An activity class for OnTrack android application(location sharing app). Describes the process when a user enters a session with a passcode to see the location of people who are in that session.

FollowActivity.java

package edu.colby.ontrack.ontrack;

import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.Toast;

import com.parse.FindCallback;
import com.parse.ParseException;
import com.parse.ParseObject;
import com.parse.ParseQuery;
import com.parse.ParseRelation;

import java.util.List;


public class FollowActivity extends ActionBarActivity {
    private EditText mUniqueID;
    private ImageButton mFollow;
    private ImageButton mCancelFollow;
    private boolean misIDUnique;
    private String muserID;



    private static ParseObject  mLeader;
    private ParseObject mcurrentSession;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_follow);
        mUniqueID = (EditText) findViewById(R.id.follower_uniqueID_edittext);
        mFollow = (ImageButton) findViewById(R.id.button_follow);
        muserID = LaunchActivity.getUser().getObjectId().toString();
        mCancelFollow = (ImageButton) findViewById(R.id.button_cancel_follow);
        mCancelFollow.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                onBackPressed();
            }
        });

        mFollow.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //check if the user has input anything
                String id = mUniqueID.getText().toString().trim();
//                Log.d("before_checking", ""+ id.equals("")) ;

                if(id.equals("")) {
//                    Log.d("after_checking", ""+ id.equals("")) ;
                    Toast.makeText(FollowActivity.this, "Please enter a valid ID", Toast.LENGTH_SHORT).show();

                }else{
                    try {
                        if (doesIDexist(id)){
                            enterSession(id);
                            Intent i = new Intent(FollowActivity.this, FollowerMapsActivity.class);
                            startActivity(i);

                        } else {
                            Toast.makeText(FollowActivity.this, "ID does not exist, try again!", Toast.LENGTH_SHORT).show();

                        }
                    } catch (ParseException e) {
                        e.printStackTrace();
                    }

                    //check if ID is unique
                }
            }

        });

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_follow, menu);
        return true;
    }

    public boolean doesIDexist(String id) throws ParseException {


        ParseQuery<ParseObject> query = ParseQuery.getQuery("Session");
        query.whereEqualTo("Title", id);
        List<ParseObject> results = query.find( );
        if (results.isEmpty()){
            return false;
        } else if (!((boolean)results.get(0).get("Active"))){
            return false;
        } else {
            mcurrentSession = results.get(0);
            mLeader = (ParseObject) mcurrentSession.get("Leader");
            return true;
        }
    }


    public void enterSession(String id){
        ParseObject currentUser = LaunchActivity.getUser();
        ParseRelation relation = mcurrentSession.getRelation("Followers");
        relation.add(currentUser);
        mcurrentSession.saveInBackground();
        currentUser.put("session", mcurrentSession);
        currentUser.saveInBackground();



    }


    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
    public static ParseObject getmLeader() {
        return mLeader;
    }

}

During HackDartmouth, I worked in a team to develop an android app which lets users share location easily across different platforms. HOW THIS APP WORKS: There's a leader who creates a session and shares a passcode with others users who want to participate in that session and share their location and learn others' location.
MY RESPONSIBiLITY: I was assigned to handle the case when the users clicks "Follow" and enters a passcode. First, I check if the code is valid and exists in the database; if it does, the transitions to a new page; if not, he/she gets a message.