Java 创建OnClickListener（Android）时出现NullPointerException

这是一个非常简单的android应用程序。我现在正在学习为android编写代码，我不确定为什么这行代码：add.setOnClickListener（new View.OnClickListener（）抛出NullPointerException

public class StartingPoint extends ActionBarActivity{

int counter;
Button add, sub;
TextView display;

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

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

    counter = 0;
    add = (Button) findViewById(R.id.bAdd);
    sub = (Button) findViewById(R.id.bSub);
    display = (TextView) findViewById(R.id.tvDisplay);

    add.setOnClickListener(new View.OnClickListener() 
    {

        @Override
        public void onClick(View v) 
        {
            counter++;
            display.setText("Your total is " + counter);
        }
    });

    sub.setOnClickListener(new View.OnClickListener() 
    {

        @Override
        public void onClick(View v) 
        {
            counter--;  
            display.setText("Your total is " + counter);
        }
    });

}

以下是XML代码：

<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.thenewboston.jaredh.StartingPoint$PlaceholderFragment" >

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:text="@string/total"
    android:textSize="40sp" 
    android:id="@+id/tvDisplay"
    />

<Button
    android:layout_width="250dp"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/tvDisplay"
    android:layout_below="@+id/tvDisplay"
    android:layout_gravity="center"
    android:text="@string/addButton"
    android:textSize="20sp"
    android:id="@+id/bAdd" />

<Button
    android:layout_width="250dp"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/bAdd"
    android:layout_below="@+id/bAdd"
    android:layout_gravity="center"
    android:text="@string/subButton"
    android:textSize="20sp" 
    android:id="@+id/bSub"/>

---

正如您所见，这是一个非常简单的应用程序，只需在单击任何一个按钮时向文本字段添加1。任何帮助都可能非常有用。只是不确定为什么会出现NullPointerException。

我猜您提供的XML（没有说明它是哪个文件，尽管XML文本甚至无效）是您在开始时添加的

占位符片段

的布局。但是，该片段不是立即添加的，而是计划由

片段管理器

添加的。因此，当您立即尝试查找其UI组件时，活动无法执行此操作，并且

将
添加
引用设置为null>findViewById

请发布logcat错误此代码是否用于

活动\u起点

？您的bAdd可能有问题并且无法实例化，发布一些logcat错误消息，我看不出它在哪里。第一组代码是java，第二组代码是xml。和

Logcat Error:
       03-29 17:44:24.930: D/AndroidRuntime(25367): Shutting down VM
03-29 17:44:24.930: W/dalvikvm(25367): threadid=1: thread exiting with uncaught exception (group=0x41831898)
03-29 17:44:24.940: E/AndroidRuntime(25367): FATAL EXCEPTION: main
03-29 17:44:24.940: E/AndroidRuntime(25367): java.lang.RuntimeException: Unable to start activity ComponentInf{com.thenewboston.jaredh/com.thenewboston.jaredh.StartingPoint}: java.lang.NullPointerException
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2359)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.app.ActivityThread.access$700(ActivityThread.java:165)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1326)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.os.Looper.loop(Looper.java:137)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.app.ActivityThread.main(ActivityThread.java:5455)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at java.lang.reflect.Method.invokeNative(Native Method)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at java.lang.reflect.Method.invoke(Method.java:525)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at dalvik.system.NativeStart.main(Native Method)
03-29 17:44:24.940: E/AndroidRuntime(25367): Caused by: java.lang.NullPointerException
03-29 17:44:24.940: E/AndroidRuntime(25367):    at com.thenewboston.jaredh.StartingPoint.onCreate(StartingPoint.java:39)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.app.Activity.performCreate(Activity.java:5372)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1104)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
03-29 17:44:24.940: E/AndroidRuntime(25367):    ... 11 more