AndroidFileOutputStreamおよびFileInputStream内部ストレージファイルの読み取り

プロジェクトディレクトリ：

MainActivity

package com.example.demo82;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import java.io.FileInputStream;
import java.io.FileOutputStream;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{
    EditText et;
    Button btn1,btn2;

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

        et=findViewById(R.id.editText);
        btn1=findViewById(R.id.button);
        btn2=findViewById(R.id.button2);
        btn1.setOnClickListener(this);
        btn2.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        int se=v.getId();
        switch(se){
            case R.id.button:
                String str=et.getText().toString();
                try{
                    FileOutputStream out=openFileOutput("text", Context.MODE_APPEND);
                    out.write(str.getBytes());
                    out.close();
                    Toast.makeText(this, "信息保存成功！", Toast.LENGTH_SHORT).show();
                }
                catch (Exception e){
                    e.printStackTrace();
                }
                break;
            case R.id.button2:
                String mess="";
                try{
                    FileInputStream in=openFileInput("text");
                    byte[] buffer=new byte[in.available()];
                    in.read(buffer);
                    mess=new String(buffer);
                    in.close();
                }
                catch(Exception e){
                    e.printStackTrace();
                }
                Toast.makeText(this,mess, Toast.LENGTH_SHORT).show();
                break;
        }
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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=".MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <EditText
            android:id="@+id/editText"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:ems="10"
            android:inputType="textPersonName"
            android:hint="请输入保存文本"
            android:text="" />

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="horizontal">

            <Button
                android:id="@+id/button"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="保存" />

            <Button
                android:id="@+id/button2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="读取" />
        </LinearLayout>
    </LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>