<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="15dp">

    <EditText
        android:id="@+id/et_name"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="输入内容"/>

    <Button
        android:id="@+id/btn_save"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="保存"
        android:textSize="20sp"/>

    <Button
        android:id="@+id/btn_show"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:text="显示"
        android:textSize="20sp"/>

    <TextView
        android:id="@+id/tv_content"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"/>

</LinearLayout>

package com.example.helloworld.datastorage;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import com.example.helloworld.R;

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

public class FileActivity extends AppCompatActivity {

    private EditText mEtName;
    private Button mBtnSave,mBtnShow;
    private TextView mTvContent;
    private final String mFileName = "test.txt";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_file);
        mEtName = findViewById(R.id.et_name);
        mBtnSave = findViewById(R.id.btn_save);
        mBtnShow = findViewById(R.id.btn_show);
        mTvContent = findViewById(R.id.tv_content);

        mBtnSave.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                save(mEtName.getText().toString());
            }
        });
        mBtnShow.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mTvContent.setText(read());
            }
        });
    }

    //存储数据
    private void save(String content){
        FileOutputStream fileOutputStream = null;
        try {
            fileOutputStream = openFileOutput(mFileName,MODE_PRIVATE);
            fileOutputStream.write(content.getBytes());
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if(fileOutputStream != null){
                try{
                    fileOutputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    //读取数据
    private String read(){
        FileInputStream fileInputStream = null;
        try {
            fileInputStream = openFileInput(mFileName);
            byte[] buff = new byte[1024];
            //用StringBuilder来实现字符串拼接
            StringBuilder sb = new StringBuilder();
            int len = 0;
            while((len = fileInputStream.read(buff)) > 0){
                sb.append(new String(buff,0,len));
            }
            return sb.toString();
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            if(fileInputStream != null){
                try{
                    fileInputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return null;
    }
}