直接上代码：

package com.example.test;import java.io.File;import java.io.FileNotFoundException;import java.io.FileOutputStream;import java.io.IOException;import android.os.Bundle;import android.os.Environment;import android.app.Activity;import android.widget.Toast;public class MainActivity extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);                //要保存的文件名称和文件内容        String fileName = "test.txt";        String content = "This is a test.";                //推断sdcard是否存在        String state = Environment.getExternalStorageState();        if(state.equals(Environment.MEDIA_MOUNTED)) {        	//获取SDCard文件夹        	File sdcardPath = Environment.getExternalStorageDirectory();        	        	File file = new File(sdcardPath, fileName);        	FileOutputStream fos;			try {				fos = new FileOutputStream(file);				fos.write(content.getBytes());				fos.close();			} catch (FileNotFoundException e) {				e.printStackTrace();			} catch (IOException e) {				e.printStackTrace();			}			Toast.makeText(this, "保存成功", Toast.LENGTH_SHORT).show();        } else {        	Toast.makeText(this, "sdcard不存在获取不可写入", Toast.LENGTH_SHORT).show();        }            }}

加入权限：