ListView.setAdapter Null 指针异常 而 adapter.getCount 显示所有记录，并且 L

Question

我有一个应用程序，它使用自定义适配器和列表中的溢出菜单项填充ListView。从MainActivity，调用自定义适配器，并调用实现onClickListner的类OnAlbumOverflowSelectedListener。在其单击功能中，我想淡化除单击的项目之外的所有列表项目。

当我尝试调用myListview.seteAdapter(Cus_aptr)时，它会抛出空指针异常。所以下面的MainActivity->CustomAdapter-> OnAlbumOverflowSelectedListener->onCLick是代码片段：

1. main activity
package com.example.shahnaii;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.snackbar.Snackbar;
import java.util.ArrayList;
public class ScrollingActivity extends AppCompatActivity {
ArrayList<DataModel> dataModels;
ListView listView;
private static CustomAdapter adapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_scrolling);
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) {
Snackbar.make(view, "Open new window to add entries", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
listView = findViewById(R.id.list_myContent);
dataModels= new ArrayList<>();
dataModels.add(new DataModel("Apple Pie", "Android 1.0", "1","September 23, 2008"));
dataModels.add(new DataModel("Banana Bread", "Android 1.1", "2","February 9, 2009"));
dataModels.add(new DataModel("Cupcake", "Android 1.5", "3","April 27, 2009"));
dataModels.add(new DataModel("Donut","Android 1.6","4","September 15, 2009"));
dataModels.add(new DataModel("Eclair", "Android 2.0", "5","October 26, 2009"));
dataModels.add(new DataModel("Froyo", "Android 2.2", "8","May 20, 2010"));
dataModels.add(new DataModel("Gingerbread", "Android 2.3", "9","December 6, 2010"));
dataModels.add(new DataModel("Honeycomb","Android 3.0","11","February 22, 2011"));
dataModels.add(new DataModel("Ice Cream Sandwich", "Android 4.0", "14","October 18, 2011"));
dataModels.add(new DataModel("Jelly Bean", "Android 4.2", "16","July 9, 2012"));
dataModels.add(new DataModel("Kitkat", "Android 4.4", "19","October 31, 2013"));
dataModels.add(new DataModel("Lollipop","Android 5.0","21","November 12, 2014"));
dataModels.add(new DataModel("Marshmallow", "Android 6.0", "23","October 5, 2015"));
//adapter= new CustomAdapter(dataModels,getApplicationContext());
adapter= new CustomAdapter(dataModels,this);
listView.setAdapter(adapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
DataModel dataModel= dataModels.get(position);
Snackbar.make(view, dataModel.getName()+"n"+dataModel.getType()+" API: "+dataModel.getVersion_number(), Snackbar.LENGTH_LONG)
.setAction("No action", null).show();
}
});

}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_scrolling, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
switch (id)
{
case R.id.action_Menu1: {
Toast.makeText(getApplicationContext(),"Menu 1 Selected",Toast.LENGTH_SHORT).show();
}

}
return super.onOptionsItemSelected(item);
}
}
2. custom Adapter
package com.example.shahnaii;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.animation.AlphaAnimation;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.PopupMenu;
import android.widget.TextView;
import com.google.android.material.snackbar.Snackbar;
import java.lang.reflect.Field;
import java.util.ArrayList;

public class CustomAdapter extends ArrayAdapter<DataModel> implements View.OnClickListener{

Context mContext;
ArrayList<DataModel> mData;
// View lookup cache
private static class ViewHolder {
TextView txtName;
TextView txtType;
TextView txtVersion;
ImageView info;
ImageView overFlow;
}
public CustomAdapter(ArrayList<DataModel> data, Context context) {
super(context, R.layout.row_item, data);
this.mContext=context;
this.mData=data;
}
@Override
public void onClick(View v) {
int position=(Integer) v.getTag();
Object object= getItem(position);
DataModel dataModel=(DataModel)object;
switch (v.getId())
{
case R.id.item_info:
Snackbar.make(v, "Release date " +dataModel.getFeature(), Snackbar.LENGTH_LONG)
.setAction("No action", null).show();
break;
}

