#6 prefer collection interfaces interfaces over concrete types

[Anonymous]

May places in the code don't use collection interfaces as the type of a variable but instead use a concrete implementation as the type. So for example within ConcreteStateModel we see:

public Transition[] getTransitionsFromState(State state) {
ArrayList resultsList = new ArrayList();

A better Java idiom is use the interface as the type of the variable:

public Transition[] getTransitionsFromState(State state) {
List resultsList = new ArrayList();

The worst place to use the concrete type of a list is in method declaration such as StandardEngines method definition here:

protected Fork addStatesFromForkPathToList(StateModel model, Transition toJoinTransition, ArrayList<State> stateList) {

As the type of the list is ArrayList<State> you lock any future use of a different type of list being passed in the future e.g. Collections.synchronisedList(someArrayList). It is recommended that the method signature is

protected Fork addStatesFromForkPathToList(StateModel model, Transition toJoinTransition, List<State> stateList) {