我有一个应用程序，当一个活动开始时显示nullpointer异常，但我无法理解错误是什么以及我做错了什么。我是android的新手，如果你能提供帮助，我将不胜感激。在此先感谢。
这是在启动AccountsActivity.java
时显示错误的活动public class AccountsActivity extends AppCompatActivity {
SharedPreferences myPrefs = PreferenceManager.getDefaultSharedPreferences(OpeningScreenActivity.getContextOfApplication());
String token = myPrefs.getString("GCMTOKEN", "");
String JSON_URL = "http://xyz.in/view_json.php?device_id=" + token;
private ListView listView;
/**
* ATTENTION: This was auto-generated to implement the App Indexing API.
* See https://g.co/AppIndexing/AndroidStudio for more information.
*/
private GoogleApiClient client;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_accounts);
listView = (ListView) findViewById(R.id.listView);
sendRequest();
// ATTENTION: This was auto-generated to implement the App Indexing API.
// See https://g.co/AppIndexing/AndroidStudio for more information.
client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
}
private void sendRequest() {
StringRequest stringRequest = new StringRequest(JSON_URL,
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
showJSON(response);
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Toast.makeText(AccountsActivity.this, error.getMessage(), Toast.LENGTH_LONG).show();
}
});
RequestQueue requestQueue = Volley.newRequestQueue(this);
requestQueue.add(stringRequest);
}
private void showJSON(String json) {
ParseJSON pj = new ParseJSON(json);
pj.parseJSON();
CustomList cl = new CustomList(this, ParseJSON.ids,ParseJSON.mem_codes,ParseJSON.created_ons);
listView.setAdapter(cl);
}
}
这是ParseJSON.java类
public class AccountsActivity extends AppCompatActivity {
SharedPreferences myPrefs = PreferenceManager.getDefaultSharedPreferences(OpeningScreenActivity.getContextOfApplication());
String token = myPrefs.getString("GCMTOKEN", "");
String JSON_URL = "http://sovran.in/sov2fa_v2/view_json.php?device_id=" + token;
private ListView listView;
/**
* ATTENTION: This was auto-generated to implement the App Indexing API.
* See https://g.co/AppIndexing/AndroidStudio for more information.
*/
private GoogleApiClient client;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_accounts);
listView = (ListView) findViewById(R.id.listView);
sendRequest();
// ATTENTION: This was auto-generated to implement the App Indexing API.
// See https://g.co/AppIndexing/AndroidStudio for more information.
client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
}
private void sendRequest() {
StringRequest stringRequest = new StringRequest(JSON_URL,
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
showJSON(response);
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Toast.makeText(AccountsActivity.this, error.getMessage(), Toast.LENGTH_LONG).show();
}
});
RequestQueue requestQueue = Volley.newRequestQueue(this);
requestQueue.add(stringRequest);
}
private void showJSON(String json) {
ParseJSON pj = new ParseJSON(json);
pj.parseJSON();
CustomList cl = new CustomList(this, ParseJSON.ids,ParseJSON.mem_codes,ParseJSON.created_ons);
listView.setAdapter(cl);
}
}
这是CustomList.java
public class CustomList extends ArrayAdapter<String> {
private String[] ids;
private String[] mem_codes;
private String[] created_ons;
private Activity context;
public CustomList(Activity context, String[] ids, String[] mem_codes, String[] created_ons) {
super(context, R.layout.list_view_layout, ids);
this.context = context;
this.ids = ids;
this.mem_codes = mem_codes;
this.created_ons = created_ons;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
LayoutInflater inflater = context.getLayoutInflater();
View listViewItem = inflater.inflate(R.layout.list_view_layout, null, true);
TextView textViewId = (TextView) listViewItem.findViewById(R.id.textViewId);
TextView textViewMemCode = (TextView) listViewItem.findViewById(R.id.textViewMem_code);
TextView textViewCreatedOn = (TextView) listViewItem.findViewById(R.id.textViewCreated_on);
textViewId.setText(ids[position]);
textViewMemCode.setText(mem_codes[position]);
textViewCreatedOn.setText(created_ons[position]);
return listViewItem;
}
}
这是错误日志
07-15 14:04:46.269 32255-32255/com.sovran.sov2fa E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.sovran.sov2fa, PID: 32255
java.lang.ExceptionInInitializerError
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1606)
at android.app.Instrumentation.newActivity(Instrumentation.java:1071)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2614)
at android.app.ActivityThread.access$800(ActivityThread.java:178)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1470)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:194)
at android.app.ActivityThread.main(ActivityThread.java:5653)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:960)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:383)
at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:378)
at com.sovran.sov2fa.AccountsActivity.<clinit>(AccountsActivity.java:20)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1606)
at android.app.Instrumentation.newActivity(Instrumentation.java:1071)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2614)
at android.app.ActivityThread.access$800(ActivityThread.java:178)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1470)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:194)
at android.app.ActivityThread.main(ActivityThread.java:5653)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:960)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
答案 0 :(得分：0)
在myPrefs - 方法
onCreate()属性
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_accounts);
myPrefs = PreferenceManager.getDefaultSharedPreferences(this);
...
}