一、从文件中读
1、首先利用openFileInput()方法，创建一个FileInputStreamFileInputStream fileInputStream=openFileInput("user.txt")2、利用fileInputStream.available()方法检测已经打开的文件的二进制数据流最大值，用于创建数组byte[] bytes=new byte[fileInputStream.available()]3、把读到的信息存储进数组fileInputStream.read(bytes)4、关闭文件fileInputStream.close()
二、向文件中写
1、利用openFileOutput方法，创建一个FileOutputStream FileOutputStream fileOutputStream=openFileOutput("user.txt",MODE_APPEND)参数MODE_APPEND表示在文件内原有内容的后面继续写参数MODE_PRIVATE表示删除文件内原有内容重新写2、剩下的工作就是把准备好的二进制文件利用fileOutputStream.write()方法写进文件，写完之后把文件关闭。
public class MainActivity extends AppCompatActivity implements View.OnClickListener {Button button1,button2;EditText editText;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);button1=findViewById(R.id.button1);button2=findViewById(R.id.button2);editText=findViewById(R.id.editTextTextPersonName);button1.setOnClickListener(this);button2.setOnClickListener(this);}@Overridepublic void onClick(View v) {switch (v.getId()){//button1 向文件中写case R.id.button1:try {FileOutputStream fileOutputStream=openFileOutput("user.txt",MODE_APPEND);String content=editText.getText().toString().trim();fileOutputStream.write(content.getBytes());fileOutputStream.close();}catch (Exception e){e.printStackTrace();}break;//button2 从文件中读case R.id.button2:try {FileInputStream fileInputStream=openFileInput("user.txt");byte[] bytes=new byte[fileInputStream.available()];fileInputStream.read(bytes);String content=new String(bytes);Log.i("fis",content);fileInputStream.close();}catch (Exception e){e.printStackTrace();}break;}}}
三、SharedPreferences适合小规模存取（一套用户名和密码）
package com.example.sharedpreferences;import androidx.appcompat.app.AppCompatActivity;import android.content.SharedPreferences;import android.os.Bundle;import android.view.View;import android.widget.Button;import android.widget.EditText;import java.util.Locale;public class MainActivity extends AppCompatActivity implements View.OnClickListener {Button button1,button2;EditText editText;SharedPreferences sharedPreferences;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);button1=findViewById(R.id.button1);button2=findViewById(R.id.button2);editText=findViewById(R.id.editText1);sharedPreferences=getSharedPreferences("A_FileName",MODE_PRIVATE);button1.setOnClickListener(this);button2.setOnClickListener(this);}@Overridepublic void onClick(View v) {switch (v.getId()){case R.id.button1:SharedPreferences.Editor editor=sharedPreferences.edit();editor.putString("name",editText.getText().toString().trim());mit();break;case R.id.button2:String username=sharedPreferences.getString("name","N/A");editText.setText(username);break;}}}
