java - NullPointerException when saving into a database -

im trying save strings are: category, title, body database, i'm getting nullpointerexception whenever try execute savestate() method. me ?

//exectued when press save button public void onclicksave(view v) {     savestate(); }  // fire creation  private void savestate() {     string title = mjoketitle.gettext().tostring();     string body = mjokebody.gettext().tostring();      long id = mdbhelper.createjoke(enteredcat, title, body);  } 

my database schema

public class notesdbadapter {  public static final string key_title = "title"; public static final string key_body = "body"; public static final string key_category = "category";  public static final string key_row_id = "_id";  private static final string tag = "notesdbadapter"; private databasehelper mdbhelper; private sqlitedatabase mdb;  private static final string database_name = "my_database"; private static final string database_table = "jokes"; private static final int database_version = 3;  private static final string database_create =         "create table " + database_table + " (_id integer primary key autoincrement, "         + "title text not null, body text not null, category text not null unique);";  public long createjoke(string category, string title, string body) {     contentvalues initialvalues = new contentvalues();     initialvalues.put(key_category, category);     initialvalues.put(key_title, title);     initialvalues.put(key_body, body);      // return rowid if successful, otherwise -1     return mdb.insert(database_table, null, initialvalues); } 

private void savestate() {     string title ="";     string body="";    if(mjoketitle.gettext()!=nll){       title = mjoketitle.gettext().tostring();    }else{        system.out.println("mjoketitle.gettext() value null need  why value null");      }     if( mjokebody.gettext()!=nll){       body = mjokebody.gettext().tostring();    }else{        system.out.println(" mjokebody.gettext() value null need why value null");      }            if(mdbhelper!=null){         long id = mdbhelper.createjoke(enteredcat, title, body);     }else{             system.out.println("mdbhelper value null need why value null");       } 

}