How to randomly select from arrayLists and add to an object?

"Implement the MenuRandomize class. The constructor receives the file name of 4 files, one for each type of meal, and uses FileManager to load data from these text files into the 4 ArrayList decleared as properties (one array for each type of dish)." Right now I am stuck on the randomMenu(). How do I randomly select from arrayLists and add to an object? I think I should return Menu but it returns an error. I have selected a random number. Do I take that index from another arrayList (that element) and store it in a different arrayList ? I have this so far.

public class MenuRandomize {

private ArrayList<Entree> entree;
private ArrayList<Salad> salad;
private ArrayList<Dessert> dessert;
private ArrayList<Side> side;

public MenuRandomize(String entreeFile, String sideFile, String saladFile, String dessertFile) {
    entree = FileManager.readEntrees(entreeFile);
    dessert = FileManager.readDesserts(dessertFile);
    side= FileManager.readSides(sideFile);
    salad= FileManager.readSalads(saladFile);
}

public randomMenu() {
    ArrayList<Menu> menu;
    int indexEntrees=(int) Math.random() * entree.size() - 1;
    entree = entrees.get(indexEntrees);

    int indexD=(int) Math.random() * dessert.size() - 1;
    dessert = desserts.get(indexD);
                             **<----add element here to arrayList <Menu>?**
    int indexSalad=(int) Math.random() * salad.size() - 1;
    salad = salads.get(indexSalad);

    int indexSide=(int) Math.random() * side.size() - 1;
    entree = sides.get(indexSide);

    return null; **<--- return Menu?**

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