List files with specified extension using listFiles() with FilenameFilter.

package com.example.androidlistfile;

import java.io.File;
import java.io.FilenameFilter;

import android.os.Bundle;
import android.os.Environment;
import android.app.Activity;
import android.widget.TextView;

public class MainActivity extends Activity {
 
 File root;
 File[] fileArray;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TextView textView = new TextView(this);
        setContentView(textView);
        
        root = new File(Environment.getExternalStorageDirectory().getAbsolutePath());
        fileArray = root.listFiles(new FilenameFilter(){

   @Override
   public boolean accept(File dir, String filename) {
    return filename.toLowerCase().endsWith(".jpg");
   }});
        
        String f = root.getAbsolutePath() + "\n\n";
        for(int i=0; i < fileArray.length; i++){
         f += fileArray[i].getName() + "\n";
        }
        textView.setText(f);
    }

}