package me.cvhc.myapplication;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @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_main, menu);
        return true;
    }

    @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);
    }
}

package me.cvhc.equationsolver;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.util.Log;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    TextView textViewEquation;
    ListView listViewVariables;
    ArrayAdapter<String> adapterVariables;
    ArrayList<String> listVariables = new ArrayList<String>();

    private final String LOG_TAG = MainActivity.class.getSimpleName();

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

        textViewEquation = (TextView)findViewById(R.id.textViewEquation);
        listViewVariables = (ListView)findViewById(R.id.listViewVariables);

        adapterVariables = new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1, listVariables);
        listViewVariables.setAdapter(adapterVariables);

        textViewEquation.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {
                String eq = s.toString();
                String[] part = eq.split("=", 2);

                Log.d(LOG_TAG, "Equation: " + eq);
                if (part.length != 2) {
                    return;
                }

                ExpressionEvaluator left = new ExpressionEvaluator(part[0]);
                ExpressionEvaluator right = new ExpressionEvaluator(part[1]);

                Log.d(LOG_TAG, "left part: " + part[0]);
                Log.d(LOG_TAG, "right part: " + part[1]);

                if (left.isError() || right.isError()) {
                    textViewEquation.setBackgroundResource(R.color.red);
                }
                else {
                    textViewEquation.setBackgroundResource(android.R.color.transparent);

                    listVariables.clear();
                    for (Character c: left.getProperty().Variables) {
                        listVariables.add(c.toString());
                    }
                    for (Character c: right.getProperty().Variables) {
                        listVariables.add(c.toString());
                    }

                    adapterVariables.notifyDataSetChanged();

                }
            }
        });
    }
}

update code in MainActivity.java