我的程序包含两个activity.i通过方法showevent调用第二个活动。但是发生错误，nullpointer异常。这是什么？ 我的程序包含两个activity.i通过方法showevent调用第二个活动。但是发生错误，nullpointer异常。这是什么
FirstActivity.java
public class FirstActivity extends FragmentActivity implements OnItemSelectedListener {
/** Called when the activity is first created. */
public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
public final static String EXTRA_MESSAGE1 = "com.example.myfirstapp.MESSAGE";
classdbOpenHelper eventsData;
TextView userSelection;
Button okButton;
Button addButton;
Button change_date_but;
TextView date;
TextView show;
EditText edittext;
public static final int Date_dialog_id = 1;
private int mYear;
private int mMonth;
private int mDay;
private static final String[] items={"Yalahanka","Rajaji nagar","Sivaji Nagar","Koramangala","RT Nagar", "Banashankari","Yashwanthpura","Hebbal"};
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_first);
okButton = (Button)findViewById(R.id.button2);
addButton = (Button)findViewById(R.id.button3);
change_date_but = (Button)findViewById(R.id.button1);
date = (TextView)findViewById(R.id.textView2);//KEY_DATE
userSelection=(TextView)findViewById(R.id.textView1);//KEY_DESC
edittext=(EditText)findViewById(R.id.editText1);//KEY_EVENT
show=(TextView)findViewById(R.id.textView5);
Spinner my_spin=(Spinner)findViewById(R.id.spinner1);
my_spin.setOnItemSelectedListener(this);
ArrayAdapter aa=new ArrayAdapter(this, android.R.layout.simple_spinner_item,items);
aa.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
my_spin.setAdapter(aa);
okButton.setOnClickListener(new clicker());
eventsData = new classdbOpenHelper(this);
change_date_but.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
DatePickerDialog DPD = new DatePickerDialog(
FirstActivity.this, mDateSetListener, mYear, mMonth,mDay);
DPD.show();
}
});
final Calendar c = Calendar.getInstance();
mYear = c.get(Calendar.YEAR);
mMonth = c.get(Calendar.MONTH);
mDay = c.get(Calendar.DAY_OF_MONTH);
updateDisplay();
}
@Override
@Deprecated
protected void onPrepareDialog(int id, Dialog dialog) {
// TODO Auto-generated method stub
super.onPrepareDialog(id, dialog);
((DatePickerDialog) dialog).updateDate(mYear, mMonth, mDay);
}
private DatePickerDialog.OnDateSetListener mDateSetListener = new DatePickerDialog.OnDateSetListener() {
public void onDateSet(DatePicker view, int year, int monthOfYear,
int dayOfMonth) {
mYear = year;
mMonth = monthOfYear;
mDay = dayOfMonth;
updateDisplay();
}
};
private void updateDisplay() {
// TODO Auto-generated method stub
date.setText(new StringBuilder()
// Month is 0 based so add 1
.append(mMonth + 1).append("-").append(mDay).append("-")
.append(mYear));
}
@Override
public void onItemSelected(AdapterView<?> arg0, View arg1, int pos, long arg3) {
userSelection.setText(items[pos]);
}
@Override
public void onNothingSelected(AdapterView<?> arg0) {
// TODO Auto-generated method stub
userSelection.setText("");
}
class clicker implements Button.OnClickListener {
public void onClick(View v) {
String datevalue = date.getText().toString();
String Userselectvalue = userSelection.getText().toString();
String Userevent = edittext.getText().toString();
SQLiteDatabase db = eventsData.getWritableDatabase();
ContentValues cv = new ContentValues();
cv.put(classdbOpenHelper.KEY_DESC, Userselectvalue);
cv.put(classdbOpenHelper.KEY_EVENT, Userevent);
cv.put(classdbOpenHelper.KEY_DATE,datevalue);
db.insert(classdbOpenHelper.DATABASE_TABLE, null, cv);
db.close();
}
}
public void ShowEvent(View view){
Intent intent = new Intent(this, Getclicker.class);
date = (TextView)findViewById(R.id.textView2);
String datevalue = date.getText().toString();
