activity_main.xmlMainActivity.java
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.codinginflow.internalstorageexample.MainActivity">

    <EditText
        android:id="@+id/edit_text"
        android:layout_width="match_parent"
        android:layout_height="200dp" />

    <Button
        android:id="@+id/button_save"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/edit_text"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="16dp"
        android:text="save"
        android:onClick="save"/>

    <Button
        android:id="@+id/button_load"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignStart="@+id/button_save"
        android:layout_below="@+id/button_save"
        android:layout_marginTop="17dp"
        android:text="load"
        android:onClick="load"/>
</RelativeLayout>
package com.codinginflow.internalstorageexample;

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

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;

public class MainActivity extends AppCompatActivity {
    private static final String FILE_NAME = "example.txt";

    EditText mEditText;

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

        mEditText = findViewById(R.id.edit_text);
    }

    public void save(View v) {
        String text = mEditText.getText().toString();
        FileOutputStream fos = null;

        try {
            fos = openFileOutput(FILE_NAME, MODE_PRIVATE);
            fos.write(text.getBytes());

            mEditText.getText().clear();
            Toast.makeText(this, "Saved to " + getFilesDir() + "/" + FILE_NAME,
                    Toast.LENGTH_LONG).show();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fos != null) {
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    public void load(View v) {
        FileInputStream fis = null;

        try {
            fis = openFileInput(FILE_NAME);
            InputStreamReader isr = new InputStreamReader(fis);
            BufferedReader br = new BufferedReader(isr);
            StringBuilder sb = new StringBuilder();
            String text;

            while ((text = br.readLine()) != null) {
                sb.append(text).append("\n");
            }

            mEditText.setText(sb.toString());

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}