Java RecyclerView在设置时引发null异常_Java_Android_Xml

Java RecyclerView在设置时引发null异常

java android xml

Java RecyclerView在设置时引发null异常,java,android,xml,Java,Android,Xml,只是为了好玩而做一个android项目。我在用户尝试登录后返回错误。我在错误所在的行附近做了一次尝试捕捉，对象本身是null。现在我知道了这一点，我正试图弄明白为什么在我执行findViewById（）时它没有被设置。我将把相关代码粘贴到下面 java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setAdapter(andr

只是为了好玩而做一个android项目。我在用户尝试登录后返回错误。我在错误所在的行附近做了一次尝试捕捉，对象本身是

null

。现在我知道了这一点，我正试图弄明白为什么在我执行

findViewById（）

时它没有被设置。我将把相关代码粘贴到下面

java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setAdapter(androidx.recyclerview.widget.RecyclerView$Adapter)' on a null object reference

MainActivity.java

import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.AbsListView;
import android.widget.Adapter;
import android.widget.SearchView;

import com.google.android.material.bottomnavigation.BottomNavigationView;

import androidx.appcompat.app.AppCompatActivity;
import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {
    MyProductAdapter myProductAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        BottomNavigationView navView = findViewById(R.id.nav_view);
        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(R.id.navigation_home, R.id.navigation_dashboard, R.id.navigation_notifications).build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
        NavigationUI.setupWithNavController(navView, navController);

        RecyclerView recyclerView;
        recyclerView = findViewById(R.id.listId);
        System.out.println("This is recyclverView" + recyclerView);
        try {
            recyclerView.setHasFixedSize(true);
            recyclerView.setLayoutManager(new LinearLayoutManager(this));
            recyclerView.setHasFixedSize(true);

        } catch (NullPointerException e) {
            System.out.println("null here" + e.getCause());
        }

        ArrayList<MyProduct> myProductData = new ArrayList<>();
        myProductData.add(new MyProduct("Panadol", "Pill  for headAche", "Price : $2.99", R.drawable.logo2));
        myProductData.add(new MyProduct("Nyquill", "Pill  for headAche", "Price : $2.99", R.drawable.logo2));
        myProductData.add(new MyProduct("ZAck", "Pill  for headAche", "Price : $2.99", R.drawable.logo2));
        myProductData.add(new MyProduct("Codeine", "Pill  for headAche", "Price : $2.99", R.drawable.logo2));


        myProductAdapter = new MyProductAdapter(myProductData, MainActivity.this);
        recyclerView.setAdapter(myProductAdapter);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        getMenuInflater().inflate(R.menu.menu_bar, menu);
        MenuItem menuItem = menu.findItem(R.id.search);
        SearchView searchView = (SearchView) menuItem.getActionView();
        searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
            @Override
            public boolean onQueryTextSubmit(String s) {
                return false;
            }

            @Override
            public boolean onQueryTextChange(String s) {

                myProductAdapter.getFilter().filter(s);
                return false;
            }
        });
        return super.onCreateOptionsMenu(menu);
    }

}

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;

import com.example.medistak.R;

public class DashboardFragment extends Fragment {

    private DashboardViewModel dashboardViewModel;

    public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        
        dashboardViewModel =
                new ViewModelProvider(this).get(DashboardViewModel.class);
        View root = inflater.inflate(R.layout.fragment_dashboard, container, false);
        dashboardViewModel.getText().observe(getViewLifecycleOwner(), new Observer<String>() {
            @Override
            public void onChanged(@Nullable String s) {

            }
        });
        return root;
    }
}

DashBoardFragment.java

import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.AbsListView;
import android.widget.Adapter;
import android.widget.SearchView;

import com.google.android.material.bottomnavigation.BottomNavigationView;

import androidx.appcompat.app.AppCompatActivity;
import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {
    MyProductAdapter myProductAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        BottomNavigationView navView = findViewById(R.id.nav_view);
        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(R.id.navigation_home, R.id.navigation_dashboard, R.id.navigation_notifications).build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
        NavigationUI.setupWithNavController(navView, navController);

        RecyclerView recyclerView;
        recyclerView = findViewById(R.id.listId);
        System.out.println("This is recyclverView" + recyclerView);
        try {
            recyclerView.setHasFixedSize(true);
            recyclerView.setLayoutManager(new LinearLayoutManager(this));
            recyclerView.setHasFixedSize(true);

