<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
     >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true"
        android:background="#008080"
        android:padding="5dp"
        android:text="Android File example"
        android:textColor="#fff" />

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true"
        android:layout_below="@+id/textView1"
        android:layout_marginTop="22dp"
        android:ems="10"
        android:layout_margin="5dp">
        <requestFocus />
    </EditText>

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true"
        android:layout_below="@+id/editText1"
        android:text="Write Text into File"
        android:onClick="WriteBtn"
        android:layout_margin="5dp"/>
    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true"
        android:layout_below="@+id/button1"
        android:text="Read Text From file"
        android:onClick="ReadBtn"
        android:layout_margin="5dp" />
</RelativeLayout>

package com.example.sudiproy.fileexample;

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

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;

public class MainActivity extends AppCompatActivity {

    EditText textmsg;
    static final int READ_BLOCK_SIZE = 100;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textmsg= (EditText) findViewById(R.id.editText1);

    }

    public void WriteBtn(){
        try{
            FileOutputStream fileOut=openFileOutput("myText.txt",MODE_PRIVATE);
            OutputStreamWriter outputWriter=new OutputStreamWriter(fileOut);
            outputWriter.write(textmsg.getText().toString());
            outputWriter.close();

            Toast.makeText(getBaseContext(), "File saved successfully!",
                    Toast.LENGTH_SHORT).show();
        }
        catch (Exception e){
            e.printStackTrace();
        }
    }

    public void ReadBtn(){
        try{
            FileInputStream fileIn=openFileInput("myText.txt");
            InputStreamReader InputRead=new InputStreamReader(fileIn);
            char[] inputBuffer=new char[READ_BLOCK_SIZE];
            String s="";
            int charRead = 0;

            while ((charRead=InputRead.read(inputBuffer))>0) {
                // char to string conversion
                String readstring=String.copyValueOf(inputBuffer,0,charRead);
                s +=readstring;
            }
            InputRead.close();
            Toast.makeText(getBaseContext(), s,Toast.LENGTH_SHORT).show();
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }
}

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />