数组适配在填充 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>

出现这个错误

E/AndroidRuntime：致命异常：main 进程：com.example.bloodbank，PID：16440 java.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)' 在 android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:392) 在 android.widget.ArrayAdapter.getView(ArrayAdapter.java:362) 在 android.widget.AbsListView.obtainView(AbsListView.java:2459) 在 android.widget.ListView.measureHeightOfChildren(ListView.java:1281) 在 android.widget.ListView.onMeasure(ListView.java:1189) 在 android.view.View.measure(View.java:17765) 在 android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:841) 在 android.widget.RelativeLayout.onMeasure(RelativeLayout.java:507) 在 android.view.View.measure(View.java:17765) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5620) 在 android.widget.FrameLayout.onMeasure(FrameLayout.java:454) 在 androidx.appcompat.widget.ContentFrameLayout.onMeasure（内容FrameLayout.java:145）在 android.view.View.measure(View.java:17765) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5620) 在 androidx.appcompat.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:496) 在 android.view.View.measure(View.java:17765) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5620) 在 android.widget.FrameLayout.onMeasure(FrameLayout.java:454) 在 android.view.View.measure(View.java:17765) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5620) 在 android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1687) 在 android.widget.LinearLayout.measureVertical(LinearLayout.java:755) 在 android.widget.LinearLayout.onMeasure(LinearLayout.java:624) 在 android.view.View.measure(View.java:17765) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5620) 在 android.widget.FrameLayout.onMeasure(FrameLayout.java:454) 在 com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2616) 在 android.view.View.measure(View.java:17765) 在 android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2349) 在 android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1373) 在 android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1597) 在 android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1251) 在 android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6438) 在 android.view.Choreographer$CallbackRecord.run(Choreographer.java:795) 在 android.view.Choreographer.doCallbacks(Choreographer.java:598) 在 android.view.Choreographer.doFrame(Choreographer.java:567) 在 android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:781) 在 android.os.Handler.handleCallback(Handler.java:810) 在 android.os.Handler.dispatchMessage(Handler.java:99) 在 android.os.Looper.loop(Looper.java:189) 在 android.app.ActivityThread.main(ActivityThread.java:5532) 在 java.lang.reflect.Method.invoke（本机方法）在 java.lang.reflect.Method.invoke(Method.java:372) 在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:950) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745)

Answer 1

在我看来，在倒数第二行，您正试图调用此构造函数：

ArrayAdapter（上下文上下文，int 资源，int textViewResourceId，T[] 对象）

请注意，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。

数组适配在填充 listView 时给出空指针异常

Array Adaption giving null pointer Exception while populating listView

android

listview

android-arrayadapter