Question

我正在构建一个联系人应用程序并为其创建了一个数据库。每次运行它时，它都会显示以下异常。

logcat错误示例：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
        at com.example.joey.sqlite.MainActivity.onResume(MainActivity.java:48)
        at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1257)
        at android.app.Activity.performResume(Activity.java:6076)
        at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3039)
        at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3081) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2447) 
        at android.app.ActivityThread.access$800(ActivityThread.java:156) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:211) 
        at android.app.ActivityThread.main(ActivityThread.java:5389) 
        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:1020) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815)

我该如何解决？

报告错误在for循环中：

@Override
protected void onResume() {

    DBHelper dbHelper = new DBHelper(this);

    namesList = new ArrayList<String>();
    for (int i=0; i< dbHelper.getAllContacts().size();i++){
        namesList.add(dbHelper.getAllContacts().get(i).getName());
    }

    adapter = new ArrayAdapter(getApplicationContext(),android.R.layout.simple_list_item_1,namesList);

    list.setAdapter(adapter);
    super.onResume();
}

数据库读取方法示例：

public ArrayList<Contact> getAllContacts() {
    SQLiteDatabase db = this.getReadableDatabase();
    ArrayList<Contact> contactArrayList = new ArrayList<Contact>();

    Cursor cur = db.rawQuery("select * from " + TABLE,null);
    cur.moveToFirst();
    while (cur.isAfterLast() == false) {
        try {
            Contact contact_new =  new Contact();
            contact_new.setName(cur.getString(cur.getColumnIndex(NAME)));
            contact_new.setEmail(cur.getString(cur.getColumnIndex(EMAIL)));
            contact_new.setPhone(cur.getString(cur.getColumnIndex(PHONE)));

            contactArrayList.add(contact_new);
            cur.moveToNext();

            return contactArrayList;

        } catch (Exception ex) {
            ex.printStackTrace();
            return null;
        }
    }
    return null;
}

Answer 1

只需在for循环上方添加此检查，以避免在数据库请求返回null时发生崩溃：

if (dbHelper.getAllContacts() != null) {
    for(...) {...}
}

从数据库读取时出现NullPointerException

1 个答案: