Pass interface between activities in intent - interface fails to be Serializable or Parcelable

You cannot "pass an interface". An "interface" is an abstract thing. What you want to pass is a concrete implementation of that interface, ie: an object (that happens to implement your interface). When you instantiate your "interface" (in your example, like this:

intent.putExtra("testInterface", new ITest() {
    @Override 
    void onSuccess(String text) {
    }
}

you are actually creating an instance of an anonymous class that implements the interface ITest. To pass this in an Intent you would need to make this class also implement Parcelable or Serializable.

However, even if you did that, it would not solve your problem. What you need to understand is that you can't pass objects (instances) by putting them as "extras" in an Intent. When you do that, Android actually serializes and then deserializes the object so that you end up with 2 objects, one is a serialized/deserialized copy of the original.

If you want ActivityB to communicate with ActivityA, you will need to use another technique. Try one of these:

• ActivityB sends a broadcast Intent, which ActivityA listens for
• ActivityA starts ActivityB using startActivityForResult() and ActivityB sends data back to ActivityA using setResult()
• Use public static (ie: global) variables to communicate
• Store data in shared preferences, or a file, or a database

What you really need to understand is that, under certain conditions, the following can occur:

• your app is running with ActivityA in the stack and ActivityB on the top of the stack
• user presses HOME
• Android moves your task to the background and eventually kills the hosting process
• User returns to your app (by starting it again or selecting it from "recent task" list
• Android creates a new process for your app, and instantiates ActivityB, then calls onCreate(), onStart() and onResume() of ActivityB.

In this case, there is no instance of ActivityA anymore. That instance is dead. So ActivityB cannot communicate with ActivityA because it no longer exists.