Question

我很难尝试理解静态变量。任何人都可以解释为什么当我调用变量＆＃39; startingDate＆＃39;在FragmentJoy中，它没有返回正确的值？假设所有其余代码都运行正常，而不是NULL值和databaseCategories工作正常。如何在“onDataChange（）＆＃39;内部保存值？方法并在FragmentJoy中使用它？

我不能删除：

@Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {)

这是仪表板活动

public class Dashboard extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

static String startingDate;
DatabaseReference databaseCategories;
ArrayList<Category> currentJoyCategories;


User currentUser; //holds the information of the current user


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_dashboard);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    . . . . . .

    databaseCategories = FirebaseDatabase.getInstance().getReference("Categories");

    currentJoyCategories = new ArrayList<>();

    databaseCategories.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

               //want to do
               startingDate = "some string";
         }

         @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
} //end of Dashboard class

在我要做的另一个课程中：

public class FragmentJoy extends Fragment {

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_joy,container,false);

         //not storing the string "some string"
        System.out.println("testing " + Dashboard.startingDate);

        return view;

} //end of onCreateView method
} //end of class

Answer 1

您需要将startingDate声明为public static String startingDate

使用此

public static String startingDate;

而不是

static String startingDate;

然后您可以在FragmentJoy这样的

中使用它

Log.e("testing ",Dashboard.startingDate);

Answer 2

在这里，我向您展示如何将一个class的静态方法用于另一个类。

e.g。我在类HomeFragment中定义了一个静态方法，

public class HomeFragment扩展了BaseFragment {

    Unbinder unbinder;
    int mode;

    public static HomeFragment getInstance(int mode) {
        HomeFragment homeFragment = new HomeFragment();
        Bundle bundle = new Bundle();
        bundle.putInt(KeyUtil.KEY_MODE, mode);
        homeFragment.setArguments(bundle);
        return homeFragment;
    }
    ....................
}

而且，我在我的static课程中使用了这个MainActivity方法，

public class MainActivity extends AppCompatActivity {

    ...........................

        /**
         * Get the fragment while need to push.
         *
         * @return: fragment
         */
        private Fragment openFragment() {
            switch (navItemIndex) {
                case 0:
                    // Here I have called static method
                    return HomeFragment.getInstance(0);
                }
         }

        ........................
    }

从一个类访问静态变量到另一个类

2 个答案: