myTasks golan

activity_main
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<LinearLayout android:layout_height="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:orientation="vertical"
    android:gravity="center"
    android:background="#DCDCDC">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:gravity="center"
        android:orientation="vertical">

        <ImageView
            android:id="@+id/imageView"
            android:layout_width="183dp"
            android:layout_height="match_parent"
            android:src="@drawable/task" />

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:layout_weight="1"
        android:gravity="center"
        android:background="#A9A9A9">

        <EditText
            android:id="@+id/userlogin"
            android:layout_width="300dp"
            android:layout_height="wrap_content"
            android:layout_marginTop="20dp"
            android:hint="Enter your user name.."/>

        <EditText
            android:id="@+id/userpass"
            android:layout_width="300dp"
            android:layout_height="wrap_content"
            android:layout_marginTop="20dp"
            android:hint="Enter your password.."
            android:inputType="textPassword"/>

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="horizontal"
        android:layout_weight="1"
        android:gravity="center"
        android:background="#A9A9A9">

        <Button
            android:id="@+id/btnLogin"
            android:layout_width="164dp"
            android:layout_height="wrap_content"
            android:text="Login"
            android:onClick="onClick"
            android:textColor="#ffffff"
            android:textSize="22sp"
            android:background="#AFEEEE"
            android:layout_margin="10dp"/>

        <Button
            android:id="@+id/btnRegister"
            android:layout_width="164dp"
            android:layout_height="wrap_content"
            android:text="Register"
            android:onClick="onClick"
            android:textColor="#ffffff"
            android:textSize="22sp"
            android:background="#AFEEEE"
            android:layout_margin="10dp"/>

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:layout_weight="1"
        android:gravity="center">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textSize="18dp"
            android:gravity="center"
            android:text="Note this app has been posted by G.V"/>

    </LinearLayout>

</LinearLayout>

main_activity.java
**************
package com.example.mytasks;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    EditText lUser,lPass;

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

    private void setPointer() {
        lUser=(EditText)findViewById(R.id.userlogin);
        lPass=(EditText)findViewById(R.id.userpass);
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()){
            case R.id.btnLogin:
                //                         Key                       Value
                if (!SharedPref.checkPass(lUser.getText().toString(),lPass.getText().toString(),this))
                {
                    Toast.makeText(this, "Invalid user name or pass", Toast.LENGTH_LONG).show();
                    return;
                }
                Intent intent = new Intent(this, Tasks.class);
                intent.putExtra("userName",lUser.getText().toString());
                startActivity(intent);
                break;

            case R.id.btnRegister:
                startActivity(new Intent(this,Register.class));
                break;

            default:
                Toast.makeText(this, "Error in button", Toast.LENGTH_SHORT).show();
        }

    }
    private void checkLogin() {
    }
}
sharedPref.java
*******************
package com.example.mytasks;

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

/**
 * Created by גולן on 27/09/2017.
 */

public class SharedPref {

    //for login screen
    public static boolean checkPass(String userName,String userPass, Context context)
    {
        //declaration of shared preferences
        SharedPreferences prefs=context.getSharedPreferences("myPrefs",Context.MODE_PRIVATE);
        //get user pass
        String spPass=prefs.getString(userName,"na");
        //check if password matches
        if (userPass.equals(spPass))
        {
            return true;
        }
        return false;
    }

    public static boolean checkUserExists(String userName, Context context)
    {
        //declaration of shared preferences
        SharedPreferences prefs=context.getSharedPreferences("myPrefs",Context.MODE_PRIVATE);
        //get user pass
        String spPass=prefs.getString(userName,"na");
        //check if password matches
        return !spPass.equals("na");
    }

    public static void registerUser(String userName, String userPass, Context context)
    {
        //declaration of shared preferences
        SharedPreferences prefs=context.getSharedPreferences("myPrefs",Context.MODE_PRIVATE);

