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android - 添加网络检测代码后FAB的空指针异常（即使定义好）

问题描述

我添加了一些代码来检测是否有互联网连接（请在下面找到它），它运行良好，但是当我只想在有互联网连接时显示 mainactivity 时，应用程序会抛出 fab 空指针异常。这是我的日志：

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.material.manohar.vblogs/com.material.manohar.vblogs.MainActivity}: 
    java.lang.NullPointerException: Attempt to invoke virtual method 'void     android.support.design.widget.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2779)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2857)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1590)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6499)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:440)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

MainActivity.java

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if(!isConnected(MainActivity.this)) buildDialog(MainActivity.this).show();
       else {
           setContentView(R.layout.activity_main);
       }

       Toolbar 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) {
               Intent intent = new Intent(Intent.ACTION_SENDTO, Uri.fromParts(
                    "mailto","xxxxx@gmail.com", null));
               String subject = "Suggest/Feedback -- VB";
               String message = "";
               intent.putExtra(Intent.EXTRA_SUBJECT, subject);
               intent.putExtra(Intent.EXTRA_TEXT, message);
               startActivity(Intent.createChooser(intent, "Choose an Email client :"));
           }
       });

       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.addDrawerListener(toggle);
       toggle.syncState();

       NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
       navigationView.setNavigationItemSelectedListener(this);
}


public boolean isConnected(Context context) {
    ConnectivityManager cm = (ConnectivityManager) context.getSystemService(context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = cm.getActiveNetworkInfo();

    if (networkInfo != null && networkInfo.isConnectedOrConnecting()) {
        android.net.NetworkInfo wifi=cm.getNetworkInfo(ConnectivityManager.TYPE_WIFI);
    android.net.NetworkInfo mobile=cm.getNetworkInfo(ConnectivityManager.TYPE_MOBILE);

        if ((mobile!= null && mobile.isConnectedOrConnecting()) || (wifi!= null && wifi.isConnectedOrConnecting())) return true;
        else return false;
    }
    else
        return false;
    }

public AlertDialog.Builder buildDialog(Context c) {

    AlertDialog.Builder builder = new AlertDialog.Builder(c);
    builder.setTitle("No Internet Connection");
    builder.setMessage("You need to have Mobile Data or wifi to access the app.     Press ok to Exit");

    builder.setPositiveButton("Ok", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            finish();
        }
    });


    return builder;
}
//Rest code is nav drawer functionalities which we get by selecting nav drawer   activity layout.

野兔是我的 activity_main.xml 文件：

 <?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout              
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:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:openDrawer="start">

<include
layout="@layout/app_bar_main"
android:layout_width="match_parent"
android:layout_height="match_parent" />

<android.support.design.widget.NavigationView
android:id="@+id/nav_view"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
android:fitsSystemWindows="true"
app:headerLayout="@layout/nav_header_main"
app:menu="@menu/activity_main_drawer" />

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

包含 fab 视图的 app_bar_main.xml 文件：

<?xml version="1.0" encoding="utf-8"?>
   <android.support.design.widget.CoordinatorLayout                
   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"
    tools:context=".MainActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:titleTextAppearance="@style/TitleBarTextAppearance"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>

    <include layout="@layout/content_main" />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        app:srcCompat="@android:drawable/ic_dialog_email" />

    </android.support.design.widget.CoordinatorLayout>

标签： androidfloating-action-button

解决方案

仔细查看您的代码，特别是这部分：

if(!isConnected(MainActivity.this)) buildDialog(MainActivity.this).show();
else {
    setContentView(R.layout.activity_main);
}

您仅setContentView()在有连接时才调用，但无论如何您都在初始化和编辑视图。如果您没有互联网，您的 if 语句将运行buildDialog().show()，_然后继续到您的其余部分onCreate()。

要解决这个问题，只需将其他所有内容放在onCreate()elsesetContentView()块中：

if(!isConnected(MainActivity.this)) buildDialog(MainActivity.this).show();
else {
    setContentView(R.layout.activity_main);

    Toolbar 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) {
            Intent intent = new Intent(Intent.ACTION_SENDTO, Uri.fromParts(
                    "mailto","xxxxx@gmail.com", null));
            String subject = "Suggest/Feedback -- VB";
            String message = "";
            intent.putExtra(Intent.EXTRA_SUBJECT, subject);
            intent.putExtra(Intent.EXTRA_TEXT, message);
            startActivity(Intent.createChooser(intent, "Choose an Email client :"));
        }
    });

    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.addDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
}

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