Java 尝试调用另一个类的函数时出现NullPoint错误

我试图调用一个函数

registerGCM（）

从my

reg.java

类进入

MainActivity.java

类

当我在

reg.java

活动中使用该函数时，该应用程序工作正常，但每当我在

main活动中调用它时，就会出现此错误

07-28 13:38:38.540  24434-24434/com.example.kmi_dev.fbloginsample E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.kmi_dev.fbloginsample, PID: 24434
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.kmi_dev.fbloginsample/com.example.kmi_dev.fbloginsample.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.kmi_dev.fbloginsample.reg.registerGCM()' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.kmi_dev.fbloginsample.reg.registerGCM()' on a null object reference
            at com.example.kmi_dev.fbloginsample.MainActivity.onCreate(MainActivity.java:158)
            at android.app.Activity.performCreate(Activity.java:5990)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

呼叫的主要活动的内容

public class MainActivity extends Activity {
    private static final String TAG = "MainActivity";
    private static final String MYPREFERENCES = "myPref";
    private CallbackManager callbackManager;
    private LoginButton fbLoginButton;
    private Button login;
    .
    .
    .
    .
    GPSTracker gps;
    reg GCM_REG;


    // flag for Internet connection status
    Boolean isInternetPresent = false;

    // Connection detector class
    ConnectionDetector cd;
    SessionManager session;
    .
    .
    String gcmId;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);


        //get global variable class
        final GlobalClass globalVariable = (GlobalClass) getApplicationContext();
       gcmId = GCM_REG.registerGCM();

注册表项的内容（）

 public String registerGCM() {
        gcm = GoogleCloudMessaging.getInstance(this);
        regId = getRegistrationId(context);

        if (TextUtils.isEmpty(regId)) {
            registerInBackground();
            Log.d("MainActivity",
                    "registerGCM - successfully registered with GCM server - regId: "
                            + regId);
        } else {
            Toast.makeText(getApplicationContext(),
                    "RegId already available. RegId: " + regId,
                    Toast.LENGTH_LONG).show();
        }
        final GlobalClass globalVariable = (GlobalClass) getApplicationContext();
        globalVariable.setGcmRegId(regId);
        return regId;
    }
    public String getRegistrationId(Context context) {
        final SharedPreferences prefs = getSharedPreferences(reg.class.getSimpleName(), Context.MODE_PRIVATE);
        String registrationId = prefs.getString(REG_ID, "");
        if (registrationId.isEmpty()) {
            Log.i(TAG, "Registration not found.");
            return "";
        }
        int registeredVersion = prefs.getInt(APP_VERSION, Integer.MIN_VALUE);
        int currentVersion = getAppVersion(context);
        if (registeredVersion != currentVersion) {
            Log.i(TAG, "App version changed.");
            return "";
        }
        return registrationId;
    }

---

你知道我为什么不能调用这个函数吗？

你从来没有初始化过

GCM\u REG

。这就是为什么会出现NullPointerException。

NullPointerException：我想对象GCM_REG没有初始化。请检查您的密码。