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我正在嘗試構建一個基本的表格視圖，其中有幾行和一列填充了textviews來表示課程時間表。我已經編寫了一些代碼，可以計算出使用日期和時間的課程，然後更改其textview的顏色。我試圖在一個單獨的類中執行此操作，因爲我希望以多種方式執行它。但是，當我運行它時，我得到由findViewById引起的nullPointerException。在findViewById上的空指針掃描
錯誤：
FATAL EXCEPTION: main
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: Process: com.example.harris.whathaveilearnt, PID: 7861
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.harris.whathaveilearnt/com.example.harris.whathaveilearnt.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread.-wrap11(ActivityThread.java)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.os.Looper.loop(Looper.java:148)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5417)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.Activity.findViewById(Activity.java:2090)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at com.example.harris.whathaveilearnt.selectLesson.nextlesson(selectLesson.java:73)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at com.example.harris.whathaveilearnt.MainActivity.onCreate(MainActivity.java:41)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.Activity.performCreate(Activity.java:6237)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread.-wrap11(ActivityThread.java)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.os.Looper.loop(Looper.java:148)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5417)
11-23 19:43:51.870 7861-7861/com.example.harris.whathaveilearnt E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
活動：
public class selectLesson extends MainActivity {
Calendar c = Calendar.getInstance();
int dayOfWeek = c.get(Calendar.DAY_OF_WEEK);
int dateDays = c.get(Calendar.DAY_OF_MONTH);
int dateMonth = c.get(Calendar.MONTH);
int weekOfMonth = c.get(Calendar.WEEK_OF_MONTH);
LocalTime currentTime = LocalTime.now();
int nextLessonNumber = 0;
String nextLessonString;
int currentLesson;
LocalTime lesson1Start = new LocalTime("09:15");
LocalTime lesson1End = new LocalTime("10:15");
LocalTime lesson2Start = new LocalTime("10:15");
LocalTime lesson3Start = new LocalTime("11:40");
LocalTime lesson4start = new LocalTime("13:45");
LocalTime lesson5Start = new LocalTime("14:45");
LocalTime endOfDay = new LocalTime ("15:50");
private static final String TAG = "selectLesson";
public void nextlesson() {
if (currentTime.isBefore(lesson1Start)) {
//p1 next
nextLessonNumber = 1;
Log.d(TAG, "It worked");
currentLesson = 0;
} else if (currentTime.isAfter(lesson1Start) && currentTime.isBefore(lesson1End)) {
//p2 next
nextLessonNumber = 2;
Log.d(TAG, "It worked");
currentLesson = 1;
} else if (currentTime.isAfter(lesson2Start) && currentTime.isBefore(lesson3Start)) {
//p3 next
nextLessonNumber = 3;
Log.d(TAG, "It worked");
currentLesson = 2;
} else if (currentTime.isAfter(lesson3Start) && currentTime.isBefore(lesson4start)) {
//p4 next
nextLessonNumber = 4;
Log.d(TAG, "It worked");
currentLesson = 3;
} else if (currentTime.isAfter(lesson4start) && currentTime.isBefore(lesson5Start)) {
//p5 next
nextLessonNumber = 5;
Log.d(TAG, "It worked");
currentLesson = 4;
} else if (currentTime.isAfter(lesson5Start) && currentTime.isAfter(endOfDay)) {
currentLesson = 5;
} else {
nextLessonNumber = 0;
Log.d(TAG, "It didn't worked");
}
TextView MonP1 = (TextView) findViewById(R.id.monP1Text);
TextView MonP2 = (TextView) findViewById(R.id.monP2Text);
TextView MonP3 = (TextView) findViewById(R.id.monP3Text);
TextView MonP4 = (TextView) findViewById(R.id.monP4Text);
TextView MonP5 = (TextView) findViewById(R.id.monP5Text);
TextView TueP1 = (TextView) findViewById(R.id.tueP1Text);
TextView TueP2 = (TextView) findViewById(R.id.tueP2Text);
TextView TueP3 = (TextView) findViewById(R.id.tueP3Text);
TextView TueP4 = (TextView) findViewById(R.id.tueP4Text);
TextView TueP5 = (TextView) findViewById(R.id.tueP5Text);
TextView WedP1 = (TextView) findViewById(R.id.wedP1Text);
TextView WedP2 = (TextView) findViewById(R.id.wedP2Text);
TextView WedP3 = (TextView) findViewById(R.id.wedP3Text);
TextView WedP4 = (TextView) findViewById(R.id.wedP4Text);
TextView WedP5 = (TextView) findViewById(R.id.wedP5Text);
TextView ThursP1 = (TextView) findViewById(R.id.thursP1Text);
TextView ThursP2 = (TextView) findViewById(R.id.thursP2Text);
TextView ThursP3 = (TextView) findViewById(R.id.thursP3Text);
TextView ThursP4 = (TextView) findViewById(R.id.thursP4Text);
TextView ThursP5 = (TextView) findViewById(R.id.thursP5Text);
TextView FriP1 = (TextView) findViewById(R.id.friP1Text);
TextView FriP2 = (TextView) findViewById(R.id.friP2Text);
TextView FriP3 = (TextView) findViewById(R.id.friP3Text);
TextView FriP4 = (TextView) findViewById(R.id.friP4Text);
TextView FriP5 = (TextView) findViewById(R.id.friP5Text);
if ((weekOfMonth == 1) || (weekOfMonth == 3)) {
Log.d(TAG, "It made it this far");
switch (dayOfWeek) {
case 1: //Sunday DO NOTHING
break;
case 2: //Monday WEEK A
if (currentLesson == 1) {
MonP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
MonP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
MonP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
MonP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
MonP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 3: //Tuesday WEEK A
if (currentLesson == 1) {
TueP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
TueP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
