Android 使用SQLite时出现Nullpointer异常_Android_Sqlite

Android 使用SQLite时出现Nullpointer异常

android sqlite

Android 使用SQLite时出现Nullpointer异常,android,sqlite,Android,Sqlite,我使用的是sqlitedatabase，我能够正确插入数据，但问题是当我试图显示插入的数据时，我的应用程序崩溃并出现nullpointer异常，有人能告诉我的代码有什么问题吗，下面是我的代码片段这行有错误 if (c1 != null & c1.getCount() != 0) { MAinActivity.java public class MainActivity extends Activity { private ListView upcominglist;

我使用的是sqlitedatabase，我能够正确插入数据，但问题是当我试图显示插入的数据时，我的应用程序崩溃并出现nullpointer异常，有人能告诉我的代码有什么问题吗，下面是我的代码片段

这行有错误

 if (c1 != null & c1.getCount() != 0) {

MAinActivity.java

public class MainActivity extends Activity {

    private ListView upcominglist;


    private ListView todays;
    private ListView eventhistory;
    private ImageView addnewevent;

    public ArrayList<ContactListItems> contactList;

    public ContactListItems contactListItems;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        upcominglist=(ListView)findViewById(R.id.listview_upcoming);
        todays=(ListView)findViewById(R.id.listview_todays);
        eventhistory=(ListView)findViewById(R.id.listview_eventhistory);

        addnewevent=(ImageView)findViewById(R.id.addneweventbutton);

        addnewevent.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(MainActivity.this, AddNewEvent.class);
                startActivity(intent);
            }
        });


       contactList = new ArrayList<ContactListItems>();
        contactList.clear();
        String query = "SELECT * FROM PHONE_CONTACTS ";
        Cursor c1 = SqlHandler.selectQuery(query);
        if (c1 != null & c1.getCount() != 0) {
            if (c1.moveToNext()) {
                do {
                    contactListItems = new ContactListItems();
                    contactListItems.setSlno(c1.getString(c1.getColumnIndex("slno")));
                    contactListItems.setNameofevent(c1.getString(c1.getColumnIndex("nameofevent")));
                    contactListItems.setDtofevent(c1.getString(c1.getColumnIndex("dtofevent")));
                    contactListItems.setTimeofevent(c1.getString(c1.getColumnIndex("timeofevent")));
                    contactListItems.setDuration(c1.getString(c1.getColumnIndex("duration")));
                    contactList.add(contactListItems);
                } while (c1.moveToNext());
            }
        }
        else
        {
            c1.close();
        }
       c1.close();

        String first=contactListItems.getSlno();
        System.out.println("First" + first);

        String second=contactListItems.getNameofevent();
        System.out.println("SEcond"+second);

        String third=contactListItems.getDtofevent();
        System.out.println("Third"+third);

        String fourth=contactListItems.getTimeofevent();
        System.out.println("Fourth"+fourth);

        String fifth=contactListItems.getDuration();
        System.out.println("Fifth"+fifth);
    }

SQL

问题出在您的

SqlHandler.selectQuery（）

中，它返回一个

null

，这里还有另一个检查结果的问题：

if (c1 != null & c1.getCount() != 0)

您使用的是位and

，而不是短路逻辑and

。在不短路完整表达式的情况下，对

null

引用上的

c1.getCount（）

进行计算。

问题出在

SqlHandler中。selectQuery（）

null

，这里还有一个检查结果的问题：

if (c1 != null & c1.getCount() != 0)

您使用的是位and

，而不是短路逻辑and

。在不短路完整表达式的情况下，对

null

引用上的

c1.getCount（）

进行计算。

问题出在

SqlHandler中。selectQuery（）

null

，这里还有一个检查结果的问题：

if (c1 != null & c1.getCount() != 0)

