android - Unfortunately, <appname> has stopped. Why is it happening? The complete code is given below -

i have written complete code in android studio. have used sqlite backend. app stops click on "add data" button. please help.

//databasehelper.java (file name) code

package com.example.android.sqlite;  import android.content.contentvalues; import android.content.context; import android.database.sqlite.sqlitedatabase; import android.database.sqlite.sqliteopenhelper;  /**  * created lenovo on 14-04-2017.  */  public class databasehelper extends sqliteopenhelper {     public static final string database_name = "user.db";     public static final string table_name = "user_info.db";     public static final string col_1 = "id";     public static final string col_2 = "name";     public static final string col_3 = "surname";     public static final string col_4 = "marks";      public databasehelper(context context) {         super(context, database_name, null, 1);      }      @override     public void oncreate(sqlitedatabase db) {         db.execsql("create table" + table_name + ("id integer primary key autoincrement,name text,surname text, marks integer"));     }      @override     public void onupgrade(sqlitedatabase db, int oldversion, int newversion) {         db.execsql("drop table if exists" + table_name);         oncreate(db);     }      public boolean insertdata(string name, string surname, string marks) {         sqlitedatabase db = this.getwritabledatabase();         contentvalues cv = new contentvalues();         cv.put(col_2, name);         cv.put(col_3, surname);         cv.put(col_4, marks);         long result = db.insert(table_name, null, cv);         if (result == -1)             return false;         else             return true;       } } 

// mainactvity.java(file name) //code

package com.example.android.sqlite;  import android.support.v7.app.appcompatactivity; import android.os.bundle; import android.view.view; import android.widget.button; import android.widget.edittext; import android.widget.toast;  public class mainactivity extends appcompatactivity {     databasehelper my_db;     edittext editn,edits,editm;     button butt;     @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main);         my_db = new databasehelper(this);         editn= (edittext)findviewbyid(r.id.text1);         edits=(edittext)findviewbyid(r.id.text2);         editm=(edittext)findviewbyid(r.id.edittext);         butt=(button)findviewbyid(r.id.button);         adddata();      }     public void adddata(){         butt.setonclicklistener(             new view.onclicklistener() {                 @override                 public void onclick(view v){                     boolean isinserted = my_db.insertdata(editn.gettext().tostring(),edits.gettext().tostring(),editm.gettext().tostring());                     if(isinserted==true)                         toast.maketext(mainactivity.this,"data inserted",toast.length_long).show();                     else                         toast.maketext(mainactivity.this,"data insertion failed",toast.length_long).show();                  }             }         );     }  } 

your table_name = "user_info.db" contains .. seems not allowed: https://stackoverflow.com/a/3694305/4322687

also, in following line: db.execsql("create table" + table_name + ("id integer primary key autoincrement,name text,surname text, marks integer")); second pair of brackets looks it's supposed inside quotation marks.