Use a custom Class in Android Studio

Add a cutom class to an Android project by going to File menu -> New -> Java Class

Usage:
Custom class…

package eu.kio.android.beeradvier;

import java.util.ArrayList;
import java.util.List;


public class BeerExpert {
    List<String> getBrands(String color){

        List<String> brands = new ArrayList<>();
        if(color.equals("amber")){
            brands.add("Jack Amber");
            brands.add("Red Moose");
        }
        else{
            brands.add("Jail Pale Ale");
            brands.add("Gout Stout");
        }
        return brands;
    }
}

Main activity

package eu.kio.android.beeradvier;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;
import android.widget.Spinner;
import java.util.List;

public class FindBeerActivity extends Activity {

    private BeerExpert expert = new BeerExpert();

    //called when the user clicks the button
    public void onClickFindBeer(View view){
        //Get a reference to the TextView
        TextView brands = (TextView)findViewById(R.id.textView);

        //Get a reference to the Spinner
        Spinner color = (Spinner)findViewById(R.id.color);

        //Get the selected item in the Spinner
        String beerType = String.valueOf(color.getSelectedItem());


        List<String> brandList = expert.getBrands(beerType);
        StringBuilder brandsFormatted = new StringBuilder();
        for(String brand : brandList){
            brandsFormatted.append(brand).append('\n');
        }

        //Display the selected item
        brands.setText(brandsFormatted);
    }

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