}
private int lastPosition = -1;
@Override
public View getView(int position, View convertView, ViewGroup parent) {
// Get the data item for this position
DataModel dataModel = getItem(position);
// Check if an existing view is being reused, otherwise inflate the view
ViewHolder viewHolder; // view lookup cache stored in tag
final View result;
if (convertView == null) {
viewHolder = new ViewHolder();
LayoutInflater inflater = LayoutInflater.from(getContext());
convertView = inflater.inflate(R.layout.row_item, parent, false);
viewHolder.txtName = (TextView) convertView.findViewById(R.id.name);
viewHolder.txtType = (TextView) convertView.findViewById(R.id.type);
viewHolder.txtVersion = (TextView) convertView.findViewById(R.id.version_number);
viewHolder.info = (ImageView) convertView.findViewById(R.id.item_info);
viewHolder.overFlow = (ImageView) convertView.findViewById(R.id.album_overflow);
//viewHolder.overFlow.setOnClickListener(new OnAlbumOverflowSelectedListener(mContext,position));
result=convertView;
convertView.setTag(viewHolder);
} else {
viewHolder = (ViewHolder) convertView.getTag();
result=convertView;
}
viewHolder.txtName.setText(dataModel.getName());
viewHolder.txtType.setText(dataModel.getType());
viewHolder.txtVersion.setText(dataModel.getVersion_number());
viewHolder.info.setOnClickListener(this);
viewHolder.info.setTag(position);
viewHolder.overFlow.setOnClickListener(new OnAlbumOverflowSelectedListener(mContext,mData,position));
//viewHolder.overFlow.setTag(position);
// Return the completed view to render on screen
return result;
}
}
3. OnAlbumOverflowSelectedListner
package com.example.shahnaii;
import android.app.Activity;
import android.content.Context;
import android.util.Log;
import android.view.View;
import android.view.animation.AlphaAnimation;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.PopupMenu;
import com.google.android.material.snackbar.Snackbar;
import java.lang.reflect.Field;
import java.util.ArrayList;
public class OnAlbumOverflowSelectedListener  implements View.OnClickListener  {
private ListView mListView;
private ArrayAdapter mArrAdapter;
private ArrayList uData;
private final AlphaAnimation mFadeOut = new AlphaAnimation(1.0f, 0.3f);
private final AlphaAnimation mFadeIn = new AlphaAnimation(0.3f, 1.0f);
//private MediaStore.Audio.Albums mAlbum;
private Context mContext;
private int mPosition;
private int mAlbum;
public OnAlbumOverflowSelectedListener(Context con,ArrayList<?>data,int pos)
{
mContext=con;
mPosition = pos;
this.uData = data;
}
@Override
public void onClick(View v) {
// This is an android.support.v7.widget.PopupMenu;
PopupMenu popupMenu = new PopupMenu(mContext, v) {
};
popupMenu.inflate(R.menu.menu);
// Force icons to show
Object menuHelper;
Class[] argTypes;
try {
Field fMenuHelper = PopupMenu.class.getDeclaredField("mPopup");
fMenuHelper.setAccessible(true);
menuHelper = fMenuHelper.get(popupMenu);
argTypes = new Class[] { boolean.class };
menuHelper.getClass().getDeclaredMethod("setForceShowIcon",
argTypes).invoke(menuHelper, true);
} catch (Exception e) {
popupMenu.show();
return;
}
// Force icons to show ends here
popupMenu.show();
/*fading process begins-*/
// Dim out all the other list items if they exist
CustomAdapter cust_adptr = new CustomAdapter(this.uData,this.mContext);
Object obj = cust_adptr.getItem(mPosition);
DataModel dm = (DataModel)obj;
Snackbar.make(v, "Release date "+cust_adptr.getItem(mPosition).getFeature(), Snackbar.LENGTH_LONG)
.setAction("No action", null).show();
mListView = v.findViewById(R.id.list_myContent);
mListView.setAdapter(cust_adptr);
int firstPos = mListView.getFirstVisiblePosition() - mListView.getHeaderViewsCount();
final int ourPos = mArrAdapter.getPosition(mPosition) - firstPos;
int count = mListView.getChildCount();
for (int i = 0; i <= count; i++) {
if (i == ourPos) {
continue;
}
final View child = mListView.getChildAt(i);
if (child != null) {
child.clearAnimation();
child.startAnimation(mFadeOut);
}
}
// Make sure to bring them back to normal after the menu is gone
popupMenu.setOnDismissListener(new PopupMenu.OnDismissListener() {
@Override
public void onDismiss(PopupMenu popupMenu) {
int count = mListView.getChildCount();
for (int i = 0; i <= count; i++) {
if (i == ourPos) {
continue;
}
final View v = mListView.getChildAt(i);
if (v != null) {
v.clearAnimation();
v.startAnimation(mFadeIn);
}
}
}
});
/*fading process ends*/
}
}

OnAlbumOverflowSelectedListener class
onlcick method()
{
....
CustomAdapter cust_adptr = new CustomAdapter(this.uData,this.mContext);
Object obj = cust_adptr.getItem(mPosition);
DataModel dm = (DataModel)obj;
Snackbar.make(v, "Release date "+cust_adptr.getItem(mPosition).getFeature(), Snackbar.LENGTH_LONG)
.setAction("No action", null).show();
mListView = v.findViewById(R.id.list_myContent);
mListView.setAdapter(cust_adptr); this code throw null pointer

Answer 1

检查activity_scrolling.xml它是否不包含 ID R.id.list_myContent的列表视图。
您也可以发布错误日志吗？

Answer 2

1. 尝试查找 id。在主要情况下，如果您错过了声明 id 并在该视图或按钮上实现任何操作，您将获得NullPointerException

2. 有时，如果您将任何值从后端设置为空值到文本视图或按钮图像视图，那么您也会得到NullPointerException。

3. 或者你可以把你的代码放在try-catch中以避免崩溃