我是Android新手，并尝试使用开源项目。项目数据库是sqlite数据库。当我运行项目并检查时，我发现android设备监视器中没有数据。请有人检查我的代码是否有错误？
package com.andisofttechnology.myapplication;
import android.app.Activity;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.util.Log;
import android.widget.Toast;
public class databaseHandler {
SQLiteDatabase database;
Activity activity;
public databaseHandler(Activity activity) {
this.activity = activity;
database = activity.openOrCreateDatabase("ASSIST", activity.MODE_PRIVATE, null);
createTable();
}
public void createTable()
{
try {
String qu = "CREATE TABLE IF NOT EXISTS STUDENT(name varchar(1000)," +
"cl varchar(100), " +
"regno varchar(100) primary key, contact varchar(100),roll integer);";
database.execSQL(qu);
}catch (Exception e)
{
Toast.makeText(activity,"Error Occured for create table",Toast.LENGTH_LONG).show();
}
try {
String qu = "CREATE TABLE IF NOT EXISTS ATTENDANCE(datex date," +
"hour int, " +
"register varchar(100) ,isPresent boolean);";
database.execSQL(qu);
}catch (Exception e)
{
Toast.makeText(activity,"Error Occured for create table",Toast.LENGTH_LONG).show();
}
try {
String qu = "CREATE TABLE IF NOT EXISTS NOTES(title varchar(100) not null," +
"body varchar(10000), cls varchar(1000), sub varchar(1000) ,datex TIMESTAMP default CURRENT_TIMESTAMP);";
database.execSQL(qu);
}catch (Exception e)
{
Toast.makeText(activity,"Error Occured for create table",Toast.LENGTH_LONG).show();
}
try {
String qu = "CREATE TABLE IF NOT EXISTS SCHEDULE(cl varchar(100),subject varchar(1000)," +
"timex time, day_week varchar(100));";
database.execSQL(qu);
}catch (Exception e)
{
Toast.makeText(activity,"Error Occured for create table",Toast.LENGTH_LONG).show();
}
}
public boolean execAction(String qu)
{
Log.i("databaseHandler", qu);
try {
database.execSQL(qu);
}catch (Exception e)
{
Log.e("databaseHandler", qu);
Toast.makeText(activity,"Error Occured for execAction",Toast.LENGTH_LONG).show();
return false;
}
return true;
}
Cursor execQuery(String qu)
{
try {
return database.rawQuery(qu,null);
}catch (Exception e)
{
Log.e("databaseHandler", qu);
// Toast.makeText(activity,"Error Occured for execAction",Toast.LENGTH_LONG).show();
}
return null;
}
}
Student_registration.class
package com.andisofttechnology.myapplication;
import android.app.Activity;
import android.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.Spinner;
import android.widget.Toast;
/*import java.util.ArrayList;*/
public class Student_Registartion extends AppCompatActivity {
private EditText name,register,contact;
Activity activity = this;
Spinner spinner;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_student__registartion);
spinner = (Spinner) findViewById(R.id.spinner);
/* ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_dropdown_item, AppBase.divisions);
spinner.setAdapter(adapter);*/
Button btn = (Button) findViewById(R.id.btnSave);
assert btn != null;
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
saveToDatabase(v);
}
});
}
public void saveToDatabase(View view) {
EditText name = (EditText)findViewById(R.id.name);
EditText roll = (EditText)findViewById(R.id.roll);
EditText register = (EditText)findViewById(R.id.register);
EditText contact = (EditText)findViewById(R.id.contact);
String classSelected = spinner.getSelectedItem().toString();
if(name.getText().length()<2||roll.getText().length()==0||register.getText().length()<2||
contact.getText().length()<2||classSelected.length()<2)
{
AlertDialog.Builder alert = new AlertDialog.Builder(activity);
alert.setTitle("Invalid");
alert.setMessage("Insufficient Data");
alert.setPositiveButton("OK", null);
alert.show();
return;
}
String qu = "INSERT INTO STUDENT VALUES('" +name.getText().toString()+ "'," +
"'" + classSelected +"',"+
"'" + register.getText().toString().toUpperCase() +"',"+
"'" + contact.getText().toString() +"',"+
"" + Integer.parseInt(roll.getText().toString()) +");";
Log.d("Student Reg" , qu);
AppBase.handler.execAction(qu);
qu = "SELECT * FROM STUDENT WHERE regno = '" + register.getText().toString() + "';";
Log.d("Student Reg" , qu);
if(AppBase.handler.execQuery(qu)!=null)
{
Toast.makeText(getBaseContext(),"Student Added", Toast.LENGTH_LONG).show();
this.finish();
}
}
}
............................................... .......................
