Develop Reference

Android: Why Parcel?

I'm a C/C++ guy, and I'm quite new to Java and Android.
So I'm creating a class instance in Activity "A" and I'm trying to use this instance to Activity "B". In C/C++ you would simply pass the address (pointer) of the instance.
Now I learned that with Android I should use a Parcel, but I don't get the idea: why would I want do all that parceling/deparceling procedure when all I want to do is passing a pointer?
Please enlighten me!

This is related to the Android activity model.
Since your app may be killed by the system to recover RAM (this should only happen when it's in the background, unless the system is in a really bad situation), an object reference just won't do. It'd be completely worthless once your process dies.
The intent you use to start activities is saved by the Activity Manager, so that it can be used to recreate your activity when the user navigates back to it. That's why all the data in your intent has to be parceled.
Keep in mind that this means that your two activities will have two similar instances -- but naturally, they will not be the same object, so changes to one won't be reflected in the other.

Parcel or parcalable is an efficient and alternative way to serialize objects in Android. Its an alternative to Serialization in core Java.
Java Serialization API allows us to convert an Object to stream that we can send over the network or save it as file or store in DB for later usage.
Its a way to exchange objects between sender and receiver(Class). Receiver than deserializes this object.
In Android parcel is used to exchange objects between different activities by using Intents.

java doesn't work like c/c++, you don't work directly with the memory. If you want to pass an object from A to B, you have to set it in a Bundle, and add that bundle to the intent. if it's a custom class, let's call it C, C should implement parcellable. Java works by value, not by reference.
custom parcelable objects
how do i pass data between activities?

Call Activity with Custom Arguments

I would like to ask about calling an Activity with arguments. I have seen many examples about it. But I would like to ask an explanation why it is somehow "not easy" or "not simple" to create(call) an activity with custom arguments.
I've seen many posts like this and others. People passing strings but not custom objects. Yes, they are passing custom objects by implementing serializable and parceable methods but that doesn't seem efficient (I might be wrong because haven't seen it). For final declared classes, it is even more work to do.
When we pass String with putExtras(String name, String value), why is a function like putExtras(String name, Object object) not defined? wouldn't it be easier to pass any custom objects by that?
And about the complexity of the process, an explanation was given in the post here
beacuse apps are in different processes, and have separate memory
address spaces, you cant just send pointer (reference) to memory block
in your process and expect it to be available in another process.
But, still, I couldn't find a reason that satisfy me or helps me to realize what I am not seeing. Okay, let's say apps are in different processes and their memory spaces are different. So are we copying the data to be passed into another activity's memory space? or at least, shouldn't the activities which belong to same application (I believe activities in an app has something common that addresses to its app) have the common memory space so that they could easily share or pass data?
I am just trying to clarify myself about this. I know Android should not be compared with Windows development but feels like it could be better if things were simpler. I know I am missing something about this, otherwise everyone knows simpler would be better. But what is the thing that prevents passing things easier?
Thanks.

why is a function like putExtras(String name, Object object) not defined? wouldn't it be easier to pass any custom objects by that?
Because startActivity() always crosses a process boundary, and therefore whatever gets passed needs to be convertible into a byte array for transport.
So are we copying the data to be passed into another activity's memory space?
We are copying data between processes.
or at least, shouldn't the activities which belong to same application (I believe activities in an app has something common that addresses to its app) have the common memory space so that they could easily share or pass data?
Except that the primary work of startActivity() is not done by your application process, but rather by a core operating system process.
I know Android should not be compared with Windows development but feels like it could be better if things were simpler.
A better analogy is Web development, where you do not pass objects between client and server, but rather data marshalled to/from strings or byte arrays.

Just as you have figured out yourself you pass data between activities through a Bundle, like this:
Intent intent = new Intent(context, MyActivity.class);
intent.putExtra(key, "some string");
intent.putExtra(key, true);
intent.putExtra(key, 1.5f);
The thing is that Intents and Bundles are a very versatile way of passing data not only between your own activities, but also to other applications, say by launching a messaging client and providing a predefined subject and body. This is very powerful but it is also a constraint. What if you like to pass MyFooClass? Well you can't do that since an Intent is an abstract way of performing an operation, not only within your own application. One might argue that Google should have made an exception for intents between your own activities, but I really don't see this as a very big problem.
You can, as you say yourself, serialize an object using serializable interface, but you could also serialize it as a JSON object. I would also say that if you need to pass complex data structures between activities you really need to rethink your design. You should probably have some global application state (in your Application or similar class) and only pass ids, keys or similar that you can use to look up the data from your application state.

Android pass persistent information in bundles or use singleton pattern?

