How to get a arraylist of all objects created of a class code example

Example: how add model class to array list

//main class
SomeClass b= new SomeClass();
list.add(b);

//subclass
 Home.MainCategory mainCategory = new Home().getInstranceMainCategory();
                mainCategory.mainCatName = getString(R.string.more);
                mainCategoryList.add(mainCategory);