        //declaration of shared preferences Editor
        SharedPreferences.Editor editor = prefs.edit();

        //put data inside, we use hash map , so we need KEY,VALUE (K,V)
        editor.putString(userName,userPass);

        //sending data to my shared prefernces file, we have to commit the changes
        editor.commit();
    }
}

activity_register.xml
************************
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:orientation="vertical"
        android:gravity="center">
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="REGISTER"
            android:layout_gravity="center"
            android:textSize="32sp"/>
    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:orientation="vertical">
        <EditText
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="Enter user name..."
            android:id="@+id/rName"/>
        <EditText
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="Enter password..."
            android:id="@+id/rpass"
            android:inputType="textPassword"/>
        <EditText
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="Retype password..."
            android:id="@+id/rCheck"
            android:inputType="textPassword"/>

    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:orientation="horizontal">
        <Button
            android:id="@+id/rCancel"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_margin="20dp"
            android:layout_weight="1"
            android:background="#009fff"
            android:onClick="onClick"
            android:text="Cancel"
            android:textColor="#ffffff"
            android:textSize="22sp"
            />

        <Button
            android:id="@+id/rRegister"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_margin="20dp"
            android:layout_weight="1"
            android:background="#009fff"
            android:onClick="onClick"
            android:text="Register"
            android:textColor="#ffffff"
            android:textSize="22sp" />
    </LinearLayout>

</LinearLayout>

register.java
******************
package com.example.mytasks;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

public class Register extends AppCompatActivity implements View.OnClickListener{

    EditText rUser, rPass, rCheck;

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

    private void setPointer() {
        rUser = (EditText) findViewById(R.id.rName);
        rPass = (EditText) findViewById(R.id.rpass);
        rCheck = (EditText) findViewById(R.id.rCheck);
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.rCancel:
                finish();
                break;

            case R.id.rRegister:
                registerUser();
                break;

            default:
                Toast.makeText(this, "Error in buttons", Toast.LENGTH_SHORT).show();
        }
    }

    private void registerUser() {
        //check if all fields filled
        if (rUser.getText().toString().isEmpty() ||
                rPass.getText().toString().isEmpty() ||
                rCheck.getText().toString().isEmpty())
        {
            Toast.makeText(this, "Please fill all fields", Toast.LENGTH_LONG).show();
            return;
        }

        //check if minimum password length > 2
        if (rPass.getText().toString().length()<3)
        {
            Toast.makeText(this, "Password is too short (3 minimum)", Toast.LENGTH_SHORT).show();
            return;
        }

        //check if password match
        if (!rPass.getText().toString().equals(rCheck.getText().toString()))
        {
            Toast.makeText(this, "Password not match", Toast.LENGTH_LONG).show();
            return;
        }

        //check if user exists
        if (SharedPref.checkUserExists(rUser.getText().toString(),this))
        {
            Toast.makeText(this, "User already exists", Toast.LENGTH_LONG).show();
            return;
        }

        //register user and finitu la comedia
        SharedPref.registerUser(rUser.getText().toString(),rPass.getText().toString(),this);
        finish();
    }
    }

activity_tasks.xml
*****************
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="hello"
        android:gravity="center"
        android:layout_marginTop="10dp"
        android:textSize="30sp"
        android:id="@+id/userName"/>

</LinearLayout>

tasks.java
***************
package com.example.mytasks;

import android.content.Context;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
import android.widget.Toast;

public class Tasks extends AppCompatActivity {

    TextView userName;
    Context context;

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

        //get the data from previous activity.
        Bundle extras = getIntent().getExtras();
        if (extras==null)
        {
            Toast.makeText(context, "Error in getting info", Toast.LENGTH_SHORT).show();
        }
        else
        {
            String uName=extras.getString("userName");
            userName.setText("Hello "+uName);
        }

    }

    private void setPointer() {
        this.context=this;
        userName=(TextView)findViewById(R.id.userlogin);
    }
}