import android.support.v.app.AppCompatActivity;
 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 {
   private EditText etName;
   private EditText etContent;
   private Button btnWrite;
   private Button btnRead;
   private String fileName = "";
   private String fileContent = "";
   @Override
   protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.activity_main);
     etName = (EditText)findViewById(R.id.etName);
     etContent = (EditText)findViewById(R.id.etContent);
     btnWrite = (Button)findViewById(R.id.btnWrite);
     btnRead = (Button)findViewById(R.id.btnRead);
     btnWrite.setOnClickListener(new View.OnClickListener() {
       @Override
       public void onClick(View view) {
         fileName = etName.getText().toString();
         fileContent = etContent.getText().toString();
         try {
           FileOutputStream foStream = openFileOutput(fileName, MODE_PRIVATE);
           byte[] buffer = fileContent.getBytes();
           foStream.write(buffer);
           Toast.makeText(MainActivity.this, "写入成功",Toast.LENGTH_SHORT).show();
           foStream.flush();
           foStream.close();
         }catch(Exception e){
           e.printStackTrace();
         }
       }
     });
     btnRead.setOnClickListener(new View.OnClickListener() {
       @Override
       public void onClick(View view) {
         fileName = etName.getText().toString();
         try{
           FileInputStream fiStream = openFileInput(fileName);
           int len = fiStream.available();
           byte[] buffer = new byte[len];
           fiStream.read(buffer);
           etContent.setText(new String(buffer));
           Toast.makeText(MainActivity.this, "读取成功",Toast.LENGTH_SHORT).show();
           fiStream.close();
         }catch(Exception e){
           e.printStackTrace();
         }
       }
     });
   }
 }
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
   android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
   android:paddingRight="@dimen/activity_horizontal_margin"
   android:paddingTop="@dimen/activity_vertical_margin"
   android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">
   <EditText
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:id="@+id/etName"
     android:layout_alignParentTop="true"
     android:layout_alignParentLeft="true"
     android:layout_alignParentStart="true"
     android:layout_alignParentRight="true"
     android:layout_alignParentEnd="true"
     android:text="文件名" />
   <EditText
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:id="@+id/etContent"
     android:layout_below="@+id/etName"
     android:layout_alignParentLeft="true"
     android:layout_alignParentStart="true"
     android:layout_alignParentRight="true"
     android:layout_alignParentEnd="true"
     android:text="文件内容" />
   <Button
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:text="保存"
     android:id="@+id/btnWrite"
     android:layout_alignTop="@+id/btnRead"
     android:layout_toLeftOf="@+id/btnRead"
     android:layout_toStartOf="@+id/btnRead" />
   <Button
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:text="读取"
     android:id="@+id/btnRead"
     android:layout_below="@+id/etContent"
     android:layout_alignParentRight="true"
     android:layout_alignParentEnd="true" />
 </RelativeLayout>