'void android.widget.ListView.setAdapter(android.widget.ListAdapter)'空对象引用

Question

我是Android的新手，我有一个应用程序，当我启动应用程序时，该应用程序已经崩溃。 错误是 ：：

尝试在空对象引用上调用虚拟方法 'void android.widget.ListView.setAdapter(android.widget.ListAdapter('

我的主要活动代码是 ：：

public class MainActivity extends AppCompatActivity {
// define channel
private static final String ChANNEL_ID = "simplified_coding";
private static final String CHANNEL_NAME = "Simplified Coding";
private static final String ChANNEL_DESC = "Simplified Coding Notification";
FloatingActionButton fab;
Button sendBtn;
Button sendBtn2;
EditText phoneEditText;
EditText plainEditText;

ListView mListView ;
ArrayList<Contacts> arrayList;
ListAdapter listAdapter;

ArrayAdapter adapter;
ArrayList<String> listItem;
MyDatabase MyDb;


//    static TextView cipherTextView;
static TextView receivedTextview;
static String pubKey="MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCJoBRPFYZb19vXqpiHzo2gPTq0" +
"D1Ey++4PohVNboTBfsscUYA8SMPX4/Jz0NmhDbtk1PL3rqOVKubcEyJumxA+l46B" +
"gNoQsnOBqXfE5Voo0GTbgP4AeN6MnkJ8mhawj3vu2TGmQqtlFYRyYbp6MZ6Q7ops" +
"kZBKULJJJSwC9qSpewIDAQAB";
static String privKey="MIICXAIBAAKBgQCJoBRPFYZb19vXqpiHzo2gPTq0D1Ey++4PohVNboTBfsscUYA8" +
"SMPX4/Jz0NmhDbtk1PL3rqOVKubcEyJumxA+l46BgNoQsnOBqXfE5Voo0GTbgP4A" +
"eN6MnkJ8mhawj3vu2TGmQqtlFYRyYbp6MZ6Q7opskZBKULJJJSwC9qSpewIDAQAB" +
"AoGATMqdimMgTS7s5DG0WlLOg5KmpsHFNQWNl/uAl2wLd0wgWPTA8OJJJLV0G1t1" +
"k3/uCB0qVTmE3T7LAhFLLRwou2wZbw3w48jF4uTXQJex6cf1xHVkXy6jBUVLlddW" +
"74MQv3pfBRs/s0NsYML4J713r55JNmTU26WDwLA2xPNiosECQQDxAQQb7i1ZmSnr" +
"9h0J6Ss3SGqpqDr/RRvUjSmCaCZtP/YFyNEd1aseQ8Wc31PpouwFgJbFYPEOAw+n" +
"h2UECEiHAkEAkjBUvSFOFNnQIeiq1TUN/piEehSa8inLdU1psroVr09GprpS6nGZ" +
"df66Nv0DhMCw6N1VjywnLNi8ru5zpg34bQJBAIXZWJzihRFT/XJtd+c/Np83evXK" +
"1ylbCn/b/ofMrDrU4SCJTYrSVn0yipf7eLEzqP3NUu9ATeDIVnNmwPWiXBUCQGCL" +
"PJs+8IMCqdqK/q8Zf6mA7hR1oTKGeZB5VHGtRP7m+wDHElU0OqXbWP1RsM4uN2dC" +
"LOwFu8V5SXq5jXS/yy0CQCpkkZ6YEHxcKJGptVyAtC5iYNCun0pYNjrJHflu6ia8" +
"pvX7FcJBo2l2Kmld6gXDlYnlJpyOSTapMDjSEs/DNr8=";


@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getWindow().getDecorView().setBackgroundColor(Color.rgb(249, 247, 209));
//      mDatabase = openOrCreateDatabase(SendActivity.DATABASE_NAME, MODE_PRIVATE, null);
MyDb = new MyDatabase(this);

//        loadContact_FromDatabase();
arrayList = new ArrayList<>();
loadDataInListView();

listItem = new ArrayList<>();
mListView = (ListView) findViewById(R.id.list_view);

// floating action button onclick
fab = (FloatingActionButton) findViewById(R.id.floatingActionButton);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent intent = new Intent(MainActivity.this, SendActivity.class);
startActivity(intent);
}
});
// Database Code
//       mDatabase = openOrCreateDatabase(DATABASE_NAME, MODE_PRIVATE, null);
//
//        CREATE_TABLE();

// handel android8 for channel
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
NotificationChannel channel = new NotificationChannel(ChANNEL_ID, CHANNEL_NAME, NotificationManager.IMPORTANCE_DEFAULT);
channel.setDescription(ChANNEL_DESC);
NotificationManager notificationManager = getSystemService(NotificationManager.class);
notificationManager.createNotificationChannel(channel);
}