您使用的是位and

，而不是短路逻辑and

。在不短路完整表达式的情况下，对

null

引用上的

c1.getCount（）

进行计算。

问题出在

SqlHandler中。selectQuery（）

null

，这里还有一个检查结果的问题：

if (c1 != null & c1.getCount() != 0)

您使用的是位and

，而不是短路逻辑and

。在不短路完整表达式（包括

c1）的情况下，对null
引用上的getCount（）

进行计算。

这里有太多内容要解释，因此我将给出导致null指针异常的缺陷。我可以看出你的编程方法是因为太过担心关闭和清理从某种程度上说，这是造成问题的原因

contactList = new ArrayList<ContactListItems>();
// You are clearing your list, it should be empty, you have just created it.
contactList.clear();
String query = "SELECT * FROM PHONE_CONTACTS ";
Cursor c1 = SqlHandler.selectQuery(query);
// As mentioned by the other answer. You need && not &
// if (c1 != null & c1.getCount() != 0) {
if (c1 != null && c1.getCount() != 0) {
    // Move to the first entry.
    c1.moveToFirst();
    //if (c1.moveToNext()) {
    //    do {
    // Continue while it has not passed the last entry.
    while (!cursor.isAfterLast())
            contactListItems = new ContactListItems();
            contactListItems.setSlno(c1.getString(c1.getColumnIndex("slno")));
            contactListItems.setNameofevent(c1.getString(c1.getColumnIndex("nameofevent")));
            contactListItems.setDtofevent(c1.getString(c1.getColumnIndex("dtofevent")));
            contactListItems.setTimeofevent(c1.getString(c1.getColumnIndex("timeofevent")));
            contactListItems.setDuration(c1.getString(c1.getColumnIndex("duration")));
            contactList.add(contactListItems);
            // Move the cursor along to the next entry.
            cursor.moveToNext();
        } 
    }
    // Close cursor after while and within if (so you know it is not null).
    c1.close();
}
else
{
    // You can't close c1 if it is Null. This will throw and error. Lose the else.
    c1.close();
}
// Move this to within your if statment.
c1.close();

您的项目中还有许多其他缺陷。比如结构，如何以及何时调用东西。您需要将其模块化，为特定任务创建方法并调用这些方法，而不是在oncreate中有大量代码。

我相信你对此会有很多疑问。但目前这个问题是针对空指针异常的，这就是我将在这里讨论的全部内容。对于与此例外情况无关的问题，请提出新问题。希望这能有所帮助。

这里有太多的内容要解释，所以我将告诉您导致空指针异常的缺陷。我可以看出你的编程方法是因为太过担心关闭和清理从某种程度上说，这是造成问题的原因

contactList = new ArrayList<ContactListItems>();
// You are clearing your list, it should be empty, you have just created it.
contactList.clear();
String query = "SELECT * FROM PHONE_CONTACTS ";
Cursor c1 = SqlHandler.selectQuery(query);
// As mentioned by the other answer. You need && not &
// if (c1 != null & c1.getCount() != 0) {
if (c1 != null && c1.getCount() != 0) {
    // Move to the first entry.
    c1.moveToFirst();
    //if (c1.moveToNext()) {
    //    do {
    // Continue while it has not passed the last entry.
    while (!cursor.isAfterLast())
            contactListItems = new ContactListItems();
            contactListItems.setSlno(c1.getString(c1.getColumnIndex("slno")));
            contactListItems.setNameofevent(c1.getString(c1.getColumnIndex("nameofevent")));
            contactListItems.setDtofevent(c1.getString(c1.getColumnIndex("dtofevent")));
            contactListItems.setTimeofevent(c1.getString(c1.getColumnIndex("timeofevent")));
            contactListItems.setDuration(c1.getString(c1.getColumnIndex("duration")));
            contactList.add(contactListItems);
            // Move the cursor along to the next entry.
            cursor.moveToNext();
        } 
    }
    // Close cursor after while and within if (so you know it is not null).
    c1.close();
}
else
{
    // You can't close c1 if it is Null. This will throw and error. Lose the else.
    c1.close();
}
// Move this to within your if statment.
c1.close();

