This what happens in the system. 1. Admin login this is in other activity but i will not post it coz it has nothing to do with this (no problem) 2. Register user in system (using database no problem) 3. Click add user button (where existing user who register must display its name in ListView) Problem: When I click adduser to see if the system registered the user, it force close.

CurrentUser.java

package com.example.istronggyminstructor;

import java.util.ArrayList;

import android.os.Bundle;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.database.Cursor;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.view.ViewGroup.LayoutParams;
import android.view.WindowManager;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.FrameLayout;
import android.widget.ListView;
import android.widget.PopupWindow;
import android.widget.TextView;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

import com.example.istronggyminstructor.registeredUserList.Users;

import android.content.ContentValues;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;

public class CurrentUsers extends Activity {
    private Button register;
    private Button adduser;
    EditText getusertext, getpass, getweight, textdisp;
    View popupview, popupview2;
    public static ArrayList<String> ArrayofName = new ArrayList<String>();
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_current_users);
        register = (Button) findViewById(R.id.regbut);
        adduser = (Button) findViewById(R.id.addbut);
        register.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View arg0) {
                LayoutInflater inflator = (LayoutInflater) getBaseContext()
                        .getSystemService(LAYOUT_INFLATER_SERVICE);
                popupview = inflator.inflate(R.layout.popup, null);
                final PopupWindow popupWindow = new PopupWindow(popupview,
                        LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
                popupWindow.showAtLocation(popupview, Gravity.CENTER, 0, 0);
                popupWindow.setFocusable(true);
                popupWindow.update();
                Button dismissbtn = (Button) popupview.findViewById(R.id.close);
                dismissbtn.setOnClickListener(new OnClickListener() {
                    @Override
                    public void onClick(View arg0) {
                        popupWindow.dismiss();
                    }
                });
                popupWindow.showAsDropDown(register, 50, -30);
            }
        });
        //Thread.setDefaultUncaughtExceptionHandler(new forceclose(this));
    }



    public void registerUser(View v) {
        EditText username = (EditText) popupview.findViewById(R.id.usertext);
        EditText password = (EditText) popupview
                .findViewById(R.id.passwordtext);
        EditText weight = (EditText) popupview.findViewById(R.id.weight);
        String getUsername = username.getText().toString();
        String getPassword = password.getText().toString();
        String getWeight = weight.getText().toString();

        dataHandler dbHandler = new dataHandler(this, null, null, 1);

        Users user = 
                  new Users(getUsername, getPassword, Integer.parseInt(getWeight));

        dbHandler.addUsers(user);

        Toast.makeText(getApplicationContext(), "Registering...",
                Toast.LENGTH_SHORT).show();
    }

    public void onClick_addUser(View v) {
        LayoutInflater inflator = (LayoutInflater) getBaseContext()
                .getSystemService(LAYOUT_INFLATER_SERVICE);
        popupview2 = inflator.inflate(R.layout.popup2, null);
        final PopupWindow popupWindow = new PopupWindow(popupview2,
                LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
        popupWindow.showAtLocation(popupview2, Gravity.CENTER, 0, -10);
        popupWindow.setFocusable(true);
        popupWindow.update();
        Button dismissbtn = (Button) popupview2.findViewById(R.id.close2);
        dismissbtn.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View arg0) {
                popupWindow.dismiss();
            }
        });
        popupWindow.showAsDropDown(register, 50, -30);


        dataHandler dbHandler = new dataHandler(this, null, null, 1);
        dbHandler.getAllUsers();
        ListView list = (ListView)findViewById(R.layout.popup2);
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1, ArrayofName);
        list.setAdapter(adapter);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.current_users, menu);
        return true;
    }

}

registeredUserList.java

package com.example.istronggyminstructor;

public class registeredUserList {

public static class Users {
      private static int _id;
      private static String _users;
      private static String _password;
      private static int _weight;
      private static String[] _workoutlists;
      private static int _score;

      public Users() {

        }

      public Users(String username, String password, int weight) {
            _users = username;
            _password = password;
            _weight = weight;
        }
      public int getId() {
        return _id;
      }

      public static void setId(int id) {
        _id = id;
      }

      public String getUsers() {
        return _users;
      }

      public static void setUsers(String users) {
        _users = users;
      }

      public String getPassword(){
          return _password;
      }

      public void setPassword(String password){
          _password = password;
      }

      public int getWeight(){
          return _weight;
      }

      public static void setWeight(int weight){
          _weight = weight;
      }

      public String[] getWorkoutLists(){
          return _workoutlists;
      }

      public void setWorkoutLists(String[] workoutlists){
          _workoutlists = workoutlists;
      }

      public int score(){
          return _score;
      }

      public void score(int score){
          _score = score;
      }


    } 
}

dataHandler.java

package com.example.istronggyminstructor;


import java.util.ArrayList;
import java.util.List;

import com.example.istronggyminstructor.registeredUserList.Users;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;

public class dataHandler extends SQLiteOpenHelper {
    private static final int DATABASE_VERSION = 1;
    private static final String DATABASE_NAME = "userInfo.db";
    public static final String TABLE_USERINFO = "user";

    public static final String COLUMN_ID = "_id";
    public static final String COLUMN_USERNAME = "username";
    public static final String COLUMN_PASSWORD = "password";
    public static final String COLUMN_WEIGHT = "weight";

    public dataHandler(Context context, String name, CursorFactory factory, int version) {
        super(context, DATABASE_NAME, factory, DATABASE_VERSION);
    }


