restapi4JsonHandling

1package pkg12restapi;2 3import java.util.LinkedHashMap;4import java.util.List;5import java.util.Map;6 7/*8 * restapi4JsonHandling.java9 * -------------------------10 * Working with JSON. Real apps use Jackson or Gson; here we show the shape of11 * JSON and a tiny hand-rolled writer/reader so the demo has NO dependencies.12 *13 * DEFINITION:14 *   JSON is the lingua franca of REST: objects {key:value}, arrays [..], strings,15 *   numbers, booleans, null. In production, map JSON <-> POJOs with a library16 *   (Jackson's ObjectMapper). This file demonstrates both worlds.17 *18 * KEY POINTS:19 *   - Production: ObjectMapper.writeValueAsString(obj) / readValue(json, Pojo.class).20 *   - Always escape special chars (" \\ newlines) when writing JSON by hand.21 *   - Prefer a library for anything beyond trivial payloads — parsing is fiddly.22 *   - Records map cleanly to/from JSON objects.23 */24public class restapi4JsonHandling {25 26    record User(int id, String name, boolean active) {}27 28    public static void main(String[] args) {29        // --- Build JSON from a record (toy serializer) ---30        User user = new User(1, "Ada \"the\" Lovelace", true);31        System.out.println("Serialized object:");32        System.out.println("  " + toJson(user));33 34        // --- Build a JSON array from a list ---35        List<User> users = List.of(new User(1, "Ada", true), new User(2, "Linus", false));36        StringBuilder arr = new StringBuilder("[");37        for (int i = 0; i < users.size(); i++) {38            if (i > 0) arr.append(',');39            arr.append(toJson(users.get(i)));40        }41        arr.append(']');42        System.out.println("\nSerialized array:");43        System.out.println("  " + arr);44 45        // --- Parse a flat JSON object into a Map (toy parser) ---46        String json = "{\"id\": 42, \"name\": \"Grace\", \"active\": false}";47        Map<String, String> parsed = parseFlat(json);48        System.out.println("\nParsed object:");49        parsed.forEach((k, v) -> System.out.println("  " + k + " = " + v));50 51        System.out.println("\nIn production, replace these helpers with Jackson:");52        System.out.println("  ObjectMapper m = new ObjectMapper();");53        System.out.println("  String s = m.writeValueAsString(user);");54        System.out.println("  User u   = m.readValue(s, User.class);");55    }56 57    /** Minimal JSON object writer for a User record (escapes quotes/backslashes). */58    static String toJson(User u) {59        return "{\"id\":" + u.id()60             + ",\"name\":\"" + escape(u.name()) + "\""61             + ",\"active\":" + u.active() + "}";62    }63 64    static String escape(String s) {65        return s.replace("\\", "\\\\").replace("\"", "\\\"")66                .replace("\n", "\\n").replace("\t", "\\t");67    }68 69    /** Toy parser for a flat {"k": v, ...} object — NOT for nested/real JSON. */70    static Map<String, String> parseFlat(String json) {71        Map<String, String> out = new LinkedHashMap<>();72        String body = json.trim().replaceAll("^\\{|}$", "");73        for (String pair : body.split(",")) {74            String[] kv = pair.split(":", 2);75            if (kv.length == 2) out.put(clean(kv[0]), clean(kv[1]));76        }77        return out;78    }79 80    static String clean(String s) { return s.trim().replaceAll("^\"|\"$", ""); }81}