1.先看activity_main.xml文件：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:android1="http://schemas.android.com/apk/res/android"

    android:layout_width="fill_parent"

    android:layout_height="fill_parent"

    android:orientation="vertical" >

    <LinearLayout

	    android:orientation="horizontal"

	    android:layout_width="fill_parent"

	    android:layout_height="wrap_content" >

	    <TextView

		    android:textSize="18dip"

		    android:layout_width="wrap_content"

		    android:layout_height="wrap_content"

		    android:text="文件名: /sdcard/" />

	    <EditText

		    android:hint="xname.txt"

		    android:id="@+id/ET_Folder"

		    android:layout_width="180dip"

		    android:layout_height="wrap_content" />

	</LinearLayout>

    <LinearLayout

	    android:orientation="horizontal"

	    android:layout_width="fill_parent"

	    android:layout_height="wrap_content" >

	   <Button

		     android:text="打开"

		     android:id="@+id/But_Open"

		     android:layout_width="wrap_content"

		     android:layout_height="wrap_content" />

	   <Button

		     android:text="清除"

		     android:id="@+id/But_Clear"

		     android:layout_width="wrap_content"

		     android:layout_height="wrap_content" />

	</LinearLayout>

    <ScrollView

		android:id="@+id/ScrollView01"

		android:layout_width="fill_parent"

		android:layout_height="wrap_content">

		<EditText

			android:editable="false"

			android:id="@+id/ET_FileContent"

			android:layout_width="fill_parent"

			android:layout_height="wrap_content" />

   </ScrollView>

</LinearLayout>

2.MainActivity.java文件

/*读取手机SD卡根目录下某个txt文件的文件内容

 * */

import java.io.BufferedReader;

import java.io.File;

import java.io.FileInputStream;

import java.io.IOException;

import java.io.InputStream;

import java.io.InputStreamReader;

import android.os.Bundle;

import android.os.Environment;

import android.app.Activity;

import android.util.Log;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

import android.widget.EditText;

import android.widget.Toast;

public class MainActivity extends Activity {

	private EditText et_folder;			//输入的文件夹名

	private Button bt_open;				//打开按钮

	private Button bt_clear;			//清除按钮

	private EditText et_filecontent;	//用于显示txt文件内容

	protected void onCreate(Bundle savedInstanceState) {

		super.onCreate(savedInstanceState);

		setContentView(R.layout.activity_main);

		et_folder = (EditText) findViewById(R.id.ET_Folder);

        et_filecontent = (EditText) findViewById(R.id.ET_FileContent);

		bt_open = (Button) findViewById(R.id.But_Open);

		bt_open.setOnClickListener(new OnClickListener(){//打开按钮监听

			public void onClick(View arg0) {

				//若输入的文件夹名为空

				if(et_folder.getText().toString().trim().equals("")){

					Toast.makeText(getApplicationContext(), "输入为空",

							Toast.LENGTH_SHORT).show();

				}else{

					//获得SD卡根目录路径   "/sdcard"

					File sdDir = Environment.getExternalStorageDirectory();

					//根目录下某个txt文件名

					File path = new File(sdDir+File.separator

					 +et_folder.getText().toString().trim());

					// 判断SD卡是否存在，并且是否具有读写权限

					if (Environment.getExternalStorageState()

						.equals(Environment.MEDIA_MOUNTED)) {

			            et_filecontent.setText("");

			            et_filecontent.setText(getFileContent(path));

					}

				}

			}

		});

		bt_clear = (Button) findViewById(R.id.But_Clear);

		bt_clear.setOnClickListener(new OnClickListener(){//清除按钮监听

			public void onClick(View arg0) {

				et_folder.setText("");

	            et_filecontent.setText("");

			}

		});

	}

	//读取指定目录下的所有TXT文件的文件内容

	protected String getFileContent(File file) {

		String content  = "";

		if (file.isDirectory() ) {	//检查此路径名的文件是否是一个目录(文件夹)

            Log.i("zeng", "The File doesn't not exist "

            	+file.getName().toString()+file.getPath().toString());

        } else {

            if (file.getName().endsWith(".txt")) {//文件格式为txt文件

            	try {

                    InputStream instream = new FileInputStream(file);

                    if (instream != null) {

                        InputStreamReader inputreader

                        	=new InputStreamReader(instream, "GBK");

                        BufferedReader buffreader = new BufferedReader(inputreader);

                        String line="";

                        //分行读取

                       while (( line = buffreader.readLine()) != null) {

                            content += line + "\n";

                        }

                        instream.close();		//关闭输入流

                    }

                }

                catch (java.io.FileNotFoundException e) {

                    Log.d("TestFile", "The File doesn't not exist.");

                }

                catch (IOException e)  {

                     Log.d("TestFile", e.getMessage());

                }

            }

		}

		return content ;

	}

}