package com.example.file;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
public class MainActivity extends AppCompatActivity {
  EditText edt;
  Button btn;
  TextView tv;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    edt = (EditText) findViewById(R.id.editText);
    btn = (Button) findViewById(R.id.button);
    tv = (TextView) findViewById(R.id.textView);
    btn.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View view) {
        WriteFiles(edt.getText().toString());
        tv.setText(readFiles());
      }
    });
  }
  //保存文件內容
  public void WriteFiles(String content){
    try {
      FileOutputStream fos = openFileOutput("a.txt",MODE_PRIVATE);
      fos.write(content.getBytes());
      fos.close();
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
  //讀取文件
  public String readFiles(){
    String content = null;
    try {
      FileInputStream fis = openFileInput("a.txt");
      ByteArrayOutputStream baos = new ByteArrayOutputStream();
      byte[]buffer = new byte[1024];
      int len = 0;
      while ((len = fis.read(buffer))!=-1)
      {
        baos.write(buffer,0,len);
      }
      content = baos.toString();
      fis.close();;
      baos.close();
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
    return content;
  }
}

<?xml version="1.0" encoding="utf-8"?>
<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:paddingBottom="@dimen/activity_vertical_margin"
  android:paddingLeft="@dimen/activity_horizontal_margin"
  android:paddingRight="@dimen/activity_horizontal_margin"
  android:paddingTop="@dimen/activity_vertical_margin"
  tools:context="com.example.file.MainActivity">
  <EditText
    android:layout_width="wrap_content"
    android:layout_height="200dp"
    android:id="@+id/editText"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true" />
  <Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="New Button"
    android:id="@+id/button"
    android:layout_below="@+id/editText"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="90dp" />
  <TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="New Text"
    android:id="@+id/textView"
    android:layout_below="@+id/button"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true"
    android:layout_alignParentBottom="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />
</RelativeLayout>