<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>

public void saveToSD(String filename,String content) throws Exception{
    //getExternalStorageDirectory()可以取得sd卡得路径
    File f=new File(Environment.getExternalStorageDirectory(),filename);
    FileOutputStream out2=new FileOutputStream(f);
    out2.write(content.getBytes());
    out2.close();
}

package org.lxh.file;
import org.lxh.service.FileService;
import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
public class FileActivity extends Activity {
  private FileService service;
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    service=new FileService(this);
    Button button=(Button)findViewById(R.id.button);
    button.setOnClickListener(new View.OnClickListener() {
      public void onClick(View v) {
        EditText filename=(EditText)findViewById(R.id.filename);
        EditText content=(EditText)findViewById(R.id.content);
        try {
          if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){
            service.saveToSD(filename.getText().toString(), content.getText().toString());
            Toast.makeText(FileActivity.this, R.string.success, 1).show();
          }else{
            Toast.makeText(FileActivity.this, R.string.sd, 1).show();
          }
          //service.saveFile(filename.getText().toString(), content.getText().toString());
        } catch (Exception e) {
          Toast.makeText(FileActivity.this, R.string.failure, 1).show();
          Log.e("FileActivity", e.getMessage());
        }
      }
    });
  }
}

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <string name="hello">Hello World, FileActivity!</string>
  <string name="app_name">文件的读取</string>
  <string name="filename">输入文件名称</string>
  <string name="content">输入文件内容</string>
  <string name="button">保存</string>
  <string name="success">文件保存成功</string>
  <string name="failure">文件保存失败</string>
  <string name="sd">sd卡不存在或不可用</string>
</resources>