//LoginActivity
--------------------
package com.example.android.loginapp;

import android.content.Context;
import android.content.Intent;
import android.graphics.Color;
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.TextView;
import android.widget.Toast;

public class LoginActivity extends AppCompatActivity {
    EditText txtUserName,txtPass;
    TextView logo;
    Button btnLogin,btnReg;
    Context context;
    private final String TAG="Login Screen";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        setPointers();
    }
    private void setPointers()
    {
        this.context=this;
        txtUserName=(EditText)findViewById(R.id.txtUserName);
        txtPass=(EditText)findViewById(R.id.txtPass);
        logo=(TextView)findViewById(R.id.txtLogo);
        btnLogin=(Button)findViewById(R.id.btnLogin);
        btnReg=(Button)findViewById(R.id.btnReg);
        logo.setOnLongClickListener(new View.OnLongClickListener()
        {
            public boolean onLongClick(View v)
            {
                Toast.makeText(context,context.getResources().getString(R.string.msgLogo),Toast.LENGTH_SHORT).show();
                return true;
            }
        });
        final int ORG_COLOR=txtUserName.getCurrentTextColor();
        btnLogin.setOnClickListener(new View.OnClickListener() {
            UtlShared utl=new UtlShared(context);
            public void onClick(View v) {
                txtPass.setHintTextColor(ORG_COLOR);
                txtUserName.setHintTextColor(ORG_COLOR);
                if(txtUserName.getText().toString().length()<2||txtPass.getText().toString().length()<2)
                {
                    Toast.makeText(context,"you had not fill the userName or the password",Toast.LENGTH_SHORT).show();
                    txtUserName.setText("");
                    txtPass.setText("");
                    txtUserName.setHintTextColor(Color.RED);
                    txtPass.setHintTextColor(Color.RED);
                    return;
                }
                if(!(utl.checkPass((txtUserName.getText().toString()),(txtPass.getText().toString()))))
                {
                    Toast.makeText(context,"the user name or the password are not correct",Toast.LENGTH_SHORT).show();
                    return;
                }
                Intent myIntent=new Intent(context,TaskActivity.class);
                myIntent.putExtra("userName",utl.getNickName(txtUserName.getText().toString()));
                myIntent.putExtra("user",txtUserName.getText().toString());
                startActivity(myIntent);
                finish();
            }
        });
        btnReg.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent myIntent=new Intent(context,RegisterActivity.class);
                startActivity(myIntent);
            }
        });
    }
}
------------------------
//UtlShared
--------------------
package com.example.android.loginapp;

import android.content.Context;
import android.content.SharedPreferences;

/**
 * Created by esra on 1/14/2017.
 */

public class UtlShared {
    Context context;
    SharedPreferences userPref;
    SharedPreferences.Editor editor;
    public UtlShared(Context context)
    {
        this.context=context;
        userPref=context.getSharedPreferences("users",Context.MODE_PRIVATE);
        editor=userPref.edit();
    }
    //return true if user excists
    //else return false
    public boolean checkUser(String userName)
    {
        return userPref.contains(userName);
    }
    //returns true if password matches the user name
    //else returns false
    public boolean checkPass(String userName,String password)
    {
        return userPref.getString(userName," ").equals(password);
    }
    public void addUser(String userName,String pass,String nickName)
    {
        editor.putString(userName,(pass+","+nickName));
        editor.commit();
    }
    public String getNickName(String userName)
    {
        String[] values=userPref.getString(userName,"n/a").split(",");
        return values[1];
    }



}
------------------
//RegisterActivity
--------------------------
package com.example.android.loginapp;
import android.content.Context;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class RegisterActivity extends AppCompatActivity {
    Context context;
    EditText txtUser, txtPass, checkPass, nickNmae;
    Button btnReg, btnCancel;
    private final String TAG="REGISTER";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_register);
        setPointers();
    }

    private void setPointers() {
        this.context = this;
        txtUser = (EditText) findViewById(R.id.regTxtUser);
        txtPass = (EditText) findViewById(R.id.regTxtPass);
        checkPass = (EditText) findViewById(R.id.checkPass);
        nickNmae = (EditText) findViewById(R.id.nickName);
        btnReg = (Button) findViewById(R.id.regBtnReg);
        btnCancel = (Button) findViewById(R.id.regBtnCancel);
        btnReg.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                btnReg();
            }
        });
        btnCancel.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                finish();
            }
        });

    }

    private void btnReg() {
        UtlShared utl = new UtlShared(context);

        if (utl.checkUser(txtUser.getText().toString())) {
            Toast.makeText(context, getResources().getString(R.string.register_toast_exists), Toast.LENGTH_SHORT).show();
            return;
        }
        if (!txtPass.getText().toString().equals(checkPass.getText().toString())) {
            Toast.makeText(context, getResources().getString(R.string.register_pass_not_match), Toast.LENGTH_SHORT).show();
            return;
        }
        if(!checkValidation())return;
        utl.addUser(txtUser.getText().toString(), txtPass.getText().toString(), nickNmae.getText().toString());
        finish();
        Log.i(TAG,"user registered");
    }

    private boolean checkValidation() {
        if (txtUser.getText().toString().length() < 3 ) {
            Toast.makeText(context, getResources().getString(R.string.register_user_short_), Toast.LENGTH_SHORT).show();
            return false;
        }
        if(nickNmae.getText().toString().length() < 3)
        {
            Toast.makeText(context,getResources().getString(R.string.regiser_nickName_short),Toast.LENGTH_SHORT);
            return false;
        }
        if (txtPass.getText().toString().length() < 3) {
            Toast.makeText(context, getResources().getString(R.string.register_pass_short), Toast.LENGTH_SHORT).show();
            return false;
        }
        return true;
    }
}
-------------------------------
//strings
-----------------------------
<resources>
    <!--activity_login layout-->
    <string name="app_name">MyLoginApp</string>
    <string name="logo_login">Log In</string>
    <string name="username_hint">Enter your name...</string>
    <string name="pass_hint">Enter your password...</string>
    <string name="btn_login">Log in</string>
    <string name="btn_reg">Register</string>
    <string name="msgLogo">Log in</string>
    <string name="user_short"></string>
    <!--activity_register layout-->
    <string name="reg_logo">Register</string>
    <string name="regHintUserName">Enter your name...</string>
    <string name="regHintPass">Enter a pssword...</string>
    <string name="checkPass">Enter your password again...</string>
    <string name="nickName">Enter your nick name...</string>
    <string name="btn_cancel">Cancel</string>
    <!--activity_task layout-->
    <string name="task_logo"></string>
    <string name="add_task">Add a Task</string>
    <!--alertv dialog new task-->
    <string name="adLogo">New Task</string>
    <string name="adTxtHint">write a new task here...</string>
    <string name="adPosBtn">OK</string>
    <string name="adNegBtn">Cancel</string>
    <!--task layout-->
    <string name="txtTask">single item..</string>
    <!--registerActivity.java-->
    <string name="register_toast_exists">User excists</string>
    <string name="register_pass_not_match">the passwords don't matches</string>
    <string name="register_user_short_">user name is too short</string>
    <string name="register_pass_short">password is too short</string>
    <string name="regiser_nickName_short">nick name is too short</string>

</resources>