运行截图

activity_main.xml（约束布局）

 <?xml version="1.0" encoding="utf-8"?> <androidx.constraintlayout.widget.ConstraintLayout xmlns:android="https://schemas.android.com/apk/res/android"     xmlns:app="https://schemas.android.com/apk/res-auto"     xmlns:tools="https://schemas.android.com/tools"     android:layout_width="match_parent"     android:layout_height="match_parent"     tools:context=".MainActivity">      <Button         android:id="@+id/button"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_marginTop="76dp"         android:onClick="btnRead"         android:text="btnRead"         app:layout_constraintEnd_toEndOf="parent"         app:layout_constraintHorizontal_bias="0.498"         app:layout_constraintStart_toStartOf="parent"         app:layout_constraintTop_toTopOf="parent" />      <Button         android:id="@+id/button2"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_marginTop="68dp"         android:onClick="btnWrite"         android:text="btnWrite"         app:layout_constraintEnd_toEndOf="parent"         app:layout_constraintHorizontal_bias="0.498"         app:layout_constraintStart_toStartOf="parent"         app:layout_constraintTop_toBottomOf="@+id/button" /> </androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.java

 package cn.lwx.readwritetemplate;  import androidx.appcompat.app.AppCompatActivity;  import android.os.Bundle; import android.view.View;  import java.io.BufferedReader; import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; import java.io.InputStreamReader;  public class MainActivity extends AppCompatActivity {      @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);     }      public void btnRead(View view) {         FileInputStream fileInputStream = null;         try {             // 1.打开文件             fileInputStream = openFileInput("aaa.txt");             // 2.读操作 字节流（byte 10001） ----->  字符流（编码）ASCLL 流操作             // Stream 字节流，reader write字符流             BufferedReader reader = new BufferedReader(new InputStreamReader(fileInputStream));             String line = null;             StringBuilder builder = new StringBuilder();             while ((line = reader.readLine()) != null) {                 builder.append(line);             }             System.out.println(builder.toString());             // String str = new String();         } catch (Exception e) {             e.printStackTrace();         } finally {             // 3.关闭文件             if (fileInputStream != null) {                 try {                     fileInputStream.close();                 } catch (IOException e) {                     e.printStackTrace();                 }             }         }     }      public void btnWrite(View view) {         FileOutputStream fileOutputStream = null;         try {             // 1.打开文件             fileOutputStream = openFileOutput("aaa.txt", MODE_PRIVATE);             // 2.写操作             fileOutputStream.write("卫星同学，好帅啊~ 我也要嫁给他~~~".getBytes());         } catch (Exception e) {             e.printStackTrace();         } finally {             if (fileOutputStream != null) {                 try {                     fileOutputStream.close();                 } catch (IOException e) {                     e.printStackTrace();                 }             }         }     }  }

谢谢观看，再走~~~   求求了~~~