What is a NullPointerException, and how do I fix it? (12个答案)
4年前关闭。
addMember.java
EditText name,paid;
Button add;
final MyDBHandler db = new MyDBHandler(this);
String unpaid, inout="0";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.add_new_member);
name=(EditText)findViewById(R.id.userName);
paid=(EditText)findViewById(R.id.PaidAmount);
add=(Button)findViewById(R.id.addMember);
add.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
unpaid=Integer.toString(250-(Integer.parseInt(paid.getText().toString())));
Log.d("Insert: ", "Inserting ..");
db.add(new backService(name.getText().toString(),paid.getText().toString(),unpaid,inout));
Intent intent=new Intent(addMember.this,MainActivity.class);
startActivity(intent);
}
});
}
backService.java
private int _id;
private String message,number,con_name,date;
public backService(){}
public backService(String msg,String num,String con_na,String dat){
this.message=msg;
this.number=num;
this.con_name=con_na;
this.date=dat;
}
//SETTERS
public void set_id(int _id) {
this._id = _id;
}
public void setMessage(String message) {
this.message = message;
}
public void setNumber(String number) {
this.number = number;
}
public void setCon_name(String con_id) {
this.con_name = con_id;
}
public void setDate(String date) {
this.date = date;
}
//END SETTERS
//GETTERS
public int get_id() {
return _id;
}
public String getMessage() {
return message;
}
public String getCon_name() {
return con_name;
}
public String getNumber() {
return number;
}
public String getDate() {
return date;
}
//END GETTERS
CustomAdapter.java
Activity context;
String name[];
String paid[];
String unpaid[];
int state[];
public CustomAdapter(Activity context, String[] name, String[] paid, String[] unpaid, int[] state) {
super();
this.context = context;
this.name = name;
this.paid = paid;
this.unpaid=unpaid;
this.state=state;
}
public int getCount() {
// TODO Auto-generated method stub
return name.length;
}
public Object getItem(int position) {
// TODO Auto-generated method stub
return null;
}
public long getItemId(int position) {
// TODO Auto-generated method stub
return 0;
}
private class ViewHolder {
TextView txtViewName;
TextView txtViewPaid;
TextView txtViewUnpaid;
Switch in_outSwitch;
}
public View getView(int position, View convertView, ViewGroup parent)
{
// TODO Auto-generated method stub
ViewHolder holder;
LayoutInflater inflater = context.getLayoutInflater();
if (convertView == null)
{
convertView = inflater.inflate(R.layout.listview_text, null);
holder = new ViewHolder();
holder.txtViewName = (TextView) convertView.findViewById(R.id.patricipants);
holder.txtViewPaid = (TextView) convertView.findViewById(R.id.money_paid);
holder.txtViewUnpaid = (TextView) convertView.findViewById(R.id.money_due);
holder.in_outSwitch= (Switch) convertView.findViewById(R.id.inorout);
convertView.setTag(holder);
}
else
{
holder = (ViewHolder) convertView.getTag();
}
holder.txtViewName.setText(name[position]);
holder.txtViewPaid.setText(paid[position]);
holder.txtViewUnpaid.setText(unpaid[position]);
if(state[position]==0){
holder.in_outSwitch.setChecked(false);
}
else{
holder.in_outSwitch.setChecked(true);
}
return convertView;
}
MainActivity.java
ListView lv;
CustomAdapter ca;
String[] name,paid,unpaid;
int[] state,ids;
MyDBHandler dbh;
int positions;
updater upd;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
name=dbh.ret_name();
paid=dbh.ret_paidAmt();
unpaid=dbh.ret_unpaidAmt();
ids=dbh.ret_id();
state=dbh.ret_inORout();
lv=(ListView) findViewById(R.id.nameSpace);
ca=new CustomAdapter(this,name,paid,unpaid,state);
lv.setAdapter(ca);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
showPopup(view);
}
});
lv.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
@Override
public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) {
showcontextmenu(view,position);
return true;
}
});
}
public void showcontextmenu(View v,int position) {
PopupMenu popupMenu=new PopupMenu(this,v);
popupMenu.setOnMenuItemClickListener(this);
MenuInflater inflater = popupMenu.getMenuInflater();
inflater.inflate(R.menu.listviewpress,popupMenu.getMenu());
positions=position;
popupMenu.show();
}
public void showPopup(View v)
{
PopupMenu popupMenu=new PopupMenu(this,v);
popupMenu.setOnMenuItemClickListener(this);
MenuInflater inflater = popupMenu.getMenuInflater();
inflater.inflate(R.menu.menu_main,popupMenu.getMenu());
popupMenu.show();
}
@Override
public boolean onMenuItemClick(MenuItem item) {
Intent intent;
//Handle ItemClicks HERE
switch(item.getItemId()) {
case R.id.delete:
dbh.deleteuser(name[positions]);
return true;
case R.id.update:
upd.name=name[positions];
upd.paid=paid[positions];
upd.id=Integer.toString(ids[positions]);
intent=new Intent(MainActivity.this,updater.class);
startActivity(intent);
return true;
case R.id.addMember:
intent=new Intent(MainActivity.this,addMember.class);
startActivity(intent);
return true;
case R.id.delAll:
dbh.deldb();
return true;
default:
return false;
}
}
@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.
