Transient field initialization when using GSON

When you use GSON to deserialize a JSON string in your Java code, the class has to have a non-argument constructor in order for the transient fields to be initialized properly:

	package gsondemo;
	import com.google.gson.Gson;
	import java.util.Arrays;
	/**
	* GSON demo. Demonstrates effect of having no non-argument constructor on
	* initialization of transient fields when creating an object from JSON string.
	* Reference: https://stackoverflow.com/a/13532237/51358
	* @author skorkmaz, April 2018, License: Public Domain
	*/
	public class GSONDemo {
	static Gson gson = new Gson();
	static class Base {
	int val1 = 1;
	//transient fields are not included in serialization:
	transient int transientVal = 3;
	transient int[] tansientArray = {1, 2, 3};
	}
	static class ClassWithImplicitNonArgConstructor extends Base {
	}
	static class ClassWithOnlyArgumentConstructor extends Base {
	//No non-argument constructor -> When creating object from JSON string,
	//transient fields will NOT be initialized
	public ClassWithOnlyArgumentConstructor(int aVal) {
	}
	}
	static class ClassWithExplicitNonArgConstructor extends Base {
	public ClassWithExplicitNonArgConstructor(int aVal) {
	}
	//Non-argument constructor --> When creating object from JSON string,
	//transient fields will be initialized
	public ClassWithExplicitNonArgConstructor() {
	}
	}
	public static void main(String[] args) {
	// Serialization:
	System.out.println("ClassWithImplicitNonArgConstructor");
	System.out.println("----------------------------------");
	Base obj11 = new ClassWithImplicitNonArgConstructor();
	obj11.val1 = 500;
	obj11.transientVal = 10;
	//Transient fields will not be included in JSON:
	String json1 = gson.toJson(obj11);
	System.out.println("json = " + json1);
	// Deserialization:
	Base obj12 = gson.fromJson(json1, ClassWithImplicitNonArgConstructor.class);
	System.out.println("value1 = " + obj12.val1);
	//The default value of 3 (not the modified value of 10) will be written:
	System.out.println("value3 = " + obj12.transientVal);
	System.out.println("intArray = " + Arrays.toString(obj12.tansientArray));
	System.out.println("\nClassWithOnlyArgumentConstructor");
	System.out.println("--------------------------------");
	Base obj21 = new ClassWithOnlyArgumentConstructor(1);
	String json2 = gson.toJson(obj21);
	System.out.println("json = " + json2);
	Base obj22 = gson.fromJson(json2, ClassWithOnlyArgumentConstructor.class);
	//will NOT have initial value in Base class:
	System.out.println("value3 = " + obj22.transientVal);
	//will NOT have initial value in Base class:
	System.out.println("intArray = " + Arrays.toString(obj22.tansientArray));
	System.out.println("\nClassWithExplicitNonArgConstructor");
	System.out.println("----------------------------------");
	Base obj31 = new ClassWithExplicitNonArgConstructor(1);
	String json3 = gson.toJson(obj31);
	System.out.println("json = " + json3);
	Base obj32 = gson.fromJson(json3, ClassWithExplicitNonArgConstructor.class);
	//will have initial value in Base class:
	System.out.println("value3 = " + obj32.transientVal);
	//will have initial value in Base class:
	System.out.println("intArray = " + Arrays.toString(obj32.tansientArray));
	}
	}

view raw gsondemo.java hosted with ❤ by GitHub

The output of the above code is as follows:

ClassWithImplicitNonArgConstructor
----------------------------------
json = {"val1":500}
value1 = 500
value3 = 3
intArray = [1, 2, 3]

ClassWithOnlyArgumentConstructor
--------------------------------
json = {"val1":1}
value3 = 0
intArray = null

ClassWithExplicitNonArgConstructor
----------------------------------
json = {"val1":1}
value3 = 3
intArray = [1, 2, 3]