Map Methods, EnumMap, and the Rest of the Family

import java.util.*;

public class MapMethods {

    // The long way. Write the null check the naive version needs.
    static Map<String, Integer> countTheLongWay(String[] words) {
        return Map.of(); // TODO
    }

    // The same answer, one line inside the loop.
    static Map<String, Integer> countWithMerge(String[] words) {
        return Map.of(); // TODO
    }

    // 0 when the name is not in the map. No null check of your own.
    static int scoreOrZero(Map<String, Integer> scores, String name) {
        return -1; // TODO
    }

    // "atlas" and "ant" both go under 'a'.
    static Map<Character, List<String>> groupByFirstLetter(String[] words) {
        return Map.of(); // TODO
    }

    public static void main(String[] args) {
        String[] words = "atlas beacon atlas cipher beacon atlas".split(" ");
        // TODO: print all four results
        // TODO: remove the null check from the long version and see what it throws
    }
}

import java.util.*;

enum Status { ACTIVE, RESERVE, RETIRED, TRAINING }

public class Enums {

    // Put rounds entries into whichever map you are handed. Return milliseconds.
    static long timePuts(Map<Status, Integer> target, int rounds) {
        return 0; // TODO
    }

    // The order a walk over the keys gives you.
    static List<Status> orderOf(Map<Status, Integer> filled) {
        return List.of(); // TODO
    }

    // Fill an EnumMap in a SCRAMBLED order: TRAINING, ACTIVE, RETIRED, RESERVE.
    static Map<Status, Integer> filledEnumMap() {
        return Map.of(); // TODO
    }

    static Set<Status> notActive()  { return Set.of(); }  // TODO: complementOf
    static Set<Status> everything() { return Set.of(); }  // TODO: allOf
    static Set<Status> nothing()    { return Set.of(); }  // TODO: noneOf

    // Try to put a null KEY. Return "ok" or the name of what came out.
    static String nullKey(Map<Status, Integer> m) {
        return "ok"; // TODO
    }

    public static void main(String[] args) {
        // TODO: time 5_000_000 puts into a HashMap and an EnumMap. Run it three times.
        //       Write down whether one of them reliably wins.
        // TODO: print the EnumMap's order and compare it with how you filled it
        // TODO: try a null key on both
    }
}

import java.util.*;

public class Identity {

    static class Node {
        final String name;
        final List<Node> links = new ArrayList<>();
        Node(String n) { name = n; }
        @Override public boolean equals(Object o) { return o instanceof Node n && n.name.equals(name); }
        @Override public int hashCode() { return name.hashCode(); }
        @Override public String toString() { return name; }
    }

    // Put new String("key") and another new String("key") into a HashMap. Size?
    static int hashMapSizeForEqualKeys() {
        return 0; // TODO
    }

    // The same two keys into an IdentityHashMap. Size?
    static int identityMapSizeForEqualKeys() {
        return 0; // TODO
    }

    // Two DIFFERENT nodes that happen to share a name, linked to each other.
    static Node[] cyclicPair() {
        return new Node[0]; // TODO
    }

    // Walk from start, counting nodes, using a normal HashSet of visited nodes.
    static int countWithEqualsSet(Node start) {
        return 0; // TODO
    }

    // Walk again, but let the visited set compare by identity.
    static int countWithIdentitySet(Node start) {
        return 0; // TODO
    }

    public static void main(String[] args) {
        // TODO: print both map sizes and both walk counts
        // TODO: say in one sentence when identity is the right question
    }
}

import java.util.*;

public class Navigate {

    // Readiness score -> unit name.
    static TreeMap<Integer, String> byScore() {
        return new TreeMap<>(); // TODO: 45 Beacon, 60 Drift, 88 Atlas, 95 Cipher
    }

    static String highest(TreeMap<Integer, String> m) { return null; }  // TODO
    static String lowest(TreeMap<Integer, String> m)  { return null; }  // TODO

    // The nearest score that does not go over. null if there is none.
    static String bestAtOrBelow(TreeMap<Integer, String> m, int score) {
        return null; // TODO
    }

    // The nearest score going the other way. null if there is none.
    static String worstAtOrAbove(TreeMap<Integer, String> m, int score) {
        return null; // TODO
    }

    // Everyone from `from` to `to`, both ends included, in score order.
    static List<String> between(TreeMap<Integer, String> m, int from, int to) {
        return List.of(); // TODO
    }

    public static void main(String[] args) {
        // TODO: answer all five questions
        // TODO: try each one on a HashMap and record why it cannot answer
    }
}