在android中分配对象以从一个活动转移到另一个活动

Question

Recently an interviewer asked me a very tricky question. There are several parts of the question.

1. Why (question is why and not how) do you need to parcel objects while sending from one activity to another and not send directly

Answer I gave -

Parcelable gives the capability to developers to restrict object creation which in a way makes it faster to use.

I was confused on the part, so decided to site difference between using serializable and parcelable :p (clever huuuhhh !), http://www.developerphil.com/parcelable-vs-serializable/ used this reference.

2. While using Bundle, when we use String, int we do not need to parcel the data, so do you think the String/int is by default internally parcelled ?

Answer I gave -

because String/int is a primitive data-type, if we had used the Wrapper class directly, might be possible we had to use parcelable(I am not sure on that part)

I did not get any useful link after googling, also I or the interviewer is not quite satisfied with the answer. If you guys can help, would be wonderful !

Answer 1

Why (question is why and not how) do you need to parcel objects while sending from one activity to another and not send directly

Parcelling/serializing objects isn't for speed as you had guessed.

When you're sending data between Activities, and especially between different applications (remember that Intent objects aren't only meant for communication between your own Activities, but are also for between yours and those of other apps as well), you cannot expect the sender and the receiver to have access to the same memory address spaces.

Android's documentation states that applications run in their own discrete memory spaces. Here's a quote to that effect from the docs:

Each process has its own virtual machine (VM), so an app's code runs in isolation from other apps.

So when you want to send an object myObject to some receiving Activity , you can't send its reference/pointer because the receiver won't necessarily have access to the location specified by the pointer. Instead you'll have to send some representation of myObject that the receiver can access and use -- this is why you need to marshall the data into a form that can be unmarshalled , and the easiest way to do so is to simply have the class of the object implement Serializable which lets Java do its best to convert the object into an array of bytes that can be easily sent to and unmarshalled by the receiver. But since Serializable uses reflection, this is slow.

You can use other ways that are faster to marshall the data -- one, for example, is converting the object into its JSON representation using a library like Gson and just sending it across since any JSON document can be represented as a String and easily converted back to a Java Object . Another way, which is probably faster in pretty much all cases is using the Parcelable interface which lets you specify exactly how you want to marshall the data and exactly how it should be unmarshalled. It basically gives you more control on the transmission of the object.

The tl:dr : Parcelling/Serializing etc is used because you can't send memory addresses across, so you have to send the actual data of the object and it has to be represented in some form.

While using Bundle, when we use String, int we do not need to parcel the data, so do you think the String/int is by default internally parcelled ?

How Bundle works internally is that it puts everything into a Map and parcels/unparcels the data as needed (ie when get/put is called). For putting Objects into a Bundle , the object's class needs to implement Serializable or Parcelable because it needs to tell the Bundle how it should be marshalled/unmarshalled internally.

But primitive types and Strings are simple enough and used often enough that the developer doesn't need to specify how that needs to happen and Bundle provides convenience methods for it. I can't give you a solid answer at the lowest level of how they works because a lot of the Parcel code is natively implemented and I couldn't find it online, but they must certainly be straightforward to convert to their representation in bytes.

Answer 2

Just to add what @uj- said, Parcelling/Serializing is needed as @uj- said it will be sent across JVMs so they need to be converted into some format so that the other party will be able to understand.

Let me take an example to explain why serializing/parcelling is needed,

you are sending data from an application written in "C++" to an application written in java, so the following are the classes,

In C++,

class Android {
    public: int dataToSend; //for example purpose making field public and omitting setter/getters
}

In Java,

class Android{
    public int dataToSend;
}

suppose the C++ code generates dynamic library (which will be generated by compiling using the standard C++ compiler and then linked), and Java code generates a jar (by compiling using the javac).

When the C++ application sends data (object of Android class) to the java application the way it is compiled and linked in C++ is completely different as compared to the way its compiled in java and hence java will be wondering what has this C++ application sent to me.

Hence to get rid of such problems serialisation/parcelling is needed which will make sure that both of the application know how the data is converting while transmitting through network (in case of android how it is transmitted to another activity, may be in same or different application).

And yea when we start comparing Serialisation and Parcelling, Parcelling gets the upper hand as we will be specifying the way the data must be converted when sending the data, else in the case of serialisation the object is converted to string using reflection and reflection always takes time. Hence Parcelling is faster compared to Serialisation.

For your second question,

if we consider the above example itself then we can say that String and int being primitive types (no user defined fields in them) and hence android will be able to handle the marshalling and unmarshalling of the data which will be sent.

I tried going through the code when we go on digging deeper we end up getting native code as said by @uj-.

Some extract from the android source code: while writing the parcel:

parcel.writeInt(BUNDLE_MAGIC);
int startPos = parcel.dataPosition();
parcel.writeArrayMapInternal(mMap);
int endPos = parcel.dataPosition();

parcel.setDataPosition(lengthPos);
int length = endPos - startPos;
parcel.writeInt(length);
parcel.setDataPosition(endPos);

while reading the parcel,

int magic = parcel.readInt();
if (magic != BUNDLE_MAGIC) {
    //noinspection ThrowableInstanceNeverThrown
    throw new IllegalStateException("Bad magic number for Bundle: 0x"
    + Integer.toHexString(magic));
}

int offset = parcel.dataPosition();
parcel.setDataPosition(offset + length);

Parcel p = Parcel.obtain();
p.setDataPosition(0);
p.appendFrom(parcel, offset, length);
p.setDataPosition(0);

mParcelledData = p;

set the magic number which will identify the start of the parcel while writing and the same will be used while we read the parcel.

Hope I answered your question.