原木猫
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.andisofttechnology.myapplication, PID: 32163
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
at com.andisofttechnology.myapplication.Student_Registartion.saveToDatabase(Student_Registartion.java:48)
at com.andisofttechnology.myapplication.Student_Registartion$1.onClick(Student_Registartion.java:37)
at android.view.View.performClick(View.java:6294)
at android.view.View$PerformClick.run(View.java:24770)
at android.os.Handler.handleCallback(Handler.java:790)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
答案 0 :(得分：0)
在OnCreate方法中执行FindViewById，这就是它创建NullPointException的原因
请参见下面的摘录，希望可以解决该问题
public class Student_Registartion extends AppCompatActivity {
private EditText name,roll,register,contact;
Activity activity = this;
Spinner spinner;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_student__registartion);
name = (EditText)findViewById(R.id.name);
roll = (EditText)findViewById(R.id.roll);
register = (EditText)findViewById(R.id.register);
contact = (EditText)findViewById(R.id.contact);
spinner = (Spinner) findViewById(R.id.spinner);
/* ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_dropdown_item, AppBase.divisions);
spinner.setAdapter(adapter);*/
Button btn = (Button) findViewById(R.id.btnSave);
assert btn != null;
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
saveToDatabase(v);
}
});
}
public void saveToDatabase(View view) {
String classSelected = spinner.getSelectedItem().toString();
if(name.getText().length()<2||roll.getText().length()==0||register.getText().length()<2||
contact.getText().length()<2||classSelected.length()<2)
{
AlertDialog.Builder alert = new AlertDialog.Builder(activity);
alert.setTitle("Invalid");
alert.setMessage("Insufficient Data");
alert.setPositiveButton("OK", null);
alert.show();
return;
}
String qu = "INSERT INTO STUDENT VALUES('" +name.getText().toString()+ "'," +
"'" + classSelected +"',"+
"'" + register.getText().toString().toUpperCase() +"',"+
"'" + contact.getText().toString() +"',"+
"" + Integer.parseInt(roll.getText().toString()) +");";
Log.d("Student Reg" , qu);
AppBase.handler.execAction(qu);
qu = "SELECT * FROM STUDENT WHERE regno = '" + register.getText().toString() + "';";
Log.d("Student Reg" , qu);
if(AppBase.handler.execQuery(qu)!=null)
{
Toast.makeText(getBaseContext(),"Student Added", Toast.LENGTH_LONG).show();
this.finish();
}
}
}
答案 1 :(得分：0)
您有很多问题，基本上都是相同的，即对象不正确/没有实例化。
以下代码有效并且已被检查。更正用 // <<<<<<<<<< / 进行注释。
请注意，为了进行测试，添加了额外的一行以为微调框提供值，并且尝试从微调框获取值的行已被注释掉。
未对 databaseHandler.java
进行任何更改Student_Registartion.java ：-
public class Student_Registartion extends AppCompatActivity {
private EditText name,roll,register,contact; //<<<<<<<<<< ADDED roll
databaseHandler handler; //<<<<<<<<<< ADDED (used instead of AppBase.handler)
Activity activity = this;
Spinner spinner;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_student__registartion);
//<<<<<<<<<< ADDED BLOCK
name = this.findViewById(R.id.name);
roll = this.findViewById(R.id.roll);
register = this.findViewById(R.id.register);
contact = this.findViewById(R.id.contact);
//<<<<<<<<<< END OF ADDED BLOCK
handler = new databaseHandler(this); //<<<<<<<<<< ADDED
spinner = (Spinner) findViewById(R.id.spinner);
/* ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_dropdown_item, AppBase.divisions);
