空指針異常DrawerLayout

Question

這是我的MainActivity.java

import android.content.Context;
import android.content.SharedPreferences;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
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;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;


public class MainActivity extends AppCompatActivity {

ListView mNavigationDrawer;


private Toolbar mToolbar;
private ActionBarDrawerToggle mDrawerToggle;
 DrawerLayout mDrawerLayout;

private boolean mUserLearnedDrawer;
private boolean mFromSavedInstanceState;

public static final String PREF_FILE_NAME = "pref";
public static final String KEY_USER_LEARNED_DRAWER = "user_learned_drawer";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (savedInstanceState != null) {
        mFromSavedInstanceState = true;
    }
    mToolbar = (Toolbar) findViewById(R.id.app_bar);
    setSupportActionBar(mToolbar);

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    mDrawerLayout = (DrawerLayout) findViewById(R.id.main_DrawerLayout);

    mNavigationDrawer = (ListView) findViewById(R.id.navigation_drawer);
    mNavigationDrawer.setAdapter(new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, getResources().getStringArray(R.array.navigation_drawer_list)));

    setDrawers(mDrawerLayout, mToolbar);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.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);
}
public void setDrawers(DrawerLayout drawerLayout, Toolbar toolbar) {
    mDrawerLayout = drawerLayout;
    mToolbar = toolbar;
    mDrawerToggle = new ActionBarDrawerToggle(this, drawerLayout, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close) {
        @Override
        public void onDrawerOpened(View drawerView) {
            super.onDrawerOpened(drawerView);
            if (!mUserLearnedDrawer)
                mUserLearnedDrawer = true;
            saveToPreference(getApplicationContext(), KEY_USER_LEARNED_DRAWER, mUserLearnedDrawer + "");
//                getActivity().invalidateOptionsMenu();

        }

        @Override
        public void onDrawerClosed(View drawerView) {
            super.onDrawerClosed(drawerView);
//                getActivity().invalidateOptionsMenu();
        }

        @Override
        public void onDrawerSlide(View drawerView, float slideOffset) {
            super.onDrawerSlide(drawerView, slideOffset);
        }
    };
    mDrawerLayout.setDrawerListener(mDrawerToggle);
    if (!mUserLearnedDrawer && !mFromSavedInstanceState)
        mDrawerLayout.openDrawer(mNavigationDrawer);

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

public static void saveToPreference(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();
//        editor.commit()
}

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

這是我的activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

<include
    android:id="@+id/app_bar"
    layout="@layout/app_bar" />

<android.support.v4.widget.DrawerLayout
    android:name="@+id/main_DrawerLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context="xymen.chetanbhagatnovels.MainActivity">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/hello_world" />

    </RelativeLayout>

    <ListView
        android:id="@+id/navigation_drawer"
        android:layout_width="@dimen/navigation_drawer_width"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:divider="@null"/>

</android.support.v4.widget.DrawerLayout>
</LinearLayout>

它在行mDrawerLayout.setDrawerListener（mDrawerToggle）;上給我錯誤 我不知道為什么。 我也有另一個項目，我從中復制了所有內容，但也沒有出錯，但是確實如此。 救命

Answer 1

根據您提供的有限信息，您的mDrawerLayout成員為null。

如果同一布局文件在多個布局文件夾中有彼此不一致的多個版本，則會發生這種情況。

我懷疑您的res / layout文件夾中有一個名為main_activity.xml的布局文件，該布局文件已經聲明了一個ID為main_DrawerLayout的DrawerLayout視圖，並且該布局文件的另一個版本（也許在您的res / layout-land文件夾中）或者沒有具有此DrawerLayout視圖，或者具有此視圖，但使用不同的視圖ID進行了聲明。

結果，findViewById（R.id.main_DrawerLayout）從您的替代main_activity.xml布局之一返回null，結果是得到了NullPointerException。

Answer 2

我只是運行您的代碼並弄清楚了。

問題是您為android.support.v4.widget.DrawerLayout指定了name而不是id 。

只需更改此：

android:name="@+id/main_DrawerLayout"

對此：

android:id="@+id/main_DrawerLayout"

進行此更改后，NullPointerException已修復，並且對我而言運行良好。