Java By Comparison

Favor Abstract Over Concrete Types

class Inventory {
    LinkedList<Supply> supplies = new LinkedList();

    void stockUp(ArrayList<Supply> delivery) {
        supplies.addAll(delivery);
    }

    LinkedList<Supply> getContaminatedSupplies() {
        LinkedList<Supply> contaminatedSupplies = new LinkedList<>();
        for (Supply supply : supplies) {
            if (supply.isContaminated()) {
                contaminatedSupplies.add(supply);
            }
        }
        return contaminatedSupplies;
    }
}

Interfaces and classes often form large type hierarchies—just look at the Java API. If you use more abstract types for variables, your code will be more flexible.

Above, you can see a part of the Inventory system. The method getContaminatedSupplies() loops through a LinkedList of Supply objects, ...