Android deletes the specified file code

package com.tware.pdfdrop;import java.io.File;import android.app.Activity;import android.graphics.Color;import android.os.Bundle;import android.view.View;import android.view.View.OnClickListener;import android.widget.TextView;public class PdfDropActivity extends Activity{    /** Called when the activity is first created. */TextView textview;@Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);         textview = (TextView)findViewById(R.id.v_file_list);        View clsbtn = findViewById(R.id.btn_clean);        clsbtn.setOnClickListener(new OnClickListener(){        public void onClick(View v)        {       File f = new File("/sdcard");              File[] fl = f.listFiles();       for (int i=0; i<fl.length; i++)       {       if(fl[i].toString().endsWith(".pdf") || fl[i].toString().endsWith(".PDF"))       {       if(fl[i].delete()){textview.setTextSize(20);textview.setTextColor(Color.GREEN);textview.append("\n" + fl[i].toString()+ "--- Success");}else{textview.setTextSize(20);textview.setTextColor(Color.RED);textview.append("\n" + fl[i].toString()+ "--- Failed");}       }       }        }        });    }}