Question

我是android开发领域的新手。我正在尝试创建NavigationDrawerFragment。

请帮我解决这个错误： Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference

package nikita.materialtst;


import android.content.Context;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.app.Fragment;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.widget.Toolbar;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;


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

public static final String PREF_FILE_NAME="testpref";
public static final String KEY_USER_LEARNED_DRAWER="user_learned_drawer";
private ActionBarDrawerToggle mDrawerToggle;
private DrawerLayout mDrawerLayout;
private View containerView;

private boolean mUserLearnedDrawer;
private boolean mFromSavedInstanceState;

public NavigationDrawerFragment() {
    // Required empty public constructor
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mUserLearnedDrawer=Boolean.valueOf( readFromPreferences(getActivity(),KEY_USER_LEARNED_DRAWER,"false"));
    if(savedInstanceState!=null){
       mFromSavedInstanceState=true;
    }

}

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

public void setUp(int fragmentId , DrawerLayout drawerLayout, Toolbar toolbar) {
    containerView=getActivity().findViewById(fragmentId);
    mDrawerLayout= drawerLayout;
    mDrawerToggle=new ActionBarDrawerToggle(getActivity(),drawerLayout,toolbar,R.string.drawer_open,R.string.drawer_close){
        @Override
        public void onDrawerOpened(View drawerView) {
            super.onDrawerOpened(drawerView);
          if(!mUserLearnedDrawer){
              mUserLearnedDrawer=true;
              saveToPreferences(getActivity(),KEY_USER_LEARNED_DRAWER,mUserLearnedDrawer+"");
          }

            getActivity().invalidateOptionsMenu();
        }

        @Override
        public void onDrawerClosed(View drawerView) {
            super.onDrawerClosed(drawerView);
            getActivity().invalidateOptionsMenu();
        }
    };
    if(!mUserLearnedDrawer && !mFromSavedInstanceState){
        mDrawerLayout.openDrawer(containerView);
    }

    mDrawerLayout.setDrawerListener(mDrawerToggle);
    mDrawerLayout.post(new Runnable() {
        @Override
        public void run() {
            mDrawerToggle.syncState();
        }
    });

}
public static  void saveToPreferences(Context context,String preferenceName,String preferenceValue){
    SharedPreferences sharedPreferences=context.getSharedPreferences(PREF_FILE_NAME,Context.MODE_PRIVATE);
    SharedPreferences.Editor editor=sharedPreferences.edit();
    editor.putString(preferenceName,preferenceValue);
    editor.apply();
}


public static String readFromPreferences(Context context, String preferenceName, String defaultValue){
    SharedPreferences sharedPreferences=context.getSharedPreferences(PREF_FILE_NAME,Context.MODE_PRIVATE);
   return sharedPreferences.getString(preferenceName,defaultValue);
}

}

日志

Process: nikita.materialtst, PID: 8089
java.lang.RuntimeException: Unable to start activity ComponentInfo{nikita.materialtst/nikita.materialtst.MainActivity}:    java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference
at nikita.materialtst.NavigationDrawerFragment.setUp(NavigationDrawerFragment.java:53)
at nikita.materialtst.MainActivity.onCreate(MainActivity.java:50)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
at android.app.ActivityThread.-wrap11(ActivityThread.java) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:148) 
at android.app.ActivityThread.main(ActivityThread.java:5417) 
at java.lang.reflect.Method.invoke(Native Method) 
at  com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
03-03 20:53:03.511 8089-8089/nikita.materialtst I/Process: Sending signal.     PID: 8089 SIG: 9

MainActivity.java

package nikita.materialtst;

import android.app.ActionBar;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.provider.ContactsContract;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity  implements View.OnClickListener{

private Toolbar toolbar;
private MainActivity mainActivity;
private Class<SubMaterialTst> subMaterialTstClass;

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

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

    TextView tv_login= (TextView) findViewById(R.id.tv_login);
    EditText et_email= (EditText) findViewById(R.id.et_email);
    EditText et_password= (EditText) findViewById(R.id.et_password);
    Button button= (Button) findViewById(R.id.button);
    TextView tv_registerLink= (TextView) findViewById(R.id.tv_registerLink);

    button.setOnClickListener(this);
    tv_registerLink.setOnClickListener(this);

    getSupportActionBar().setDisplayShowHomeEnabled(true);

    NavigationDrawerFragment drawerFragment =new NavigationDrawerFragment();
            getSupportFragmentManager().findFragmentById(R.id.fragment_navigation_drawer);
    drawerFragment.setUp(R.id.fragment_navigation_drawer, (DrawerLayout) findViewById( R.id.drawer_layout),toolbar);
}

@Override
public void onClick(View v) {
    switch(v.getId()){
        case R.id.button:
            startActivity(new Intent(this, Profile.class) );
            break;

        case R.id.tv_registerLink:
            startActivity(new Intent(this, SubMaterialTst.class) );
            break;
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id=item.getItemId();
    if(id== R.menu.menu_main) {
        Toast.makeText(MainActivity.this, "you just clicked option settings", Toast.LENGTH_SHORT).show();
        return true;
    }

       if(id==R.id.navigate){
            startActivity(new Intent(this, SubMaterialTst.class) );

    }
    return super.onOptionsItemSelected(item);
}

}

Answer 1

您的问题出在此处：containerView=getActivity().findViewById(fragmentId);

找不到视图。请检查此变量fragmentId，以便传递给此方法的内容：public void setUp(int fragmentId , DrawerLayout drawerLayout, Toolbar toolbar)

Answer 2

正如已经说过的那样，代码不是这里的问题。事实是，有时，当您在此示例containerView上将View声明为时，该视图不会同步，而在运行应用程序时，它将引发此异常。要摆脱这种情况，您需要重构代码并再次运行该应用程序。

尝试在空对象引用上调用虚方法'android.view.View android.app.Activity.findViewById（int）'

2 个答案: