如何在 android 中的空对象引用上解决"void android.widget.ListView.setAdapter(android.widget.ListAdapter)"？

Question

我正在实现包含所有其他活动的工具栏的"BaseActivity"。 此基本活动需要在其他活动中扩展以添加工具栏。现在的问题是我有一个名为"ElectronicsMainActivity"的活动，其中包含列表视图。此活动需要扩展"基本活动"，例如

public class ElectronicsMainActivity 扩展 BaseActivity{

但是启动此活动时，我的应用程序在运行时崩溃。

错误是：

java.lang.NullPointerException：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"ElectronicsMainActivity"代码是：

public class  ElectronicsMainActivity extends BaseActivity{
private ListView lvComments;
private ModelWomen comment;
private AdapterCommentList adpComment;
private SparseArray<View> mapViewHolder;
private ArrayList<ModelWomen> alComment;
int i;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_electronics_main);
lvComments = (ListView) findViewById(R.id.lvListview);
alComment = new ArrayList<>();
mapViewHolder = new SparseArray<View>();
comment = new ModelWomen();
comment = (ModelWomen) getIntent().getSerializableExtra("id");

initUi();
new AsyncGetWomencategories().execute();
}
public void initUi() {
adpComment = new AdapterCommentList(getApplicationContext(), alComment);
lvComments.setAdapter(adpComment);
final String usr =getIntent().getStringExtra("userid");
lvComments.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
if(i==0) {
Intent intent = new Intent(getApplicationContext(), CellPhones.class);
intent.putExtra("userid",usr);
startActivity(intent);
}
if(i==1) {
Intent intent = new Intent(getApplicationContext(), Cameres.class);
intent.putExtra("userid",usr);
startActivity(intent);
}
if(i==2) {
Intent intent = new Intent(getApplicationContext(), MP3_Players.class);
intent.putExtra("userid",usr);
startActivity(intent);
}
if(i==3) {
Intent intent = new Intent(getApplicationContext(), Car_Electronics.class);
intent.putExtra("userid",usr);
startActivity(intent);
}
if(i==4) {
Intent intent = new Intent(getApplicationContext(), Tv_Video.class);
intent.putExtra("userid",usr);
startActivity(intent);
}
if(i==5) {
Intent intent = new Intent(getApplicationContext(), Computers.class);
intent.putExtra("userid",usr);
startActivity(intent);
}
}
});
}
private class AdapterCommentList extends BaseAdapter {
private Context context;
private List<ModelWomen> alComments;
private LayoutInflater inflater;
public AdapterCommentList(Context context, List<ModelWomen> alComments) {
this.context = context;
this.alComments = alComments;
inflater = LayoutInflater.from(this.context);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
AdapterCommentList.ViewHolder holder = null;
if (mapViewHolder.get(position) == null) {
convertView = inflater.inflate(R.layout.women_sub_categories, null);
holder = initHolder(convertView, position);
attachEvents(holder, position);
convertView.setTag(holder);
mapViewHolder.put(position, convertView);
} else {
holder = (AdapterCommentList.ViewHolder) mapViewHolder.get(position).getTag();
}
updateHolder(holder, position);
return mapViewHolder.get(position);
}
@Override
public int getCount() {
return alComments.size();
}
@Override
public ModelWomen getItem(int position) {
return alComments.get(position);
}
@Override
public long getItemId(int position) {
return position;
}
private class ViewHolder {
private TextView tvName;
}
private AdapterCommentList.ViewHolder initHolder(View convertView, int pos) {
AdapterCommentList.ViewHolder holder = new AdapterCommentList.ViewHolder();

holder.tvName = (TextView) convertView.findViewById(R.id.tvName);
return holder;
}

private void updateHolder(AdapterCommentList.ViewHolder holder, int pos) {
holder.tvName.setText(alComment.get(pos).getCategory());

}
private void attachEvents(AdapterCommentList.ViewHolder holder, final int position) {
}