        } catch (NullPointerException e) {
            System.out.println("null here" + e.getCause());
        }

        ArrayList<MyProduct> myProductData = new ArrayList<>();
        myProductData.add(new MyProduct("Panadol", "Pill  for headAche", "Price : $2.99", R.drawable.logo2));
        myProductData.add(new MyProduct("Nyquill", "Pill  for headAche", "Price : $2.99", R.drawable.logo2));
        myProductData.add(new MyProduct("ZAck", "Pill  for headAche", "Price : $2.99", R.drawable.logo2));
        myProductData.add(new MyProduct("Codeine", "Pill  for headAche", "Price : $2.99", R.drawable.logo2));


        myProductAdapter = new MyProductAdapter(myProductData, MainActivity.this);
        recyclerView.setAdapter(myProductAdapter);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        getMenuInflater().inflate(R.menu.menu_bar, menu);
        MenuItem menuItem = menu.findItem(R.id.search);
        SearchView searchView = (SearchView) menuItem.getActionView();
        searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
            @Override
            public boolean onQueryTextSubmit(String s) {
                return false;
            }

            @Override
            public boolean onQueryTextChange(String s) {

                myProductAdapter.getFilter().filter(s);
                return false;
            }
        });
        return super.onCreateOptionsMenu(menu);
    }

}

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;

import com.example.medistak.R;

public class DashboardFragment extends Fragment {

    private DashboardViewModel dashboardViewModel;

    public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        
        dashboardViewModel =
                new ViewModelProvider(this).get(DashboardViewModel.class);
        View root = inflater.inflate(R.layout.fragment_dashboard, container, false);
        dashboardViewModel.getText().observe(getViewLifecycleOwner(), new Observer<String>() {
            @Override
            public void onChanged(@Nullable String s) {

            }
        });
        return root;
    }
}

导入android.os.Bundle；
导入android.view.LayoutInflater；
导入android.view.view；
导入android.view.ViewGroup；
导入android.widget.TextView；
导入androidx.annotation.NonNull；
导入androidx.annotation.Nullable；
导入androidx.fragment.app.fragment；
导入androidx.lifecycle.Observer；
导入androidx.lifecycle.ViewModelProvider；
导入com.example.medistak.R；
公共类DashboardFragment扩展了该片段{
专用仪表板视图模型仪表板视图模型；
公共视图onCreateView（@NonNull layoutiner充气机，
视图组容器，捆绑包savedInstanceState）{
仪表板视图模型=
新建ViewModelProvider（this.get）（DashboardViewModel.class）；
视图根=充气机。充气（R.layout.fragment_仪表板，容器，错误）；
dashboardViewModel.getText（）.observe（getViewLifecycleOwner（），new Observer（））{
@凌驾
公共void onChanged（@Nullable String s）{
}
});
返回根；
}
}

Edit（在OP之后添加

DashboardFragment

类）将

MyProductAdapter

和

recyclerView

代码从

MainActivity

移动到

DashboardFragment

onCreateView（）

方法，如下所示：

RecyclerView recyclerView = findViewById(R.id.listId);
recyclerView.setHasFixedSize(true);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerView.setAdapter(myProductAdapter);

您的

RecyclerView

listId

驻留在

仪表板片段中，而您正在MainActivity
类中初始化和设置它。出现空对象引用
错误的原因是活动
及其底层片段
添加到backbackback
的顺序
首先设置main活动。然后跟随MainActivity
中的Fragments
，在您的示例中，是仪表板Fragments
。因此，如果您要设置仪表板片段的recyclerView
，从MainActivity的角度来看，它甚至不存在，而且尚未创建，，当然会导致NPE

您的问题的解决方案是从main活动
类中取出所有recyclerView
设置代码，并将其转移到DashboardFragment
中，检查R.id.listId
是否已分配给R.layout.activity\u main2中的recyclerView
，请参阅更新的代码plz.I已更新我的DashBoardFragment类。请建议我应该添加/删除哪些内容。getting无法解析Dashboard fragment中的方法findViewById
任何想法@karan Dhilo您不能在片段中直接调用findViewById（）
。调用getView（）.findViewById（）
。非常感谢@karan dhillon