尝试在非活动类中调用方法时的 Java NPE

Question

我已经在这里和那里搜索了两天，但似乎没有任何解决方案不起作用。问题出在Fitness.getSessionsClient(this, Objects.requireNonNull(GoogleSignIn.getLastSignedInAccount(2ndclass.mContext))) 及其传递的活动 (this) 和上下文 (2ndClass.mContext) 变量中，我可以通过许多找到的方法填充（变量或非空或空通过调试器检查）。仍然 - 我总是得到 NPE (java.lang.NullPointerException) 或 getApplicationContext() 为空。有什么想法吗？

截图，如果有帮助的话：

崩溃日志行：

E/<SOME TAG>: Error during PUT connection... Data: {"coord":{"lon":-0.13,"lat":51.51},"weather":[{"id":803,"main":"Clouds","description":"broken clouds","icon":"04d"}],"base":"stations","main":{"temp":13.11,"pressure":1033,"humidity":77,"temp_min":10,"temp_max":16.11},"visibility":10000,"wind":{"speed":3.1,"deg":330},"clouds":{"all":69},"dt":1568357213,"sys":{"type":1,"id":1414,"message":0.0113,"country":"GB","sunrise":1568352700,"sunset":1568398909},"timezone":3600,"id":2643743,"name":"London","cod":200} Message: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference

代码片段：

第一个（MainActivity）类：

public class 1stClass extends AppCompatActivity {
    public static 1stClass instance;
    public static Context mContext;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        instance = this;
        mContext = getApplicationContext();
    }
public final void 1stmethod(){
        2ndClass.2ndmethod();
}
/**
 * @return instance
 */
@Contract(pure = true)
public static 1stClass getInstance() { return instance; } // return.getInstance();

public static Context getContext() {
    //  return instance.getApplicationContext();
    return mContext;
}
}

二等：

public class 2ndClass extends 1stClass{
  static 2ndClass instance;
  public final void 2ndmethod() {
  instance = this;
    3rdClass MFP = new 3rdClass();
    MFP.3rdmethod(..);
  }
/**
 * @return instance
 */
@Contract(pure = true)
public static 2ndClass getInstance() { return instance; } //return.getInstance();
}

非活动（第三）类：

public final class 3rdClass extends 2ndClass {
static 3rdClass instance;

public void 3rdmethod() {
    instance = this;
Fitness.getSessionsClient(this, Objects.requireNonNull(GoogleSignIn.getLastSignedInAccount(2ndclass.mContext))) // <---- ERROR HERE
                .insertSession(request)
                .addOnSuccessListener(new OnSuccessListener<Void>() {
                    @Override
                    public void onSuccess(Void aVoid) {
                        Log.i(TAG, "Session insert was successful!");
                    }
                })
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        Log.wtf(TAG, "There was a problem inserting the session: " +
                                e.getLocalizedMessage());
                    }
                });
/**
 * @return instance
 */
@Contract(pure = true)
public static MFPalInterface getInstance() {
    return instance;
} // return.getInstance();

}

Answer 1

我想我明白你在做什么，尽管代码是一团糟。

您正在尝试在 2ndClass 的方法“2ndmethod()”中创建自己的活动，然后在新创建的 3rdClass 上调用该方法。

public final void 2ndmethod() {
    3rdClass MFP = new 3rdClass();
    MFP.3rdmethod(..);
}

即使您从AppCompatActivity 扩展了3rdClass，您也没有要求系统创建 Activity 实例并尝试自己创建它。这意味着新创建的实例没有上下文，系统也不知道。

---

你需要做什么

通过调用您当前正在运行的Activity 的startActivity() 方法（在本例中我认为是2ndClass）并传递一个意图来启动活动。像这样：

Intent intent = new Intent(this, 3rdClass.class);    // this is your current Activity's reference.
currentActivity.startActivity(intent);