Convert Json from/to object/tree/inputstream/reader/file with Gson in Java

TestGson.java

import com.google.gson.Gson;
import com.google.gson.JsonElement;
import java.io.ByteArrayInputStream;
import java.io.InputStreamReader;
import org.junit.jupiter.api.Test;

/**
 * Convert Json from/to object/tree/inputstream/reader/file with Gson in Java
 * 
 * ref https://www.javadoc.io/doc/com.google.code.gson/gson/2.9.0/com.google.gson/com/google/gson/Gson.html
 *
 */
class BagOfPrimitives {

  private int value1 = 1;
  private String value2 = "abc";
  private transient int value3 = 3;

  BagOfPrimitives() {
    // no-args constructor
  }

  @Override
  public String toString() {
    return "BagOfPrimitives{" +
        "value1=" + value1 +
        ", value2='" + value2 + '\'' +
        ", value3=" + value3 +
        '}';
  }
}

public class TestGson {

  @Test
  void serialize() {
    // obj to str
    BagOfPrimitives obj = new BagOfPrimitives();
    Gson gson = new Gson();
    String json = gson.toJson(obj);
    System.out.println(json);
  }

  @Test
  void deserialize() {
    Gson gson = new Gson();
    BagOfPrimitives obj;

    //  str to obj
    obj = gson.fromJson("{\"value1\":1,\"value2\":\"abc\"}", BagOfPrimitives.class);
    System.out.println(obj);

    // reader (inputstream/file) to obj
    //    InputStream is = TestGson.class.getResourceAsStream("/xxx.json")
    InputStream is = new ByteArrayInputStream(
        "{\"value1\":1,\"value2\":\"abc\"}".getBytes());
    obj = gson.fromJson(new InputStreamReader(is), BagOfPrimitives.class);
    System.out.println(obj);
  }

  /**
   * JsonElement is a map/tree like data structure.
   * 
   *        JsonElement 
   *          /     \
   *         /  Gson \             
   *        /         \
   *   Json doc ---- Object
   * 
  **/
  @Test
  void jsonElement() {
    BagOfPrimitives obj = new BagOfPrimitives();
    Gson gson = new Gson();

    // obj to tree
    JsonElement jsonTree = gson.toJsonTree(obj);
    System.out.println(jsonTree);

    // tree to obj
    obj = gson.fromJson(jsonTree, BagOfPrimitives.class);
    System.out.println(obj);

    // tree to str
    String json = gson.toJson(jsonTree);
    System.out.println(json);

    // str to tree
    jsonTree = gson.fromJson(json, JsonElement.class);
  }
}