onMenuItemClick(item);
return super.onOptionsItemSelected(item);
}
MyDBHandler.java
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "picnic.db";
private static final String TABLE_NAME="members";
private static final String COLUMN_ID="_id";
private static final String COLUMN_MSG="_name";
private static final String COLUMN_NUM="_paid";
private static final String COLUMN_NAME="_unpaid";
private static final String COLUMN_TIME="_inORout";
public long s;
public int size;
public MyDBHandler(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
// FINDS THE SIZE OF A COLUMN IN A DATABASE
public void get_dbSize(){
SQLiteDatabase db= getWritableDatabase();
s= DatabaseUtils.queryNumEntries(db, TABLE_NAME);
}
@Override
public void onCreate(SQLiteDatabase db) {
String query= "CREATE TABLE " + TABLE_NAME + " ( " +
COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT " +
COLUMN_NUM + " TEXT " +
COLUMN_NAME + " TEXT " +
COLUMN_MSG + " TEXT " +
COLUMN_TIME + " TEXT " +
");";
db.execSQL(query);
get_dbSize();
size=(int)s;
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS "+ TABLE_NAME);
onCreate(db);
}
public void deldb(){
SQLiteDatabase db =getWritableDatabase();
db.execSQL("DROP TABLE IF EXISTS "+ TABLE_NAME);
}
// ADD A NEW DATA IN THE DATA BASE
public void add(backService BackService){
ContentValues values =new ContentValues();
values.put(COLUMN_NUM, BackService.getNumber());
values.put(COLUMN_NAME, BackService.getCon_name());
values.put(COLUMN_MSG, BackService.getMessage());
values.put(COLUMN_TIME,BackService.getDate());
SQLiteDatabase db =getWritableDatabase();
db.insert(TABLE_NAME,null,values);
db.close();
}
public void deleteuser(String name){
SQLiteDatabase db =getWritableDatabase();
db.execSQL("DELETE FROM "+ TABLE_NAME + " WHERE "+ COLUMN_MSG + "= \"" + name +"\";");
}
public void updateuser(String Value1, String Value2, String id){
SQLiteDatabase db=getWritableDatabase();
db.execSQL("UPDATE "+TABLE_NAME+" SET "+COLUMN_MSG+ " = \'"+Value1+"\' WHERE "+COLUMN_ID+" = \'"+id+"\' ;");
db.execSQL("UPDATE "+TABLE_NAME+" SET "+COLUMN_NUM+ " = \'"+Value2+"\' WHERE "+COLUMN_ID+" = \'"+id+"\' ;");
}
/*
//
//
//
*/
public String[] ret_paidAmt(){
String[] phn_num=new String[size];
SQLiteDatabase sqLiteDatabase = getWritableDatabase();
String query = "SELECT "+COLUMN_NUM+" FROM " + TABLE_NAME + " WHERE 1"; `enter code here`
int count=0;
Cursor cursor = sqLiteDatabase.rawQuery(query, null); //Cursor point to a location in results.
cursor.moveToFirst(); //Move to the first row in results.
while (!cursor.isAfterLast()) {
if (cursor.getString(cursor.getColumnIndex(COLUMN_NUM)) != null) {
phn_num[count] = cursor.getString(cursor.getColumnIndex(COLUMN_NUM));
count++;
}
cursor.moveToNext();
}
cursor.close();
sqLiteDatabase.close();
return phn_num;
}
// GET NAME FROM THE DB
public String[] ret_name(){
String[] phn_msg=new String[size];
SQLiteDatabase sqLiteDatabase = getWritableDatabase();
String query = "SELECT "+COLUMN_MSG+" FROM " + TABLE_NAME + " WHERE 1";
//Select every column, select every row.
int count=0;
Cursor cursor = sqLiteDatabase.rawQuery(query, null); //Cursor point to a location in results.
cursor.moveToFirst(); //Move to the first row in results.