    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_USER_TABLE = "CREATE TABLE " +
                 TABLE_USERINFO + " ("
                 + COLUMN_ID + " INTEGER PRIMARY KEY, " + COLUMN_USERNAME 
                 + " TEXT," + COLUMN_PASSWORD + " TEXT, " + COLUMN_WEIGHT + " INTEGER " + ");";
          db.execSQL(CREATE_USER_TABLE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_USERINFO);
          onCreate(db);
    }

    public void addUsers(Users user) {
        ContentValues values = new ContentValues();
        values.put(COLUMN_USERNAME, user.getUsers());
        values.put(COLUMN_PASSWORD, user.getPassword());
        values.put(COLUMN_WEIGHT, user.getWeight());

        SQLiteDatabase db = this.getWritableDatabase();

        db.insert(TABLE_USERINFO, null, values);
        db.close();
}
    public Users findUsers(String username) {
        String query = "Select * FROM " + TABLE_USERINFO + " WHERE " + COLUMN_USERNAME
                + " =  \"" + username + "\"";

        SQLiteDatabase db = this.getWritableDatabase();

        Cursor cursor = db.rawQuery(query, null);

        Users user = new Users();

        if (cursor.moveToFirst()) {
            cursor.moveToFirst();
            Users.setUsers(cursor.getString(1));
            //Users.setWeight(Integer.parseInt(cursor.getString(3))); not yet needed
            cursor.close();
        } else {
            user = null;
        }
            db.close();
        return user;
    }

    public List<Users> getAllUsers(){
        List<Users> user = new ArrayList();
        String selectQuery = "SELECT * FROM " + TABLE_USERINFO;
        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);
        if (cursor.moveToFirst()) {
            do {
                Users users = new Users();
                users.setUsers(cursor.getString(1));

                String name = cursor.getString(1);
                CurrentUsers.ArrayofName.add(name);
                // Adding contact to list
                user.add(users);
            } while (cursor.moveToNext());
        }

        // return user list
        return user;

    }


    public boolean deleteUsers(String username) {

        boolean result = false;

        String query = "Select * FROM " + TABLE_USERINFO + " WHERE " + COLUMN_USERNAME + " =  \"" 
        + username + "\"";

        SQLiteDatabase db = this.getWritableDatabase();

        Cursor cursor = db.rawQuery(query, null);

        Users user = new Users();

        if (cursor.moveToFirst()) {
            Users.setId(Integer.parseInt(cursor.getString(0)));
            db.delete(TABLE_USERINFO, COLUMN_ID + " = ?",
                    new String[] { String.valueOf(user.getId()) });
            cursor.close();
            result = true;
        }
            db.close();
        return result;
    }
}

Logcat

08-20 03:23:23.293: E/AndroidRuntime(16363): FATAL EXCEPTION: main
08-20 03:23:23.293: E/AndroidRuntime(16363): java.lang.IllegalStateException: Could not execute method of the activity
08-20 03:23:23.293: E/AndroidRuntime(16363):    at android.view.View$1.onClick(View.java:3599)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at android.view.View.performClick(View.java:4204)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at android.view.View$PerformClick.run(View.java:17355)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at android.os.Handler.handleCallback(Handler.java:725)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at android.os.Handler.dispatchMessage(Handler.java:92)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at android.os.Looper.loop(Looper.java:137)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at android.app.ActivityThread.main(ActivityThread.java:5041)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at java.lang.reflect.Method.invokeNative(Native Method)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at java.lang.reflect.Method.invoke(Method.java:511)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at dalvik.system.NativeStart.main(Native Method)
08-20 03:23:23.293: E/AndroidRuntime(16363): Caused by: java.lang.reflect.InvocationTargetException
08-20 03:23:23.293: E/AndroidRuntime(16363):    at java.lang.reflect.Method.invokeNative(Native Method)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at java.lang.reflect.Method.invoke(Method.java:511)
08-20 03:23:23.293: E/AndroidRuntime(16363):    at android.view.View$1.onClick(View.java:3594)
08-20 03:23:23.293: E/AndroidRuntime(16363):    ... 11 more
08-20 03:23:23.293: E/AndroidRuntime(16363): Caused by: java.lang.NullPointerException
08-20 03:23:23.293: E/AndroidRuntime(16363):    at com.example.istronggyminstructor.CurrentUsers.onClick_addUser(CurrentUsers.java:118)
08-20 03:23:23.293: E/AndroidRuntime(16363):    ... 14 more