Just wondering what is a better practice to pass information between activites, adding it to a bundle or using a singleton class to store and access this data. I have used both in the past for various android side projects, but I am now working on an android project that is of much larger scale, so would prefer to do things right towards the beginning.
My application authenticates users and then will have to do various queries based on it's id. To minimize coupling between activities, I would think just adding the id to the bundle, and then letting each activity query for the information that it needs, would be the best bet; however to increase responsiveness, I was leaning towards using a singleton class to store persistent information, preventing more queries than need be.

Personally, I would create an extension of Application to store the state of your app and share data between the different activities. The Application acts as the context for your whole app and Android guarantees there will always only be one instance across your app. Hence it works similar to defining your own Singleton, but using Application will allow Android to take control of the life cycle of your shared data and basically do the memory management for you.
Here are some more details. If you go down this path, you can simply add any getter/setter (or other) method to your application extension to store/retrieve data and do operations on it. Especially the latter can become quite a pain to manage (and keep consistent) when using Bundles passed back and forth between activities. If would only use a Bundle if the data is needed in just one or two places that are neighbours in the activity flow and does not need any (complex) operations to be run on it.

The better way to go for you is to use SharedPreferences to keep userId you need to keep and reuse. Of course you can use singleton approach or even Application class, but the data will be lost after application is killed.

The only time I pass data between Activities via bunlde is if it's something that I won't need to access for a while(i.e the the resID of a resource I want to use only once in the calling activity, etc). I would also think the difference in responsiveness would be very minimal, so that shouldn't be of concern. I suggest the singleton approach

Passing bundles is a tedious job. You'll have to pass a bundle for every change in activity to make sure that the value is not lost, even if you're not using the value in the called activity.

Singleton pattern have some bad results. For example:From Main activity you call secondary activity. Phone call interrupted your work.After ending phone call Android is trying to bring secondary activity to screen. Here is my nightmare - many users complaint about exceptions - Google reported to me NULL pointers in my singleton. So you have to provide not only singleton, but all data inside singleton have to be as singleton too. This maked come very complicated :(

Within an application what is the best way to pass custom objects between activities?

I know this question has been asked multiple number of times and i have been through a lot of these questions......almost all of these questions throw up the use of the parcelable interface for your class.
However in a couple of questions i came across a quotation:
"NOTE: Seeing Parcelable might have triggered the question, why is Android not using the
built-in Java serialization mechanism? It turns out that the Android team came to the conclusion
that the serialization in Java is far too slow to satisfy Android’s interprocess-communication
requirements. So the team built the Parcelable solution. The Parcelable approach requires
that you explicitly serialize the members of your class, but in the end, you get a much faster
serialization of your objects.
Also realize that Android provides two mechanisms that allow you to pass data to another
process. The first is to pass a bundle to an activity using an intent, and the second is to pass a
Parcelable to a service. These two mechanisms are not interchangeable and should not be
confused. That is, the Parcelable is not meant to be passed to an activity. If you want to start
an activity and pass it some data, use a bundle. Parcelable is meant to be used only as part of
an AIDL definition."
This quote can also be found in the book Pro Android 2.
Now seeing that all activities within the same application run in the same process(Every Activity in Android is a Process,or One Application is one process),unless otherwise specified in the manifest,communication within the activities of the same application is not Interprocess communication per se.So is it really faster to use the parcelable class or is it just enough to pass your object attributes through bundle via intent ?
Shedding any light on this aspect will be largely appreciated.
Cheers !!

There's a FAQ for that. :)
The short answer is that the Android team recommends three techniques for passing data between activities and services within an app: a singleton class; a public static field or method; a HashMap of WeakReferences to Objects (and you pass the key in the intent). The major issue to keep in mind is how your data is going to behave under various lifecycle events. (For instance, if the user turns the phone, by default your activities will be destroyed and recreated; your data handling method needs to be designed with that in mind.)

The Parcelable construct is designed to be very fast at passing data across application memory boundaries: within an application you are MUCH better served using Bundle because all the memory locations the data is stored in are accessible to both the sender and the receiver. Since the in-memory objects are accessible there is no need to incur the cost of reconstruction: just use the Bundle, which is really just a glorified HashMap with type-specific put/get methods.
For AIDL and IPC purposes you can't (by default) share memory locations so you need an efficient way of moving data: this is where Parcelable kicks in. Unless one of the components of your application is using the remote process capability then there is no need to use Parcelable.

