My shared / Version 1.1

//==========================================MainActivity
package com.example.com.myshared;

import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends Activity
{
    EditText txtUser;
    EditText txtPass;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        txtUser=(EditText)findViewById(R.id.user);
        txtPass=(EditText)findViewById(R.id.pass);
        if (checkUser())
        {
            //if key found , start the second activity.
            SharedPreferences pref=getApplicationContext().getSharedPreferences("myPref",MODE_PRIVATE);
            Intent iEntry = new Intent(this,Entry.class);
            iEntry.putExtra("putUser", pref.getString("userLogged", "error"));
            this.startActivity(iEntry);
        }
    }


    private boolean checkUser()
    {
        //declaration of shared preferences
        SharedPreferences pref=getApplicationContext().getSharedPreferences("myPref",MODE_PRIVATE);
        //getting value of key "user"
        String userName = pref.getString("userLogged","error");
        //return false if key "user" not exists
        return (!userName.equals("error"));
    }

    public void btnLogin(View v)
    {
        SharedPreferences pref = getApplicationContext().getSharedPreferences("myPref",MODE_PRIVATE);

        String userName=txtUser.getText().toString().toLowerCase();
        String password= txtPass.getText().toString();
        String passDB=pref.getString(userName,"ErrorP");

        if(userName.equals("") || password.equals("")) {
            Toast.makeText(this, "Please type username and password", Toast.LENGTH_LONG).show();
            return;
        }
            else if(!pref.contains(userName)) {
                Toast.makeText(this, "Please register", Toast.LENGTH_LONG).show();
                return;
            }
                else if(password.equals(passDB)) {
                    Toast.makeText(this, "Good to see you", Toast.LENGTH_SHORT).show();
                }
                    else
                    {
                        Toast.makeText(this, "Wrong user name or password", Toast.LENGTH_LONG).show();
                        return;
                    }

        //declaration of editor to edit shared preferences
        SharedPreferences.Editor editor=pref.edit();
        //create key "user" with value of userName, if key exists, it will be overwritten
        editor.putString("userLogged",userName);
        //commit the changes
        editor.commit();

        Intent iEntry = new Intent(this,Entry.class);
        iEntry.putExtra("putUser", pref.getString("userLogged","error"));
        this.startActivity(iEntry);
    }

    public void btnReg(View v)
    {
        Intent regIntent = new Intent(this, Register.class);
        this.startActivity(regIntent);
    }
}

//======================================REGISTER CLASS
package com.example.com.myshared;

import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;


public class Register extends Activity
{
    EditText txtUser, txtPass1, txtPass2;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.register);
        setPointer();
    }

    public void setPointer()
    {
        txtUser=(EditText)findViewById(R.id.user);
        txtPass1=(EditText)findViewById(R.id.pass1);
        txtPass2=(EditText)findViewById(R.id.pass2);
    }


    public void btnRegister(View v)
    {
        SharedPreferences pref=getApplicationContext().getSharedPreferences("myPref", MODE_PRIVATE);
        //declaration of editor to edit shared preferences
        SharedPreferences.Editor editor=pref.edit();
        //insert into string value from EditText
        String userName=txtUser.getText().toString().toLowerCase();
        String password1=txtPass1.getText().toString();
        String password2=txtPass2.getText().toString();

        if(userName.equals("")||password1.equals("")){
            Toast.makeText(this, "Please type username and password", Toast.LENGTH_LONG).show();
            return;
        }
            else if(pref.contains(userName)) {
                Toast.makeText(this, "The username exist, please type another username", Toast.LENGTH_LONG).show();
            return;
            }
                else if(!password1.equals(password2)){
                    Toast.makeText(this, "The password must be equals", Toast.LENGTH_LONG).show();
                    return;
                }
        //create key "user name" with value of your password
        editor.putString(userName, password1);

        //commit the changes
        editor.commit();

        Intent iLogin = new Intent(this, MainActivity.class);
        this.startActivity(iLogin);
    }

    public void btnCancel(View v)
    {
        Intent iLogin = new Intent(this, MainActivity.class);
        this.startActivity(iLogin);
    }
}

//===========================================ENTRY CLASS
package com.example.com.myshared;

import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

public class Entry extends Activity {

    TextView txt;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.entry);
        txt=(TextView)findViewById(R.id.text);
        txt.setText("Hello " + getIntent().getStringExtra("putUser"));
    }


    public void btnLogout(View v)
    {
        //declaration of shared preferences
        SharedPreferences pref=getApplicationContext().getSharedPreferences("myPref",MODE_PRIVATE);
        //declaration of editor
        SharedPreferences.Editor editor=pref.edit();
        //remove key user and pass
        editor.remove("userLogged");
        //commit changes
        editor.commit();
        //start another window - main window
        Intent iMain = new Intent(this,MainActivity.class);
        this.startActivity(iMain);
    }
}