓:paddingTop="@dimen/activity_vertical_margin"
    安卓:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.nes.安卓.emplacements.MainActivity$PlaceholderFragment"
    安卓:focusable="true">


    <EditText
        安卓:layout_width="fill_parent"
        安卓:layout_height="80px"
        安卓:id="@+id/editText2"
        安卓:inputType="text"
        安卓:layout_below="@+id/textView"
        安卓:background="#cccccc"
        安卓:autoText="false"
        安卓:textSize="20px"
        安卓:textIsSelectable="true" />

    <TextView
        安卓:layout_width="wrap_content"
        安卓:layout_height="wrap_content"
        安卓:textAppearance="?安卓:attr/textAppearanceLarge"
        安卓:text="@string/text_emplacement"
        安卓:id="@+id/textView"
        安卓:layout_below="@+id/textView3"
        安卓:layout_alignParentLeft="true"
        安卓:layout_alignParentStart="true"
        安卓:layout_marginTop="46dp"
        安卓:layout_alignParentRight="true"
        安卓:layout_alignParentEnd="true" />    

</RelativeLayout>

Answer 1

试试清洁&清洁；在eclipse中构建项目以重新生成R文件

并确保Activy_主布局包含id为editText2的元素

更新：我看到id为editText2的视图实际上在片段布局中。因此，我建议您将处理此编辑文本视图的代码移动到Fragment类中。例如，在片段类的这个方法中：

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

       //retrieve your fragment views here