class FSM {
    Set<String> S;      // the states
    String s₀;          //  start state    s₀ ∈ S      S.contains(s₀)
    Set<Character> Σ;   // the input alphabet
    BiFunction<String,Character, String> τ;   /* Note: #allowable inputs is:      S.size() * Σ.size() */
    Set<String> F;     // F subset  S     S.containsAll(F)
    }


// Btw, in Python, we'd have tau be a map<pair<state,character>,state>


new FSM( new Set<String>{"q0","q1","q2","q4"},
         "q0",
         new Set<Character>{ 'a', 'b' }
         new BiFunction<String,Character, String> {
             String apply( String currState, Character c ) {
                 return switch (currState) {
                     case "q0" -> switch (c) { case 'a' -> "q4"; case 'b' -> "q1"; };
                     case "q1" -> switch (c) { case 'a' -> "q4"; case 'b' -> "q2"; };
                     case "q2" -> switch (c) { case 'a' -> "q2"; case 'b' -> "q1"; };
                     case "q4" -> "q4";
                     }
                 }
             }
         new Set<String>{"q1"}
         )