android - 抽屉导航 :- NullPointerException: Attempt to invoke virtual method 'java. lang.String android.content.Context.getPackageName()

Question

DRAWER IMAGE你好 friend 我在我的项目中使用抽屉导航但是在尝试打开 listview-listitem 的 Activity onclick 时遇到上述错误(第二次)当我们点击列表项时应用程序在第一次运行完美但是如果我们返回并单击抽屉菜单的任何项目，然后如果我们尝试单击列表项，则会发生错误 LIST VIEW

FRAGMENT.java


package com.forever.technology.bscit;

import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.ListFragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import com.forever.technology.bscit.R;

/**
 * A simple {@link Fragment} subclass.
 */
public class Sem1Fragment_list extends ListFragment {




    public Sem1Fragment_list() {
        // Required empty public constructor
    }
    ListView lv;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_sem1_list, container, false);
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        ArrayAdapter adapter = ArrayAdapter.createFromResource(getActivity(),
                R.array.sem1, android.R.layout.simple_list_item_1);
        setListAdapter(adapter);

    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        super.onListItemClick(l, v, position, id);
        switch (position)
        {
            case 0:
                Intent intent = new Intent(getContext(),Subject1Activity.class);
                startActivity(intent);
        }

    }
}

主要 Activity

package com.forever.technology.bscit;

import android.content.ComponentName;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    NavigationView navigationView = null;
    Toolbar toolbar = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //Set the fragment initially
        HomeFragment fragment = new HomeFragment();
        android.support.v4.app.FragmentTransaction fragmentTransaction =
                getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.fragment_container, fragment);
        fragmentTransaction.commit();

        toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);

        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        navigationView = (NavigationView) findViewById(R.id.nav_view);

        //How to change elements in the header programatically

      /*  View headerView = navigationView.getHeaderView(0);
        TextView emailText = (TextView) headerView.findViewById(R.id.email);
        emailText.setText("newemail@email.com"); */

        navigationView.setNavigationItemSelectedListener(this);
    }

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_home) {
            //Set the fragment initially
            HomeFragment fragment = new HomeFragment();
            android.support.v4.app.FragmentTransaction fragmentTransaction =
                    getSupportFragmentManager().beginTransaction();
            fragmentTransaction.replace(R.id.fragment_container, fragment);
            fragmentTransaction.commit();
            // Handle the camera action
        } else if (id == R.id.nav_sem1) {
            //Set the fragment initially
            Sem1Fragment fragment = new Sem1Fragment();
            android.support.v4.app.FragmentTransaction fragmentTransaction =
                    getSupportFragmentManager().beginTransaction();
            fragmentTransaction.replace(R.id.fragment_container, fragment);
            fragmentTransaction.commit();
            // Handle the camera action
        } else if (id == R.id.nav_sem2) {
            //Set the fragment initially
            Sem2Fragment fragment = new Sem2Fragment();
            android.support.v4.app.FragmentTransaction fragmentTransaction =
                    getSupportFragmentManager().beginTransaction();
            fragmentTransaction.replace(R.id.fragment_container, fragment);
            fragmentTransaction.commit();

        } else if (id == R.id.nav_sem3) {

        } else if (id == R.id.nav_sem4) {

        } else if (id == R.id.nav_sem5) {

        } else if (id == R.id.nav_sem6) {

        }
        else if (id == R.id.nav_blogs) {

        }
        else if (id == R.id.nav_site) {

        }

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }
}

Subject1.activity

package com.forever.technology.bscit;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class Subject1Activity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.subject1);
    }
}

洛格卡特

03-23 01:06:33.111 11547-11547/com.compscitutorials.basigarcia.navigationdrawervideotutorial E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.compscitutorials.basigarcia.navigationdrawervideotutorial, PID: 11547
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
   at android.content.ComponentName.<init>(ComponentName.java:128)
   at android.content.Intent.<init>(Intent.java:4449)
   at com.forever.technology.bscit.Sem1Fragment_list.onListItemClick(Sem1Fragment_list.java:50)
   at android.support.v4.app.ListFragment$2.onItemClick(ListFragment.java:58)
   at android.widget.AdapterView.performItemClick(AdapterView.java:310)
   at android.widget.AbsListView.performItemClick(AbsListView.java:1145)
   at android.widget.AbsListView$PerformClick.run(AbsListView.java:3066)
   at android.widget.AbsListView$3.run(AbsListView.java:3903)
   at android.os.Handler.handleCallback(Handler.java:739)
   at android.os.Handler.dispatchMessage(Handler.java:95)
   at android.os.Looper.loop(Looper.java:148)
   at android.app.ActivityThread.main(ActivityThread.java:5417)
   at java.lang.reflect.Method.invoke(Native Method)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Answer 1

问题出在 Sem1Fragment.java 代码上。您拥有膨胀 View 的静态引用“rootView”。再次从 NavigationDrawer 选择相同的 fragment 时，您将再次实例化相同的 fragment ，但由于您的膨胀 View 引用是静态的，因此它不会被刷新并将包含陈旧的上下文，因此 getContext () 为空。因此，更改您的代码 -

static View rootView;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    if (rootView == null) {
        rootView = inflater.inflate(R.layout.fragment_sem1, container, false);
    }
    return rootView;
}

由

  @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
       return inflater.inflate(R.layout.fragment_sem1, container, false);
    }

One more improvement

• 您应该在 onNavigationItemSelected() 中检查您尝试实例化的 fragment 是否已经可见，然后不要再次实例化它。