TueP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
TueP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
TueP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 4: //Wednesday WEEK A
if (currentLesson == 1) {
WedP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
WedP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
WedP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
WedP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
WedP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 5: //Thursday WEEK A
if (currentLesson == 1) {
ThursP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
ThursP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
ThursP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
ThursP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
ThursP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 6: //Friday WEEK A
if (currentLesson == 1) {
FriP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
FriP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
FriP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
FriP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
FriP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 7: //Saturday DO NOTHING
break;
}
} else {
switch (dayOfWeek) {
case 1: //Sunday DO NOTHING
break;
case 2: //Monday WEEK A
if (currentLesson == 1) {
MonP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
MonP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
MonP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
MonP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
MonP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 3: //Tuesday WEEK A
if (currentLesson == 1) {
TueP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
TueP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
TueP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
TueP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
TueP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 4: //Wednesday WEEK A
if (currentLesson == 1) {
WedP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
WedP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
WedP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
WedP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
WedP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 5: //Thursday WEEK A
if (currentLesson == 1) {
ThursP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
ThursP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
ThursP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
ThursP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
ThursP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 6: //Friday WEEK A
if (currentLesson == 1) {
FriP1.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 2) {
FriP2.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 3) {
FriP3.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 4) {
FriP4.setTextColor(this.getResources().getColor(R.color.colorAccent));
} else if (currentLesson == 5) {
FriP5.setTextColor(this.getResources().getColor(R.color.colorAccent));
}
break;
case 7: //Saturday DO NOTHING
break;
}
}
}
}
主要活動：
package com.example.harris.whathaveilearnt;
import android.os.Bundle;
import android.provider.CalendarContract;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.Toolbar;
import android.text.format.Time;
import android.util.Log;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import org.joda.time.LocalTime;
import java.util.Calendar;
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MyActivity";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
selectLesson selectLesson = new selectLesson();
Calendar c = Calendar.getInstance();
int dayOfWeek = c.get(Calendar.DAY_OF_WEEK);
int dateDays = c.get(Calendar.DAY_OF_MONTH);
int dateMonth = c.get(Calendar.MONTH);
LocalTime currentTime = LocalTime.now();
selectLesson.nextlesson();
TextView NextLessonTextView = (TextView) findViewById(R.id.NextLessonText);
TextView DayOfTheWeek = (TextView) findViewById(R.id.dayOfWeekText);
String dayOfWeekText;
// DayOfTheWeek.setText(String.valueOf(dayOfWeek) + String.valueOf(dateDays));
switch(dayOfWeek){
case 1: dayOfWeekText = "Sunday";
DayOfTheWeek.setText(dayOfWeekText +" " +dateDays +" " + dateMonth);
break;
case 2: dayOfWeekText = "Monday";
DayOfTheWeek.setText(dayOfWeekText +" " +dateDays +" " + dateMonth);
break;
case 3: dayOfWeekText = "Tuesday";
DayOfTheWeek.setText(dayOfWeekText +" " +dateDays +" " + dateMonth);
break;
case 4: dayOfWeekText = "Wednesday";
DayOfTheWeek.setText(dayOfWeekText +" " +dateDays +" " + dateMonth);
break;
case 5: dayOfWeekText = "Thurday";
DayOfTheWeek.setText(dayOfWeekText +" " +dateDays +" " + dateMonth);
break;
case 6: dayOfWeekText = "Friday";
DayOfTheWeek.setText(dayOfWeekText +" " +dateDays +" " + dateMonth);
break;
case 7: dayOfWeekText = "Saturday";
DayOfTheWeek.setText(dayOfWeekText +" " +dateDays +" " + dateMonth);
break;
}
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("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_main, 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();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
你可以發佈你的'MainActivity'代碼嗎？ nextlesson（）從哪裏調用？在我看來，在你開始調用'findViewById（）'之前，你還沒有調用過'setContentView（R.layout.your_layout）'。 – ehehhh
也許你還沒有在onCreate（）中調用'setContentView（）'？ –