while (!cursor.isAfterLast()) {
if (cursor.getString(cursor.getColumnIndex(COLUMN_MSG)) != null) {
phn_msg[count] = cursor.getString(cursor.getColumnIndex(COLUMN_MSG));
count++;
}
cursor.moveToNext();
}
cursor.close();
sqLiteDatabase.close();
return phn_msg;
}
// GET UNPAID AMOUNT FROM DB
public String[] ret_unpaidAmt(){
String[] phn_conna=new String[size];
SQLiteDatabase sqLiteDatabase = getWritableDatabase();
String query = "SELECT "+COLUMN_NAME+" FROM " + TABLE_NAME + " WHERE 1";
//Select every column, select every row.
int count=0;
Cursor cursor = sqLiteDatabase.rawQuery(query, null); //Cursor point to a location in results.
cursor.moveToFirst(); //Move to the first row in results.
while (!cursor.isAfterLast()) {
if (cursor.getString(cursor.getColumnIndex(COLUMN_NAME)) != null) {
phn_conna[count] = cursor.getString(cursor.getColumnIndex(COLUMN_NAME));
count++;
}
cursor.moveToNext();
}
cursor.close();
sqLiteDatabase.close();
return phn_conna;
}
// GET ID OF CONTACT FROM DB
public int[] ret_id(){
int[] con_id=new int[size];
SQLiteDatabase sqLiteDatabase = getWritableDatabase();
String query = "SELECT "+COLUMN_ID+" FROM " + TABLE_NAME + " WHERE 1";
//Select every column, select every row.
int count=0;
Cursor cursor = sqLiteDatabase.rawQuery(query, null);
//Cursor point to a location in results.
cursor.moveToFirst(); //Move to the first row in results.
while (!cursor.isAfterLast()) {
if (cursor.getString(cursor.getColumnIndex(COLUMN_ID)) != null) {
con_id[count] = Integer.parseInt(cursor.getString(cursor.getColumnIndex(COLUMN_ID)));
count++;
}
cursor.moveToNext();
}
cursor.close();
sqLiteDatabase.close();
return con_id;
}
// GET IN OR OUT FROM DB
public int[] ret_inORout(){
int[] rec_time=new int[size];
String temp;
SQLiteDatabase sqLiteDatabase = getWritableDatabase();
String query = "SELECT "+COLUMN_TIME+" FROM " + TABLE_NAME + " WHERE 1";
//Select every column, select every row.
int count=0;
Cursor cursor = sqLiteDatabase.rawQuery(query, null); //Cursor point to a location in results.
cursor.moveToFirst(); //Move to the first row in results.
while (!cursor.isAfterLast()) {
if (cursor.getString(cursor.getColumnIndex(COLUMN_TIME)) != null) {
temp = cursor.getString(cursor.getColumnIndex(COLUMN_TIME));
rec_time[count] = Integer.parseInt(temp.trim());
count++;
}
cursor.moveToNext();
}
cursor.close();
sqLiteDatabase.close();
return rec_time;
}
更新程序
EditText nameField,paymentField;
Button updateButton;
TextView textView;
String name,paid,id;
MyDBHandler dbh;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.updatemember);
nameField=(EditText) findViewById(R.id.nameOfMember);
paymentField=(EditText) findViewById(R.id.amtPaidByUser);
updateButton=(Button) findViewById(R.id.updateButton);
textView=(TextView) findViewById(R.id.Remaining);
nameField.setText(name);
paymentField.setText(paid);
updatetext();
paymentField.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence s, int start, int count, int after) {
}
@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {
}
@Override
public void afterTextChanged(Editable s) {
if (!paymentField.getText().toString().equalsIgnoreCase("")){
updatetext();
}
}
});
updateButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
name=nameField.getText().toString();
paid=paymentField.getText().toString();
dbh.updateuser(name,paid,id);
Toast.makeText(getApplicationContext(),name+" is updated",Toast.LENGTH_SHORT).show();
Intent intent=new Intent(updater.this,MainActivity.class);
}
});
}
public void updatetext(){
textView.setText(Integer.toString((250-Integer.parseInt(paid))));
}
logCat
>FATAL EXCEPTION: main
Process: com.example.neil.picnicrecord, PID: 22647
Theme: themes:{}
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.neil.picnicrecord/com.example.neil.picnicrecord.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String[] com.example.neil.picnicrecord.MyDBHandler.ret_name()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2462)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2522)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5471)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String[] com.example.neil.picnicrecord.MyDBHandler.ret_name()' on a null object reference
at com.example.neil.picnicrecord.MainActivity.onCreate(MainActivity.java:37)
at android.app.Activity.performCreate(Activity.java:7125)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2415)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2522)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5471)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) >
我有一个异常(exception)，如果有更多错误，我不知道它在哪里发生以及如何处理。请告诉我。