}
public class AsyncGetWomencategories extends AsyncTask<Void, Void, AsyncTaskResult<Object>> {
@Override
protected void onPreExecute() {
super.onPreExecute();
findViewById(R.id.rlProgress).setVisibility(View.VISIBLE);
}
@Override
protected AsyncTaskResult<Object> doInBackground(Void... params) {
BufferedReader in = null;
try {
HttpParams httpParameters = new BasicHttpParams();
HttpConnectionParams
.setConnectionTimeout(httpParameters, 20000);
HttpConnectionParams.setSoTimeout(httpParameters, 20000);
HttpClient client = new DefaultHttpClient(httpParameters);
String url = AppConstants.URL_GET_ELECTRONICS_CATEGORY_MAINLEVEL2;
HttpGet request = new HttpGet(url);
request.setHeader("Accept", "application/json");
request.setHeader("Content-type", "application/json");
request.setHeader("Authorization", "Basic " + "my_key");
HttpResponse response = client.execute(request);
in = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));
StringBuffer sb = new StringBuffer("");
String line = "";
String NL = System.getProperty("line.separator");
while ((line = in.readLine()) != null) {
sb.append(line + NL);
}
in.close();
String result = sb.toString();
if (AppConstants.DEBUG)
Log.v(AppConstants.DEBUG_TAG, "REPLY COMMENTRESPONSE : " + result);
return new AsyncTaskResult<Object>(result);
} catch (Exception e) {
e.printStackTrace();
} finally {
if (in != null) {
try {
in.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
return new AsyncTaskResult<Object>("OK");
}
@Override
protected void onPostExecute(AsyncTaskResult<Object> result) {
super.onPostExecute(result);
findViewById(R.id.rlProgress).setVisibility(View.GONE);
if (result.getError() != null) {
} else {
String response = result.getResult().toString();
try {
alComment.clear();
alComment.addAll(ParseJson.ParseWomen(response));
adpComment.notifyDataSetChanged();
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
}

这是我的"基本活动"代码：

public abstract class BaseActivity extends AppCompatActivity {
Toolbar toolbar;
String toknlink;
LibFile libFile;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
initData();
initUi();
}
protected void initData() {
libFile = LibFile.getInstance(getApplicationContext());
}
protected void initUi() {
if (libFile.getToken() != null) {
toknlink = libFile.getToken();
}else {
}
}

protected boolean useToolbar() {
return true;
}
@Override
public void setContentView(int layoutResID) {
View view = getLayoutInflater().inflate(layoutResID, null);
configureToolbar(view);
super.setContentView(view);
}
private void configureToolbar(View view) {
toolbar = (Toolbar) view.findViewById(R.id.toolbartest);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}

@Override
public void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
}
public boolean onPrepareOptionsMenu(Menu menu) {
MenuItem login = menu.findItem(R.id.action_login);
MenuItem logout = menu.findItem(R.id.action_logout);
if (toknlink.length() == 0) {
login.setVisible(true);
logout.setVisible(false);
} else {
login.setVisible(false);
logout.setVisible(true);
}
return true;
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.othermenu, menu);
MenuItem item1 = menu.findItem(R.id.action_searchicon);

MenuItem item = menu.findItem(R.id.action_cart);
int ids = item.getItemId();
ImageView imageView = (ImageView)findViewById(R.id.cartimage);
return true;
}
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.action_cart) {
Intent i = new Intent(getApplicationContext(), Cart.class);
startActivity(i);
overridePendingTransition(R.anim.lefttoright, R.anim.right_to_left);
}
if (id == R.id.action_wishlist) {
Intent i = new Intent(getApplicationContext(), MyWishList.class);
i.putExtra("userid", toknlink);
startActivity(i);
}
if (id == R.id.action_logout) {
new AlertDialog.Builder(this)
.setMessage("Are you sure you want to logout?")
.setCancelable(false)
.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int id) {
finishAffinity();
Intent i = new Intent(getApplicationContext(), MainActivity.class);
startActivity(i);
logout();
}
})
.setNegativeButton("No", null)
.show();
return true;
}
if (id == R.id.action_login) {
finish();
Intent i = new Intent(getApplicationContext(), MainActivity.class);
startActivity(i);
}
if (id == R.id.action_home) {
finish();
Intent i = new Intent(getApplicationContext(), Main2Activity.class);
startActivity(i);
}
if (id == android.R.id.home) {
this.finish();
return true;
}
return super.onOptionsItemSelected(item);
}
public void logout() {
libFile.clearCache();
Toast.makeText(getApplicationContext(), " Logout  Successfully ", Toast.LENGTH_SHORT).show();
}
}