userSelection=(TextView)findViewById(R.id.textView1);
String Userselectvalue = userSelection.getText().toString();
intent.putExtra(EXTRA_MESSAGE, datevalue);
intent.putExtra(EXTRA_MESSAGE1, Userselectvalue);
startActivity(intent);
}
}
Getclicker.java
package example.events1;
import android.app.ListActivity;
import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.support.v4.widget.SimpleCursorAdapter;
import android.widget.ListView;
public class Getclicker extends ListActivity {
Intent intent = getIntent();
String datevalue = intent.getStringExtra(FirstActivity.EXTRA_MESSAGE);
String Userselectvalue = intent.getStringExtra(FirstActivity.EXTRA_MESSAGE1);
classdbOpenHelper eventsData1;
Cursor cursor1;
ListView listView ;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second); /*String datevalue = date.getText().toString();
String Userselectvalue = userSelection.getText().toString();
String showevent = eventsData.getContact(datevalue,Userselectvalue);
show.setText(showevent);*/
cursor1 = eventsData1.getContact(datevalue,Userselectvalue);
String[] fromColumns = {classdbOpenHelper.KEY_EVENT};
int[] toViews = {R.id.event};
SimpleCursorAdapter adapter = new SimpleCursorAdapter(this,R.layout.events, cursor1, fromColumns, toViews,0 );
listView = (ListView) findViewById(android.R.id.list);
listView.setAdapter(adapter);
}
public void onDestroy() {
eventsData1.close();
}
}
logcat：
08-07 10:00:58.396: E/AndroidRuntime(889): FATAL EXCEPTION: main
08-07 10:00:58.396: E/AndroidRuntime(889): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{example.events1/example.events1.Getclicker}: java.lang.NullPointerException
08-07 10:00:58.396: E/AndroidRuntime(889): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2106)
08-07 10:00:58.396: E/AndroidRuntime(889): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
08-07 10:00:58.396: E/AndroidRuntime(889): at android.app.ActivityThread.access$600(ActivityThread.java:141)
08-07 10:00:58.396: E/AndroidRuntime(889): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
08-07 10:00:58.396: E/AndroidRuntime(889): at android.os.Handler.dispatchMessage(Handler.java:99)
08-07 10:00:58.396: E/AndroidRuntime(889): at android.os.Looper.loop(Looper.java:137)
08-07 10:00:58.396: E/AndroidRuntime(889): at android.app.ActivityThread.main(ActivityThread.java:5041)
08-07 10:00:58.396: E/AndroidRuntime(889): at java.lang.reflect.Method.invokeNative(Native Method)
08-07 10:00:58.396: E/AndroidRuntime(889): at java.lang.reflect.Method.invoke(Method.java:511)
08-07 10:00:58.396: E/AndroidRuntime(889): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
08-07 10:00:58.396: E/AndroidRuntime(889): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
08-07 10:00:58.396: E/AndroidRuntime(889): at dalvik.system.NativeStart.main(Native Method)
08-07 10:00:58.396: E/AndroidRuntime(889): Caused by: java.lang.NullPointerException
08-07 10:00:58.396: E/AndroidRuntime(889): at example.events1.Getclicker.<init>(Getclicker.java:12)
08-07 10:00:58.396: E/AndroidRuntime(889): at java.lang.Class.newInstanceImpl(Native Method)
08-07 10:00:58.396: E/AndroidRuntime(889): at java.lang.Class.newInstance(Class.java:1319)
08-07 10:00:58.396: E/AndroidRuntime(889): at android.app.Instrumentation.newActivity(Instrumentation.java:1054)
08-07 10:00:58.396: E/AndroidRuntime(889): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097)
08-07 10:00:58.396: E/AndroidRuntime(889): ... 11 more
答案 0 :(得分：5)
移动这些：
Intent intent = getIntent();
String datevalue = intent.getStringExtra(FirstActivity.EXTRA_MESSAGE);
String Userselectvalue = intent.getStringExtra(FirstActivity.EXTRA_MESSAGE1);
进入onCreate()方法。我想是因为Intent还没到那里。
希望这有帮助。