contactList = new ArrayList<ContactListItems>();
// You are clearing your list, it should be empty, you have just created it.
contactList.clear();
String query = "SELECT * FROM PHONE_CONTACTS ";
Cursor c1 = SqlHandler.selectQuery(query);
// As mentioned by the other answer. You need && not &
// if (c1 != null & c1.getCount() != 0) {
if (c1 != null && c1.getCount() != 0) {
    // Move to the first entry.
    c1.moveToFirst();
    //if (c1.moveToNext()) {
    //    do {
    // Continue while it has not passed the last entry.
    while (!cursor.isAfterLast())
            contactListItems = new ContactListItems();
            contactListItems.setSlno(c1.getString(c1.getColumnIndex("slno")));
            contactListItems.setNameofevent(c1.getString(c1.getColumnIndex("nameofevent")));
            contactListItems.setDtofevent(c1.getString(c1.getColumnIndex("dtofevent")));
            contactListItems.setTimeofevent(c1.getString(c1.getColumnIndex("timeofevent")));
            contactListItems.setDuration(c1.getString(c1.getColumnIndex("duration")));
            contactList.add(contactListItems);
            // Move the cursor along to the next entry.
            cursor.moveToNext();
        } 
    }
    // Close cursor after while and within if (so you know it is not null).
    c1.close();
}
else
{
    // You can't close c1 if it is Null. This will throw and error. Lose the else.
    c1.close();
}
// Move this to within your if statment.
c1.close();

contactList = new ArrayList<ContactListItems>();
// You are clearing your list, it should be empty, you have just created it.
contactList.clear();
String query = "SELECT * FROM PHONE_CONTACTS ";
Cursor c1 = SqlHandler.selectQuery(query);
// As mentioned by the other answer. You need && not &
// if (c1 != null & c1.getCount() != 0) {
if (c1 != null && c1.getCount() != 0) {
    // Move to the first entry.
    c1.moveToFirst();
    //if (c1.moveToNext()) {
    //    do {
    // Continue while it has not passed the last entry.
    while (!cursor.isAfterLast())
            contactListItems = new ContactListItems();
            contactListItems.setSlno(c1.getString(c1.getColumnIndex("slno")));
            contactListItems.setNameofevent(c1.getString(c1.getColumnIndex("nameofevent")));
            contactListItems.setDtofevent(c1.getString(c1.getColumnIndex("dtofevent")));
            contactListItems.setTimeofevent(c1.getString(c1.getColumnIndex("timeofevent")));
            contactListItems.setDuration(c1.getString(c1.getColumnIndex("duration")));
            contactList.add(contactListItems);
            // Move the cursor along to the next entry.
            cursor.moveToNext();
        } 
    }
    // Close cursor after while and within if (so you know it is not null).
    c1.close();
}
else
{
    // You can't close c1 if it is Null. This will throw and error. Lose the else.
    c1.close();
}
// Move this to within your if statment.
c1.close();

这是nullpointer的原因？这是nullpointer的原因？这是nullpointer的原因？这是nullpointer的原因？正如您所看到的，select查询用于主活动中..insert在addnewevent classi中放置（&N）错误在此处。。else{c1.close（）；}我不清楚一件事..当我运行应用程序mainactivity时，它将运行..并且在表中没有数据的情况下，选择查询如何工作，正如您所看到的，选择查询在main activity中使用..并且insert在addnewevent classi中放置（&now）错误在这里。。else{c1.close（）；}我不清楚一件事..当我运行应用程序mainactivity时，它将运行..并且在表中没有数据的情况下，选择查询如何工作，正如您所看到的，选择查询在main activity中使用..并且insert在addnewevent classi中放置（&now）错误在这里。。else{c1.close（）；}我没有清除一件事..当我运行app main时，活动将运行..并且没有数据在其中