我正在使用SQLite数据库，每当我调用它时，我都需要将上下文传递给它。我通常使用getApplicationContext（）来执行此操作，该工作在其他活动中。 在我当前的类中，它只返回null。救命啊！
04-06 16:46:19.523: E/AndroidRuntime(4279): Process: com.example.schoolandrevisiontimetable, PID: 4279
04-06 16:46:19.523: E/AndroidRuntime(4279): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.schoolandrevisiontimetable/com.example.schoolandrevisiontimetable.Input_slessontime}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2225)
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2388)
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.app.ActivityThread.access$800(ActivityThread.java:148)
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292)
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.os.Handler.dispatchMessage(Handler.java:102)
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.os.Looper.loop(Looper.java:135)
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.app.ActivityThread.main(ActivityThread.java:5312)
04-06 16:46:19.523: E/AndroidRuntime(4279): at java.lang.reflect.Method.invoke(Native Method)
04-06 16:46:19.523: E/AndroidRuntime(4279): at java.lang.reflect.Method.invoke(Method.java:372)
04-06 16:46:19.523: E/AndroidRuntime(4279): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
04-06 16:46:19.523: E/AndroidRuntime(4279): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)
04-06 16:46:19.523: E/AndroidRuntime(4279): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:105)
04-06 16:46:19.523: E/AndroidRuntime(4279): at com.example.schoolandrevisiontimetable.Input_slessontime.getAmountOfDaysUsed(Input_slessontime.java:95)
04-06 16:46:19.523: E/AndroidRuntime(4279): at com.example.schoolandrevisiontimetable.Input_slessontime.<init>(Input_slessontime.java:30)
04-06 16:46:19.523: E/AndroidRuntime(4279): at java.lang.reflect.Constructor.newInstance(Native Method)
04-06 16:46:19.523: E/AndroidRuntime(4279): at java.lang.Class.newInstance(Class.java:1572)
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.app.Instrumentation.newActivity(Instrumentation.java:1088)
04-06 16:46:19.523: E/AndroidRuntime(4279): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2215)
04-06 16:46:19.523: E/AndroidRuntime(4279): ... 10 more
这是它返回null的类 package com.example.schoolandrevisiontimetable;
import java.util.List;
import com.astuetz.PagerSlidingTabStrip;
import Database.DatabaseHelper;
import Database.MC_sschoolday;
import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.util.TypedValue;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
public class Input_slessontime extends FragmentActivity {
DatabaseHelper db;
String[] days = new String[getAmountOfDaysUsed()];
private PagerSlidingTabStrip tabs;
private ViewPager pager;
private MyPagerAdapter adapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.fragment_input_slessontime);
tabs = (PagerSlidingTabStrip) findViewById(R.id.tabs);
pager = (ViewPager) findViewById(R.id.pager);
adapter = new MyPagerAdapter(getSupportFragmentManager());
final int pageMargin = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 4, getResources()
.getDisplayMetrics());
pager.setPageMargin(pageMargin);
tabs.setViewPager(pager);
if (savedInstanceState == null) {
getFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.input_slessontime, 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();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_input_slessontime, container, false);
return rootView;
}
}
public int getAmountOfDaysUsed(){
db = new DatabaseHelper(getApplicationContext());
List<MC_sschoolday> MC_sschoolday = db.getAllSSchoolDays(); //gets all the lessons and stores them in a list
int amountOfDaysUsed=0;
MC_sschoolday[] sday = new MC_sschoolday[7];
for (int i =0; i<7; i++){
sday[i]=MC_sschoolday.get(i); //stores the lessons in an array
}
for (int i =0; i<7; i++){
if(sday[i].getUsed().equals("y")){
amountOfDaysUsed++;
}
}
db.close();
return amountOfDaysUsed;
}
public void getTitles(){
db = new DatabaseHelper(getApplicationContext());
MC_sschoolday[] sday = new MC_sschoolday[7];
List<MC_sschoolday> schoolday = db.getAllSSchoolDays();
int usedCount2=0;
for (int i =0; i< db.getAllSSchoolDays().size(); i++){
sday[i]=schoolday.get(i);
if(sday[i].getUsed().equals("y")){
days[usedCount2] = sday[i].getschool_day();
usedCount2++;
}
}
db.close();
}
public class MyPagerAdapter extends FragmentPagerAdapter {
public MyPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public CharSequence getPageTitle(int position) {
return days[position];
}
@Override
public int getCount() {
return getAmountOfDaysUsed();
}
public android.support.v4.app.Fragment switchFragmentDay(int num) {
DatabaseHelper db;
db = new DatabaseHelper(getApplicationContext());
MC_sschoolday[] sday = new MC_sschoolday[7];
List<MC_sschoolday> schoolday = db.getAllSSchoolDays();
int usedCount1=getAmountOfDaysUsed();
int [] dayID = new int[usedCount1];
int usedCount2=0;
for (int i =0; i< db.getAllSSchoolDays().size(); i++){
sday[i]=schoolday.get(i);
if(sday[i].getUsed().equals("y")){
dayID[usedCount2] = (int) sday[i].getSchool_day_id();
usedCount2++;
}
}
db.close();
switch (dayID[num]){
case 1:
return new Input_slessontime_monday();
case 2:
return new Input_slessontime_tuesday();
case 3:
return new Input_slessontime_wednesday();
case 4:
return new Input_slessontime_thursday();
case 5:
return new Input_slessontime_friday();
case 6:
return new Input_slessontime_saturday();
case 7:
return new Input_slessontime_sunday();
}
return null;
}
@Override
public android.support.v4.app.Fragment getItem(int position) {
switch (position){
case 0:
return switchFragmentDay(0);
case 1:
return switchFragmentDay(1);
case 2:
return switchFragmentDay(2);
case 3:
return switchFragmentDay(3);
case 4:
return switchFragmentDay(4);
case 5:
return switchFragmentDay(5);
case 6:
return switchFragmentDay(6);
}
return null;
}
}
}
感谢您的帮助！
答案 0 :(得分：6)
String[] days = new String[getAmountOfDaysUsed()];
您在成员变量初始化时过早地将活动用作Context。您需要等到活动生命周期中的onCreate()或更晚。