File Manager Code

package net.samaysoftware.sampleapplication12345;

import android.content.Intent;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.*;

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;
public class FMMainActivity extends AppCompatActivity implements AdapterView.OnItemClickListener {
    ListView lv;
    String[] from = {"fname","ftype"};
    int[] to = {android.R.id.text1, android.R.id.text2};
    ArrayList<HashMap<String, String>> data = new ArrayList<>();
    @Override    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fmmain);
        lv = (ListView) findViewById(R.id.fmlv);
        generateData();
        SimpleAdapter sa = new SimpleAdapter(this, data, android.R.layout.simple_list_item_2,from, to);
        lv.setAdapter(sa);
        lv.setOnItemClickListener(this);
    }
    private void generateData() {
        File froot = null;
        if(getIntent().getStringExtra("abcd")==null) {
            froot = Environment.getExternalStorageDirectory();
        }
        else{
            String fpath = getIntent().getStringExtra("abcd");
            froot = new File(fpath);
            setTitle(froot.getName());
        }
        File[] arr = froot.listFiles();
        for(File f: arr){
            String name = f.getName();
            String type = f.isFile()?"File":"Folder";
            HashMap<String, String> map = new HashMap<>();
            map.put("fname", name);
            map.put("ftype", type);
            map.put("fpath", f.getAbsolutePath());
            data.add(map);
        }
    }

    @Override    public void onItemClick(AdapterView<?> adapterView, View view, int pos, long l) {
        String fpath = data.get(pos).get("fpath");
        File cf = new File(fpath);
        if(cf.isDirectory()){
            Intent i = new Intent(this, FMMainActivity.class);
            i.putExtra("abcd", fpath);
            startActivity(i);
        }else{

        }

    }
}

Note: Add permission in Manifest file as below:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"></uses-permission>
just before the <application> tag and inside the <manifest> tag.