请告诉我我哪里错了。我已经在另一个活动中实现了这个基本活动，但该活动没有列表视图。"ElectronicsMainActivity"包含列表视图。请建议。

Answer 1

java.lang.NullPointerException： Try to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter(' on a 空对象引用

首先声明列表视图。

lvComments = (ListView) findViewById(R.id.lvListview);       
alComment = new ArrayList<>();
mapViewHolder = new SparseArray<View>();
comment = new ModelWomen();
comment = (ModelWomen) getIntent().getSerializableExtra("id");

编辑

@Override
public void setContentView(int layoutResID) {
View view = getLayoutInflater().inflate(layoutResID, null);
configureToolbar(view);
super.setContentView(view);
}

为什么你调用setContentView((两次？

Answer 2

试试这个：用这个替换你的 BaseActivity

public class BaseActivity extends AppCompatActivity {
@Nullable
@BindView(R.id.toolbar)
Toolbar toolbar;
@Nullable
@BindView(R.id.ivLogo)
ImageView ivLogo;
private MenuItem inboxMenuItem;
@Override
public void setContentView(int layoutResID) {
super.setContentView(layoutResID);
bindViews();
}
protected void bindViews() {
ButterKnife.bind(this);
setupToolbar();
}
public void setContentViewWithoutInject(int layoutResId) {
super.setContentView(layoutResId);
}
protected void setupToolbar() {
if (toolbar != null) {
setSupportActionBar(toolbar);
toolbar.setNavigationIcon(R.drawable.ic_menu_white);
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.menu_main, menu);
inboxMenuItem = menu.findItem(R.id.action_inbox);
inboxMenuItem.setActionView(R.layout.menu_item_view);
return true;
}
public Toolbar getToolbar() {
return toolbar;
}
public MenuItem getInboxMenuItem() {
return inboxMenuItem;
}
public ImageView getIvLogo() {
return ivLogo;
}}

使用此依赖项调用 BindView

compile 'com.jakewharton:butterknife:8.4.0'
annotationProcessor 'com.jakewharton:butterknife-compiler:8.4.0'

创建新工具栏.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary"
app:elevation="@dimen/default_elevation"
app:layout_scrollFlags="scroll|enterAlways"
app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">
<TextView
android:id="@+id/ivLogo"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_gravity="center"
android:scaleType="center"
/>
</android.support.v7.widget.Toolbar>

在活动中使用此工具栏

<include
android:id="@+id/toolbar"
layout="@layout/toolbar.xml" />

Answer 3

上述代码的问题在于，在尝试从 BaseActivity 查找列表视图后，您正在设置活动的内容视图 因此，在调用super.onCreate()方法之前设置内容视图

setContentView(R.layout.activity_electronics_main);
super.onCreate(savedInstanceState);

这将解决问题

编辑从 BaseActivity 中删除 setContentView 覆盖方法，并通过findViewById(R.id.toolbar_id)onCreate()找到工具栏

Answer 4

这是因为您正在调用BaseActivityoncreate并且它正在调用initUI()但您UI尚未初始化，因此会导致空指针。 您可以在ListView初始化后执行以下操作super.onCreate()

如下所示

@Override
protected void onCreate(Bundle savedInstanceState) {
setContentView(R.layout.activity_electronics_main);
lvComments = (ListView) findViewById(R.id.lvListview);
alComment = new ArrayList<>();
mapViewHolder = new SparseArray<View>();
comment = new ModelWomen();
comment = (ModelWomen) getIntent().getSerializableExtra("id");
super.onCreate(savedInstanceState);
// initUi(); <-- Redundant method invocation as it is already invoked from base
new AsyncGetWomencategories().execute();
}