空指针异常尝试在列表视图上实现搜索

Question

>我使用 AsyncTask 创建了一个 SQL 数据库，该数据库在一个活动中接收输入并在另一个活动中显示它们。现在我正在尝试学习如何使用搜索视图在列表视图中搜索数据库。

我有一个绑定到列表视图的适配器类。其中有一个过滤功能，用于过滤搜索视图，这是我认为错误的来源;因为错误发生在适配器上

。我的错误：

7167-7167/com.example.andrewg.sqltutorial E/AndroidRuntime: FATAL EXCEPTION: 
main
Process: com.example.andrewg.sqltutorial, PID: 7167
java.lang.NullPointerException: Attempt to invoke virtual method 'void 
com.example.andrewg.sqltutorial.ProductAdapter.filter(java.lang.String)' on a 
null object reference
at
com.example.andrewg.sqltutorial.DisplayProduct$1.onQueryTextChange
(DisplayProduct.java:47)
at android.widget.SearchView.onTextChanged(SearchView.java:1250)
at android.widget.SearchView.access$2100(SearchView.java:98)
at android.widget.SearchView$10.onTextChanged(SearchView.java:1776)
at android.widget.TextView.sendOnTextChanged(TextView.java:9754)
at android.widget.TextView.handleTextChanged(TextView.java:9851)
at android.widget.TextView$ChangeWatcher.onTextChanged(TextView.java:12512)
at android.text.SpannableStringBuilder.sendTextChanged(SpannableStringBuilder.java:1263)
at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:575)
at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:506)
at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:36)
at android.view.inputmethod.BaseInputConnection.replaceText(BaseInputConnection.java:843)
at android.view.inputmethod.BaseInputConnection.setComposingText(BaseInputConnection.java:616)
at com.android.internal.view.IInputConnectionWrapper.executeMessage(IInputConnectionWrapper.java:396)
at com.android.internal.view.IInputConnectionWrapper$MyHandler.handleMessage(IInputConnectionWrapper.java:85)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6649)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:826)

显示数据库.java

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.TextUtils;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ListView;
import android.widget.SearchView;
public class DisplayProduct extends AppCompatActivity {
ListView listView;
ProductAdapter adapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.display_product_layout);
listView = findViewById(R.id.Display_listview);
BackgroundTask backgroundTask = new BackgroundTask(this);
backgroundTask.execute("get_info");
//-----------------------------EDIT--------------------------------
adapter = new ProductAdapter(this, R.layout.display_product_row);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.menu, menu);
MenuItem myActionMenuItem = menu.findItem(R.id.action_search);
SearchView searchView = (SearchView) myActionMenuItem.getActionView();
searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
@Override
public boolean onQueryTextSubmit(String s) {
return false;
}
@Override
public boolean onQueryTextChange(String s) {
if(TextUtils.isEmpty(s)){
adapter.filter("");
listView.clearTextFilter();
}
else{
adapter.filter(s);
}
return true;
}
});
return super.onCreateOptionsMenu(menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if(id == R.id.action_settings){
//do your functionality here
return true;
}
return super.onOptionsItemSelected(item);
}
}

产品适配器.java

import android.content.Context;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;
import java.util.ArrayList;
import java.util.List;
import java.util.Locale;
public class ProductAdapter extends ArrayAdapter {
List list = new ArrayList();
//Menu Search
Context context;
ArrayList<Product> productArrayList = new ArrayList<>();

