import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.*;
import android.widget.AdapterView.OnItemClickListener;
import java.io.File;

public class MainLayoutActivity extends AppCompatActivity
    {
     ListView listView;
     File currentTargetDir;
     int level=0;

     class Listener implements OnItemClickListener
        {
         @Override
         public void onItemClick(AdapterView<?> parent,View view,int position,long id)
            {
             File selected= new File(currentTargetDir.getAbsolutePath() +"/"+ ( (TextView)view).getText().toString() );
             if(selected.exists() && selected.isDirectory() )
                {
                 currentTargetDir=selected;
                 initAdapter();
                 level++;
                 }
             }
         }

     void initAdapter()
        {
         String fileList[]= currentTargetDir.list();
         ArrayAdapter<String> adapter= new ArrayAdapter<>(this,android.R.layout.simple_list_item_1,fileList);
         listView.setAdapter(adapter);
         }

     @Override
     protected void onCreate(Bundle savedInstanceState)
        {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.main_layout);

         listView= (ListView)findViewById(R.id.list);
         listView.setOnItemClickListener(new Listener());
         if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))
            {
             currentTargetDir= new File(Environment.getExternalStorageDirectory().getAbsolutePath() );
             initAdapter();
             }
         else
             Toast.makeText(this,"Карта не найдена",Toast.LENGTH_SHORT).show();
         }

     @Override
     public void onBackPressed()
        {
         if(level==0)
             finish();
         else
            {
             level--;
             currentTargetDir= currentTargetDir.getParentFile();
             initAdapter();
             }
         }
     }