Java Reflection : avoid fields with default values

I have the following class structure:

public Class A{
private int a;
private String b;
private B binst;
}

public Class B{
private int x;
private String y;
}

All the getters and setters are defined. I use Java reflection to invoke as follows :

method.invoke(ClassAObj, ClassBObj);

Now, before invoking this, I had only set y and not x. I convert this ClassAObj into JSON and find that the default value of 0 is set for x, and it appears in the JSON. I don't want x field to appear in the JSON. How should I avoid this?

Interestingly, if I set x and not y, the tag y doesn't appear in the JSON.