GsonSample.java

package dummy.json;

import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonParser;
import com.google.gson.reflect.TypeToken;
import java.lang.reflect.Type;
import java.util.Map;
import java.util.Objects;

public class GsonSample {

  public static void main(String... args) {
    String json = "{'k1':'apple','k2':123, 'k3':123.0, 'k4':123}";
    // test0(json);
    test1(json);
  }

  public static void test0(String json) {
    Gson gson = new Gson();
    Type type = new TypeToken<Map<String, Object>>() {}.getType();
    Map<String, Object> myMap = gson.fromJson(json, type);
    System.out.println(myMap);
    myMap.forEach(
        (String key, Object val) -> {
          System.out.println("key:" + key);
        });

    // Double cannot cast to Long.
    // Long val2 = (Long) myMap.get("k2");

    Double d1 = Double.valueOf(12.345);
    Double d2 = Double.valueOf(12.345);
    Double d3 = Double.valueOf(12.3456);

    System.out.println("d1 == d2: " + d1.equals(d2));
    System.out.println("d1 == d3: " + d1.equals(d3));

    Double d4 = Double.valueOf(12.0);
    Double d5 = Double.valueOf(12);
    System.out.println("d4 == d5: " + d4.equals(d5));

    //    JsonObject elem = gson.fromJson("{'k1':'apple','k2':123}", JsonObject.class);
    //    elem.getAsJsonObject()
    //        .entrySet()
    //        .forEach((kv) -> System.out.println(kv.getKey() + kv.getValue().));
  }

  public static void test1(String json) {
    Gson gson = new Gson();
    JsonParser parser = new JsonParser();
    JsonElement root = parser.parse(json);
    root.getAsJsonObject()
        .entrySet()
        .forEach(
            entry -> {
              System.out.println(entry.getKey());
              System.out.println(entry.getValue());
              System.out.println("isJsonPrimitive(): " + entry.getValue().isJsonPrimitive());
              System.out.println("isNumber(): " + entry.getValue().getAsJsonPrimitive().isNumber());
            });

    Number k2 = null;
    Number k3 = null;
    Number k4 = null;
    for (Map.Entry<String, JsonElement> ent : root.getAsJsonObject().entrySet()) {
      if (ent.getKey().equals("k2")) k2 = ent.getValue().getAsNumber();
      if (ent.getKey().equals("k3")) k3 = ent.getValue().getAsNumber();
      if (ent.getKey().equals("k4")) k4 = ent.getValue().getAsNumber();
    }
    Objects.requireNonNull(k2, "k2 is null");
    Objects.requireNonNull(k3, "k3 is null");
    Objects.requireNonNull(k4, "k4 is null");
    System.out.println("k2 == K3: " + (k2 == k3)); // false
    System.out.println("k2 == K4: " + (k2 == k4)); // false
    System.out.println("k2.equals(k3): " + k2.equals(k3)); // false
    System.out.println("k2.equals(k4): " + k2.equals(k4)); // true
    // Numberは整数と浮動小数点は == もequalsも等しくならないが、
    // 数値型が同じで数値が同じ場合はequalsメソッドはtrueを返す。
    // でもインスタンスは異なるので == はfalseを返す。
  }
}