创建OnClickListener（Android）时的java NullPointerException

Question

这是一个非常简单的安卓应用程序。我现在正在学习为安卓系统编写代码，我不确定为什么会有这样一行：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: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:paddingBottom="@dimen/activity_vertical_margin"
安卓:paddingLeft="@dimen/activity_horizontal_margin"
安卓:paddingRight="@dimen/activity_horizontal_margin"
安卓:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.thenewboston.jaredh.StartingPoint$PlaceholderFragment" >

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

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

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

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 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at 安卓.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2359)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at 安卓.app.ActivityThread.access$700(ActivityThread.java:165)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1326)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at 安卓.os.Handler.dispatchMessage(Handler.java:99)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at 安卓.os.Looper.loop(Looper.java:137)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at 安卓.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.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at com.安卓.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 安卓.app.Activity.performCreate(Activity.java:5372)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at 安卓.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1104)
03-29 17:44:24.940: E/AndroidRuntime(25367):    at 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
03-29 17:44:24.940: E/AndroidRuntime(25367):    ... 11 more

正如你所见，这是一个非常简单的应用程序，只需在点击任何一个按钮时向文本字段添加1。任何帮助都是值得感激的。只是不确定为什么会导致NullPointerException