This interface is implemented to create instances of a class that does not define a no-args constructor. If you can modify the class, you should instead add a private, or public no-args constructor. However, that is not possible for library classes, such as JDK classes, or a third-party library that you do not have source-code of. In such cases, you should define an instance creator for the class. Implementations of this interface should be registered with GsonBuilder#registerTypeAdapter(Type, Object) method before Gson will be able to use them.

Let us look at an example where defining an InstanceCreator might be useful. The Id class defined below does not have a default no-args constructor.

public class Id<T> {
  private final Class<T> clazz;
  private final long value;
  public Id(Class<T> clazz, long value) {
    this.clazz = clazz;
    this.value = value;
  }
}

If Gson encounters an object of type Id during deserialization, it will throw an exception. The easiest way to solve this problem will be to add a (public or private) no-args constructor as follows:

private Id() {
  this(Object.class, 0L);
}

However, let us assume that the developer does not have access to the source-code of the Id class, or does not want to define a no-args constructor for it. The developer can solve this problem by defining an InstanceCreator for Id:

class IdInstanceCreator implements InstanceCreator<Id> {
  public Id createInstance(Type type) {
    return new Id(Object.class, 0L);
  }
}

Note that it does not matter what the fields of the created instance contain since Gson will overwrite them with the deserialized values specified in JSON. You should also ensure that a new object is returned, not a common object since its fields will be overwritten. The developer will need to register IdInstanceCreator with Gson as follows:

Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdInstanceCreator()).create();