From docs:
Parcelables
The Parcelable protocol provides an extremely efficient (but
low-level) protocol for objects to write and read themselves from
Parcels. You can use the direct methods writeParcelable(Parcelable,
int) and readParcelable(ClassLoader) or writeParcelableArray(T[], int)
and readParcelableArray(ClassLoader) to write or read. These methods
write both the class type and its data to the Parcel, allowing that
class to be reconstructed from the appropriate class loader when later
reading.
There are also some methods that provide a more efficient way to work
with Parcelables: writeTypedArray(T[], int), writeTypedList(List),
readTypedArray(T[], Parcelable.Creator) and readTypedList(List,
Parcelable.Creator). These methods do not write the class information
of the original object: instead, the caller of the read function must
know what type to expect and pass in the appropriate
Parcelable.Creator instead to properly construct the new object and
read its data. (To more efficient write and read a single Parceable
object, you can directly call Parcelable.writeToParcel and
Parcelable.Creator.createFromParcel yourself.)
Bundles
A special type-safe container, called Bundle, is available for
key/value maps of heterogeneous values. This has many optimizations
for improved performance when reading and writing data, and its
type-safe API avoids difficult to debug type errors when finally
marshalling the data contents into a Parcel. The methods to use are
writeBundle(Bundle), readBundle(), and readBundle(ClassLoader).

Good alternatives to share a complex tree of objects between activities in Android?

This is a question I've now had for a few different apps I've built, and I have yet to be satisified with any of the solutions I've come up with. I thought I'd put it out there to the community to see other solutions there might be.
Let's say you have an Activity that downloads a complex tree of data (in this case via json, but it could be anything), unmarshalls that data to a set of java objects (in this case using gson, but again, could be whatever), then spawns additional activities to view different parts of that data. There might be one activity to view Trips in your response, and another to view Flights in those trips, and maybe another to view Passengers of those flights.
My initial implementation of this app was to unmarshall all the Trips in the first activity, then pass them by value (as an extra in the intent) to the TripActivity. The TripActivity then passes individual flights to the FlightActivity, and so on.
The problem with this is that there's a noticeable pause between activities while the app serializes and deserializes the data. We're talking several seconds. The pause is quite noticeable when my tree uses Serialization or Parcelable to pass data around. Initial performance testing with using google's Parcelable instead show a roughly 30% speedup over serialization, but Parcelable is difficult to work with and doesn't seem to handle circular object references well like Serialization does, and besides it still pauses for almost as many seconds, so I've put that experiment on the backburner while I try other things.
So then I tried moving the tree of objects directly into the Application class. Each activity just gets the tree directly from the app whenever it needs it. This makes performance quite snappy, but handling corner cases like unexpected activity start/stops (either due to activity crashes or because the activity has been closed temporarily to make more memory available, or whatever other cause) seems tricky. Perhaps it's no more than implementing onSaveInstanceState(), I'm not sure, but the solution seems a bit hacky so I haven't investigated further yet.
So in search of a less cobbled-together solution, I tried creating a custom ContentProvider to store and retrieve my objects. Since ContentProviders can be configured to run in-process using multiprocess=true, I thought that would be an excellent way to avoid serialization costs while doing something more "standard" than storing data in the Application object. However, ContentProviders were clearly not intended to return arbitrary object types -- they only support types such as numbers, strings, booleans, etc. It appears I can finagle one to store arbitrary objects by using ContentResolver.getContentProviderClient().getLocalContentProvider() and accessing my custom class directly, but I'm not sure that's less hacky than storing data in the Application object.
Surely someone must have a good solution to this problem. What am I doing wrong?

In addition to fiXedd's solution, another one is to use a local service. Have the service "own" the objects, with activities calling service APIs to get whatever it needs. The service can also be responsible for fetching and parsing the data, encapsulating that bit of logic.
The Application object is the "red-headed step-child" of Android components. Members of the core Android team have come out against the practice of creating custom Application subclasses, though it is certainly supported by the API. Having engineered one ADC2 200 application that leveraged a custom Application subclass, I can say that I should have gone with a service in my case as well. Live and learn...
By using the local binding pattern, your service will automatically be created and destroyed as needed, so you don't have to worry about that. And, by definition, a local service runs in the same process/VM as your activities, so you don't have to worry about marshaling overhead like you would in the ContentProvider scenario.

The way I'm handling this in one of my apps is downloading the data then shoving it into a database. This way I don't have to carry all those objects around (which, IIRC, eat about 1kb each just for the object instantiation) and I can easily pull just the data that I need. I don't know if this will work for you, but it worked for my use-case.

Another approach would be to save the data objects to a shared preferences file. That's how we implemented one of our apps, but I didn't like that approach because it seems too slow.
It's bad coding practice, but the fastest way may be to just use a service to parse the data and save the data to a static class that you can use for the rest of the app's life.