检查数据库是否为空 [英] Check whether database is empty
本文介绍了检查数据库是否为空的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我想检查一个SQLite数据库是空的使用
公共布尔chkDB(){
布尔CHK = FALSE;
光标mCursor = db.rawQuery(SELECT * FROM+ DATABASE_TABLE,NULL);
如果(mCursor!= NULL){
mCursor.moveToFirst();
如果(mCursor.getInt(0)== 0){
CHK = FALSE;
}
}其他{
CHK =真;
}
返回CHK;
}
但每次调用该方法,我得到空指针异常
我的logcat显示了这个
06-28 22:35:19.519:E / AndroidRuntime(441):在com.android.id.DBAdapter.chkDB(DBAdapter.java:82)
06-28 22:58:06.269:E / AndroidRuntime(621):致命异常:主要
06-28 22:58:06.269:E / AndroidRuntime(621):java.lang.RuntimeException的:无法启动的活动ComponentInfo {com.android.id/com.android.id.MainActivity}:显示java.lang.NullPointerException
06-28 22:58:06.269:E / AndroidRuntime(621):在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
06-28 22:58:06.269:E / AndroidRuntime(621):在android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
06-28 22:58:06.269:E / AndroidRuntime(621):在android.app.ActivityThread.access $ 2300(ActivityThread.java:125)
06-28 22:58:06.269:E / AndroidRuntime(621):在android.app.ActivityThread $ H.handleMessage(ActivityThread.java:2033)
06-28 22:58:06.269:E / AndroidRuntime(621):在android.os.Handler.dispatchMessage(Handler.java:99)
06-28 22:58:06.269:E / AndroidRuntime(621):在android.os.Looper.loop(Looper.java:123)
06-28 22:58:06.269:E / AndroidRuntime(621):在android.app.ActivityThread.main(ActivityThread.java:4627)
06-28 22:58:06.269:E / AndroidRuntime(621):在java.lang.reflect.Method.invokeNative(本机方法)
06-28 22:58:06.269:E / AndroidRuntime(621):在java.lang.reflect.Method.invoke(Method.java:521)
06-28 22:58:06.269:E / AndroidRuntime(621):在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run(ZygoteInit.java:868)
06-28 22:58:06.269:E / AndroidRuntime(621):在com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
06-28 22:58:06.269:E / AndroidRuntime(621):在dalvik.system.NativeStart.main(本机方法)
06-28 22:58:06.269:E / AndroidRuntime(621):由:显示java.lang.NullPointerException
06-28 22:58:06.269:E / AndroidRuntime(621):在com.android.id.DBAdapter.chkDB(DBAdapter.java:82)
06-28 22:58:06.269:E / AndroidRuntime(621):在com.android.id.MainActivity.enterDB(MainActivity.java:66)
06-28 22:58:06.269:E / AndroidRuntime(621):在com.android.id.MainActivity.onCreate(MainActivity.java:23)
06-28 22:58:06.269:E / AndroidRuntime(621):在android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
06-28 22:58:06.269:E / AndroidRuntime(621):在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
06-28 22:58:06.269:E / AndroidRuntime(621):11 ...更多
解决方案
mCursor.moveToFirst()
返回是否成功找到一个元素或不是一个布尔值。用它移动到第一行中的光标,并在同一时间检查如果一个行实际存在
光标mCursor = db.rawQuery(SELECT * FROM+ DATABASE_TABLE,NULL);
布尔rowExists;
如果(mCursor.moveToFirst())
{
// DO SOMETHING光标
rowExists = TRUE;
} 其他
{
//我空虚
rowExists = FALSE;
}
您试图访问游标中的行无论是否存在与否。
I am trying to check if a sqlite database is empty using
public boolean chkDB(){
boolean chk = false;
Cursor mCursor = db.rawQuery("SELECT * FROM " + DATABASE_TABLE, null);
if (mCursor != null){
mCursor.moveToFirst();
if (mCursor.getInt(0) == 0){
chk = false;
}
}else{
chk = true;
}
return chk;
}
but every time i call that method i get null pointer exception
My Logcat shows this
06-28 22:35:19.519: E/AndroidRuntime(441): at com.android.id.DBAdapter.chkDB(DBAdapter.java:82)
06-28 22:58:06.269: E/AndroidRuntime(621): FATAL EXCEPTION: main
06-28 22:58:06.269: E/AndroidRuntime(621): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android.id/com.android.id.MainActivity}: java.lang.NullPointerException
06-28 22:58:06.269: E/AndroidRuntime(621): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
06-28 22:58:06.269: E/AndroidRuntime(621): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
06-28 22:58:06.269: E/AndroidRuntime(621): at android.app.ActivityThread.access$2300(ActivityThread.java:125)
06-28 22:58:06.269: E/AndroidRuntime(621): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
06-28 22:58:06.269: E/AndroidRuntime(621): at android.os.Handler.dispatchMessage(Handler.java:99)
06-28 22:58:06.269: E/AndroidRuntime(621): at android.os.Looper.loop(Looper.java:123)
06-28 22:58:06.269: E/AndroidRuntime(621): at android.app.ActivityThread.main(ActivityThread.java:4627)
06-28 22:58:06.269: E/AndroidRuntime(621): at java.lang.reflect.Method.invokeNative(Native Method)
06-28 22:58:06.269: E/AndroidRuntime(621): at java.lang.reflect.Method.invoke(Method.java:521)
06-28 22:58:06.269: E/AndroidRuntime(621): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
06-28 22:58:06.269: E/AndroidRuntime(621): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
06-28 22:58:06.269: E/AndroidRuntime(621): at dalvik.system.NativeStart.main(Native Method)
06-28 22:58:06.269: E/AndroidRuntime(621): Caused by: java.lang.NullPointerException
06-28 22:58:06.269: E/AndroidRuntime(621): at com.android.id.DBAdapter.chkDB(DBAdapter.java:82)
06-28 22:58:06.269: E/AndroidRuntime(621): at com.android.id.MainActivity.enterDB(MainActivity.java:66)
06-28 22:58:06.269: E/AndroidRuntime(621): at com.android.id.MainActivity.onCreate(MainActivity.java:23)
06-28 22:58:06.269: E/AndroidRuntime(621): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
06-28 22:58:06.269: E/AndroidRuntime(621): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
06-28 22:58:06.269: E/AndroidRuntime(621): ... 11 more
解决方案
mCursor.moveToFirst()
Returns a boolean of whether it successfully found an element or not. Use it to move to the first row in the cursor and at the same time check if a row actually exists.
Cursor mCursor = db.rawQuery("SELECT * FROM " + DATABASE_TABLE, null);
Boolean rowExists;
if (mCursor.moveToFirst())
{
// DO SOMETHING WITH CURSOR
rowExists = true;
} else
{
// I AM EMPTY
rowExists = false;
}
You are trying to access a row in the cursor regardless of whether one exists or not.
这篇关于检查数据库是否为空的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!
查看全文