Typeface face = Typeface.createFromAsset(getAssets(),
"fonts/BRoya.ttf");
}
private void loadDataInListView() {

arrayList = MyDb.loadPHone_FromDb();
listAdapter = new ListAdapter(this, arrayList);
mListView.setAdapter(listAdapter);
listAdapter.notifyDataSetChanged();
}
private void loadContact_FromDatabase() {
try {

Cursor cursor = MyDb.loadContact_FromDatabase();
if (cursor.getCount() == 0) {
Toast.makeText(this, "No Data To Show", Toast.LENGTH_SHORT).show();
} else {
while (cursor.moveToNext()) {
listItem.add(cursor.getString(1));
}
adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listItem);
mListView.setAdapter(adapter);
}
} catch (Exception ex) {
Toast.makeText(this, "load data",
Toast.LENGTH_LONG).show();
}
}
public String encryptBeforeSend(String inputString) {
// encrypt
byte[] userData = inputString.getBytes();
try {
byte[] encodeData = myRSA.encryptByPublicKey(userData, pubKey);
//            String encodeStr = new BigInteger(1, encodeData).toString(16);
String encodeStr = Base64.encodeToString(encodeData, Base64.DEFAULT);
return encodeStr;
}
catch (Exception ex){
ex.printStackTrace();
return ex.toString();
}
}

public static String decryptAfterReceive(String receivedMsg) {
byte[] userData = Base64.decode(receivedMsg, Base64.DEFAULT);
try {
byte[] decodeData = myRSA.decryptByPrivateKey(userData,privKey);
String decodeStr = new String(decodeData);
return decodeStr;
}catch (Exception e){
e.printStackTrace();
return e.toString();
}
}

public  void receiverHandler(Context context, String sender, String receivedMsg) {
if(sender == null)
{
receivedTextview.setText("sender is null ");
return;
}
if(receivedMsg == null)
{
receivedTextview.setText("msg is null ");
return;
}
try {
String decodedText = decryptAfterReceive(receivedMsg);
if (decodedText == null) {
decodedText = receivedMsg;
}
String presentableText =  "فرستنده:  " + sender + " nn " + decodedText;
if(receivedTextview == null)
{
Intent resultIntent = new Intent(context, MessageActivity.class);
PendingIntent resultPendingIntent = PendingIntent.getActivity(context,1, resultIntent, PendingIntent.FLAG_UPDATE_CURRENT);

NotificationCompat.Builder mBuilder =
new NotificationCompat.Builder(context.getApplicationContext(), ChANNEL_ID)
.setSmallIcon(R.drawable.tic_tac_toe)
.setContentTitle("دوز")
.setContentText(presentableText)
.setAutoCancel(true)
.setContentIntent(resultPendingIntent)
.setPriority(NotificationCompat.PRIORITY_DEFAULT);

NotificationManagerCompat mNotificationMgr = NotificationManagerCompat.from(context.getApplicationContext());
mNotificationMgr.notify(1, mBuilder.build());
}
else{
receivedTextview.setText(presentableText);
}
MyDb.addContacts(sender, decodedText, context);
}
catch (Exception ex)
{
Toast.makeText(context, ex.toString(),
Toast.LENGTH_LONG).show();
}

}
}

我的列表适配器代码是 ：：

public class ListAdapter extends BaseAdapter {

Context context;
ArrayList<Contacts> arrayList;

public ListAdapter(Context context, ArrayList<Contacts> arrayList){
this.context = context;
this.arrayList = arrayList;
}

@Override
public long getItemId(int position) {
return position;
}

@Override
public Object getItem(int position) {
return arrayList.get(position);
}
@Override
public View getView(int position, View convertview, ViewGroup parent) {

LayoutInflater inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
convertview = inflater.inflate(R.layout.list_item, null);
TextView phoneNumber = (TextView)convertview.findViewById(R.id.list_item_textview);
Contacts contacts = arrayList.get(position);
phoneNumber.setText(contacts.getPhoneNumber());
return convertview;
}
@Override
public int getCount() {
return this.arrayList.size();
}
}

我的主活动.xml代码是 ：：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
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="maa.tic_tac_to.MainActivity"
tools:layout_editor_absoluteY="81dp"
tools:layout_editor_absoluteX="0dp">

<com.google.android.material.floatingactionbutton.FloatingActionButton
android:id="@+id/floatingActionButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/add_new_contact"
android:layout_alignParentBottom="true"
android:layout_alignParentEnd="true"
android:layout_margin="16dp"
android:background="@color/colorAccent"/>

<ListView
android:id="@+id/list_view"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_alignParentStart="true"
android:layout_alignParentTop="true">
</ListView>

</RelativeLayout>

list_item.xml是 ：：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/list_item_textview"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</LinearLayout>

现在，当我想转到主活动时，程序崩溃了!!!! 谁能帮我!?????

Answer 1

你正在打电话

loadDataInListView();

以前

mListView = (ListView) findViewById(R.id.list_view);

这样，当您使用mListView.setAdapter(listAdapter);设置适配器时mListView为 null