数组适应在填充listView时给出空指针异常

Question

Bank.java文件代码

package com.example.bloodbank;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Spinner;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import java.util.ArrayList;

public class Bank extends AppCompatActivity {
private Spinner spBloodGroup,spCity;
private String bgArray[]=new String[] {"A+","A-","B+","B-","O+","O-","AB+","AB-"};
private DBConnection db;
private ListView lvDonors;
private ArrayList<String> city,name;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.acitvity_bank);
//Initialisation spinner
spBloodGroup=findViewById(R.id.spBloodGroup);
spCity=findViewById(R.id.spCity);
//Initialisation ListView
lvDonors=findViewById(R.id.lvDonors);
//ArrayAdapter to spinner blood group
ArrayAdapter<String> adapter =new ArrayAdapter<>(this,R.layout.bank_spinner,bgArray);
spBloodGroup.setAdapter(adapter);
//Initialisation of Arraylist
city=new ArrayList<String>();
name=new ArrayList<>();
//Initialisation of database onject
db=new DBConnection(this);
//Populating City spinner
populateList();
ArrayAdapter<String> adapter1=new ArrayAdapter<>(this,R.layout.spinner_city,city);
spCity.setAdapter(adapter1);

spBloodGroup.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
filterDonor();
}
@Override
public void onNothingSelected(AdapterView<?> adapterView) {
}
});
spCity.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
filterDonor();
}
@Override
public void onNothingSelected(AdapterView<?> adapterView) {
}
});

}
private void populateList() {
SQLiteDatabase sqldb=db.getWritableDatabase();
Cursor cursor=sqldb.rawQuery("select city from registration",null);
while(cursor.moveToNext()) {
city.add(cursor.getString(cursor.getColumnIndex("city")));
}
}
private void filterDonor() {
name.clear();
SQLiteDatabase sqldb=db.getWritableDatabase();
Cursor cursor=sqldb.rawQuery("select name from registration natural join donor where donor.bloodgroup='"+spBloodGroup.getSelectedItem().toString()+"' and registration.city='"+spCity.getSelectedItem().toString()+"'",null);
while(cursor.moveToNext()) {
name.add(cursor.getString(cursor.getColumnIndex("name")));
}
ArrayAdapter<String> adapter=new ArrayAdapter<>(this,R.layout.custom_listview,R.id.lvDonors,name);
lvDonors.setAdapter(adapter);
}
}

Custom_listview.xml布局

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:padding="15dp">
<TextView
android:id="@+id/lvtvName"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="34sp"
android:textColor="#000000"/>
</RelativeLayout>

activity_bank.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:tools="http://schemas.android.com/tools"
tools:context=".Bank">
<TextView
android:id="@+id/tvDonarHeader"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Donate"
android:textSize="44sp"
android:textColor="#FF0000"
android:layout_centerHorizontal="true"/>
<TextView
android:id="@+id/tvBLoodGroup"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Blood Group"
android:layout_below="@+id/tvDonarHeader"
android:textSize="20sp"
android:textColor="#000000"
android:layout_marginRight="5dp"/>
<Spinner
android:id="@+id/spBloodGroup"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_toRightOf="@+id/tvBLoodGroup"
android:layout_below="@+id/tvDonarHeader"/>
<TextView
android:id="@+id/tvCity"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="City"
android:layout_below="@+id/tvDonarHeader"
android:textSize="20sp"
android:layout_toRightOf="@+id/spBloodGroup"
android:textColor="#000000"
android:layout_marginRight="5dp"/>
<Spinner
android:id="@+id/spCity"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_toRightOf="@+id/tvCity"
android:layout_below="@+id/tvDonarHeader"/>
<ListView
android:id="@+id/lvDonors"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_below="@+id/spBloodGroup"
android:divider="@color/teal_200"
android:dividerHeight="2dp"
android:listSelector="@color/teal_200"/>
</RelativeLayout>