spinner.setAdapter(adapter);*/
Button btn = (Button) findViewById(R.id.btnSave);
assert btn != null;
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
saveToDatabase(v);
}
});
}
public void saveToDatabase(View view) {
//<<<<<<<<<<< REMOVED BLOCK
EditText name = (EditText)findViewById(R.id.name);
EditText roll = (EditText)findViewById(R.id.roll);
EditText register = (EditText)findViewById(R.id.register);
EditText contact = (EditText)findViewById(R.id.contact);
//<<<<<<<<<<< END OF REMOVED BLOCK
//String classSelected = spinner.getSelectedItem().toString(); //<<<<<<<<<< commeneted out
String classSelected = "MyCLass"; //<<<<<<<<<< Temporary get around for no spinner data
if(name.getText().length()<2||roll.getText().length()==0||register.getText().length()<2||
contact.getText().length()<2||classSelected.length()<2)
{
AlertDialog.Builder alert = new AlertDialog.Builder(activity);
alert.setTitle("Invalid");
alert.setMessage("Insufficient Data");
alert.setPositiveButton("OK", null);
alert.show();
return;
}
String qu = "INSERT INTO STUDENT VALUES('" +name.getText().toString()+ "'," +
"'" + classSelected +"',"+
"'" + register.getText().toString().toUpperCase() +"',"+
"'" + contact.getText().toString() +"',"+
"" + Integer.parseInt(roll.getText().toString()) +");";
Log.d("Student Reg" , qu);
handler.execAction(qu); //<<<<<<<<<< changed
qu = "SELECT * FROM STUDENT WHERE regno = '" + register.getText().toString() + "';";
Log.d("Student Reg" , qu);
if(handler.execQuery(qu)!=null) //<<<<<<<<< changed
{
Toast.makeText(getBaseContext(),"Student Added", Toast.LENGTH_LONG).show();
this.finish();
}
}
}
以下是成功添加数据的运行日志（Toasts execAction发生错误，重复，Toasts Student已添加，并且在没有输入数据时显示对话框）：-
09-18 20:45:23.160 1379-1379/so52386209.so52386209 D/Student Reg: INSERT INTO STUDENT VALUES('harold','MyCLass','BLAH','blah',54321);
09-18 20:45:23.160 1379-1379/so52386209.so52386209 I/databaseHandler: INSERT INTO STUDENT VALUES('harold','MyCLass','BLAH','blah',54321);
09-18 20:45:23.160 1379-1379/so52386209.so52386209 E/databaseHandler: INSERT INTO STUDENT VALUES('harold','MyCLass','BLAH','blah',54321);
09-18 20:45:23.164 1379-1379/so52386209.so52386209 D/Student Reg: SELECT * FROM STUDENT WHERE regno = 'blah';
09-18 20:45:23.212 1379-1379/so52386209.so52386209 W/IInputConnectionWrapper: showStatusIcon on inactive InputConnection
09-18 20:45:23.680 1379-1379/so52386209.so52386209 D/OpenGLRenderer: TextureCache::flush: target size: 2284
TextureCache::callback: name, removed size, mSize = 57, 1728, 2080
09-18 20:49:49.900 1379-1379/so52386209.so52386209 W/EGL_genymotion: eglSurfaceAttrib not implemented
09-18 20:49:49.916 1379-1379/so52386209.so52386209 D/OpenGLRenderer: TextureCache::get: create texture(0xb8f77208): name, size, mSize = 158, 1728, 3808
09-18 20:49:58.896 1379-1382/so52386209.so52386209 D/dalvikvm: GC_CONCURRENT freed 223K, 9% free 6603K/7239K, paused 10ms+0ms, total 14ms
09-18 20:50:24.516 1379-1379/so52386209.so52386209 D/Student Reg: INSERT INTO STUDENT VALUES('ian','MyCLass','77889900','something',12345);
09-18 20:50:24.516 1379-1379/so52386209.so52386209 I/databaseHandler: INSERT INTO STUDENT VALUES('ian','MyCLass','77889900','something',12345);
09-18 20:50:24.520 1379-1379/so52386209.so52386209 D/Student Reg: SELECT * FROM STUDENT WHERE regno = '77889900';