public ProductAdapter(@NonNull Context context, int resource) {
super(context, resource);
this.context = context;
this.productArrayList.addAll(list);
}
public void addList(ArrayList<Product> Oldarraylist){
this.productArrayList = Oldarraylist;
}
public void add(Product object) {
list.add(object);
super.add(object);
}
@Override
public int getCount() {
return list.size();
}
@Nullable
@Override
public Object getItem(int position) {
return list.get(position);
}
@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
//Referencing each row as a View, which is an object
View row = convertView;
ProductHolder productHolder;
if(row == null){
//There is no view at this position, so we are creating a new one
//In this case we are inflating an XML layout
LayoutInflater layoutInflater = (LayoutInflater) this.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
row = layoutInflater.inflate(R.layout.display_product_row, parent, false);
//Locate the views in row.xml
productHolder = new ProductHolder();
productHolder.id = row.findViewById(R.id.ID);
productHolder.name = row.findViewById(R.id.NAME);
productHolder.price = row.findViewById(R.id.PRICE);
productHolder.quantity = row.findViewById(R.id.QUANTITY);
/*setTag is very useful for custom ArrayAdapter using. It is some kind of optimization.
There setTag used as reference to object that references on some parts of layout
(that displaying in ListView) instead of findViewById.
*/
row.setTag(productHolder);
}
else{
//We are recycling a view that already exists
productHolder = (ProductHolder) row.getTag();
}
//Once we have a reference to the View we are returning, we set its values
//Setting results into textviews
Product product = (Product) getItem(position);
productHolder.id.setText(product.getID().toString());
productHolder.name.setText(product.getName().toString());
productHolder.price.setText(Integer.toString(product.getPrice()));
productHolder.quantity.setText(Integer.toString(product.getQuantity()));
//Menu
//Listview item clicks
row.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
}
});
return row;
}
//Menu
public void filter(String charText){
charText = charText.toLowerCase(Locale.getDefault());
list.clear();
if(charText.length() == 0){
list.addAll(productArrayList);
}
else{
for (Product product : productArrayList){
if(product.getName().toLowerCase(Locale.getDefault()).contains(charText)){
list.add(product);
}
}
}
notifyDataSetChanged();
}
static class ProductHolder{
TextView id, name, price, quantity;
}
}

我的异步任务线程中的相关代码

public class BackgroundTask extends AsyncTask<String,Product,String> {
Context context;
ProductAdapter productAdapter;
Activity activity;
ListView listView;
ArrayList<Product> arrayList = new ArrayList<>();
@Override
protected String doInBackground(String... params) {
listView = activity.findViewById(R.id.Display_listview);
SQLiteDatabase db = dbOperations.getReadableDatabase();
Cursor cursor = dbOperations.getInformation(db);
productAdapter = new ProductAdapter(context, 
R.layout.display_product_row);
String id, name;
int price, quantity;
while(cursor.moveToNext()){
id = 
cursor.getString(cursor.getColumnIndex(ProductContract.ProductEntry.ID));
name = 
cursor.getString(cursor.getColumnIndex(ProductContract.ProductEntry.NAME));
price = 
cursor.getInt(cursor.getColumnIndex(ProductContract.ProductEntry.PRICE));
quantity = 
cursor.getInt(cursor.getColumnIndex(ProductContract.ProductEntry.QUANTITY));
Product product = new Product(id, name, price, quantity);
publishProgress(product);
}
return "get_info";
}
@Override
protected void onPostExecute(String s) {
//Bind the array adapter to the listview
listView.setAdapter(productAdapter);
productAdapter.addList(arrayList);
@Override
protected void onProgressUpdate(Product... values) {
productAdapter.add(values[0]);
arrayList.add(values[0]);
}

Answer 1

正如@Psypher指出的，您必须初始化 ProductAdapter。但是，仅在 AsyncTask 中添加ProductAdapter adapter = new ProductAdapter()是行不通的，因为它引用的不是您之前创建的同一对象。

您需要先创建适配器，然后将其传递到 AsyncTask 中(或将其设为静态(。

Answer 2

在调用 product.filter(( 方法之前，您尚未初始化ProductAdapter adapter;。当您在 onQueryTextChange 中调用 filter(( 时，适配器为 null，因此会出现错误。

如果 ProductAdapter 类被声明为静态，您现在拥有的代码可能已经有效，因为您不必在调用类中的方法之前创建该类的新实例。

在调用筛选器方法之前，需要具有ProductAdapter adapter= new ProductAdapter()。

在BackgroundTask类中也是如此，在调用 add 或 addList 之前，您尚未初始化适配器变量。