package example.com.myapplication;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.PrintStream;
public class MainActivity extends Activity
{
  final String FILE_NAME = "test.txt";
  @Override
  public void onCreate(Bundle savedInstanceState)
  {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    System.out.println(new StringBuilder("a").append("b").append("c")
        .toString());
    // 获取两个按钮
    Button read = (Button) findViewById(R.id.read);
    Button write = (Button) findViewById(R.id.write);
    // 获取两个文本框
    final EditText edit1 = (EditText) findViewById(R.id.edit1);
    final EditText edit2 = (EditText) findViewById(R.id.edit2);
    // 为write按钮绑定事件监听器
    write.setOnClickListener(new View.OnClickListener()
    {
      @Override
      public void onClick(View source)
      {
        // 将edit1中的内容写入文件中
        write(edit1.getText().toString());
        edit1.setText("");
      }
    });
    read.setOnClickListener(new View.OnClickListener()
    {
      @Override
      public void onClick(View v)
      {
        // 读取指定文件中的内容，并显示出来
        edit2.setText(read());
      }
    });
  }
  private String read()
  {
    try
    {
      // 打开文件输入流
      FileInputStream fis = openFileInput(FILE_NAME);
      byte[] buff = new byte[1024];
      int hasRead = 0;
      StringBuilder sb = new StringBuilder("");
      while ((hasRead = fis.read(buff)) > 0)
      {
        sb.append(new String(buff, 0, hasRead));
      }
      return sb.toString();
    }
    catch (Exception e)
    {
      e.printStackTrace();
    }
    return null;
  }
  private void write(String content)
  {
    try
    {
      // 以追加模式打开文件输出流
      FileOutputStream fos = openFileOutput(FILE_NAME, MODE_APPEND);
      // 将FileOutputStream包装成PrintStream
      PrintStream ps = new PrintStream(fos);
      // 输出文件内容
      ps.println(content);
      ps.close();
    }
    catch (Exception e)
    {
      e.printStackTrace();
    }
  }
}

<!--?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">
  <EditText
    android:id="@+id/edit1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:lines="4"/>
    <Button
      android:id="@+id/write"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="write"/>
  <EditText
    android:id="@+id/edit2"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:editable="false"
    android:lines="4"/>
  <Button
    android:id="@+id/read"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="read"/>
  </LinearLayout>