得到这个错误

E/AndroidRuntime: FATAL EXCEPTION: maincom . example。过程:血库，PID: 16440java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法'void android.widget.TextView.setText(java.lang.CharSequence)'android.widget.ArrayAdapter.createViewFromResource (ArrayAdapter.java: 392)android.widget.ArrayAdapter.getView (ArrayAdapter.java: 362)android.widget.AbsListView.obtainView (AbsListView.java: 2459)android.widget.ListView.measureHeightOfChildren (ListView.java: 1281)android.widget.ListView.onMeasure (ListView.java: 1189)android.view.View.measure (View.java: 17765)android.widget.RelativeLayout.measureChildHorizontal (RelativeLayout.java: 841)android.widget.RelativeLayout.onMeasure (RelativeLayout.java: 507)android.view.View.measure (View.java: 17765)android.view.ViewGroup.measureChildWithMargins (ViewGroup.java: 5620)android.widget.FrameLayout.onMeasure (FrameLayout.java: 454)androidx.appcompat.widget.ContentFrameLayout.onMeasure (ContentFrameLayout.java: 145)android.view.View.measure (View.java: 17765)android.view.ViewGroup.measureChildWithMargins (ViewGroup.java: 5620)androidx.appcompat.widget.ActionBarOverlayLayout.onMeasure (ActionBarOverlayLayout.java: 496)android.view.View.measure (View.java: 17765)android.view.ViewGroup.measureChildWithMargins (ViewGroup.java: 5620)android.widget.FrameLayout.onMeasure (FrameLayout.java: 454)android.view.View.measure (View.java: 17765)android.view.ViewGroup.measureChildWithMargins (ViewGroup.java: 5620)android.widget.LinearLayout.measureChildBeforeLayout (LinearLayout.java: 1687)android.widget.LinearLayout.measureVertical (LinearLayout.java: 755)android.widget.LinearLayout.onMeasure (LinearLayout.java: 624)android.view.View.measure (View.java: 17765)android.view.ViewGroup.measureChildWithMargins (ViewGroup.java: 5620)android.widget.FrameLayout.onMeasure (FrameLayout.java: 454)com.android.internal.policy.impl.PhoneWindow DecorView.onMeasure美元(PhoneWindow.java: 2616)android.view.View.measure (View.java: 17765)android.view.ViewRootImpl.performMeasure (ViewRootImpl.java: 2349)android.view.ViewRootImpl.measureHierarchy (ViewRootImpl.java: 1373)android.view.ViewRootImpl.performTraversals (ViewRootImpl.java: 1597)android.view.ViewRootImpl.doTraversal (ViewRootImpl.java: 1251)android.view.ViewRootImpl TraversalRunnable.run美元(ViewRootImpl.java: 6438)android.view.Choreographer CallbackRecord.run美元(Choreographer.java: 795)android.view.Choreographer.doCallbacks (Choreographer.java: 598)android.view.Choreographer.doFrame (Choreographer.java: 567)android.view.Choreographer FrameDisplayEventReceiver.run美元(Choreographer.java: 781)android.os.Handler.handleCallback (Handler.java: 810)android.os.Handler.dispatchMessage (Handler.java: 99)android.os.Looper.loop (Looper.java: 189)android.app.ActivityThread.main (ActivityThread.java: 5532)在java.lang.reflect.Method。调用(本地方法)java.lang.reflect.Method.invoke (Method.java: 372)com.android.internal.os.ZygoteInit MethodAndArgsCaller.run美元(ZygoteInit.java: 950)com.android.internal.os.ZygoteInit.main (ZygoteInit.java: 745)

Answer 1

在我看来，在倒数第二行，您试图调用这个构造函数:

ArrayAdapter(Context Context, int resource, int textViewResourceId, T[] objects)

注意第三个参数应该是对TextView的引用。但是你的第三个参数是对ListView的引用。