Android API Design – Why Bundle API Accepts ArrayLists

androidandroid-developmentapi-designjava

I'm new to Android and while going through a tutorial on saving Activity state to a Bundle, I noticed that instead of accepting the more generic List interface, Bundle's put methods are expecting ArrayLists.

Example:

Bundle.putCharSequenceArrayList(key, value)
Bundle.putIntegerArrayList(key, value)
Bundle.putParcelableArrayList(key, value)
Bundle.putStringArrayList(key, value)

Most of us are familiar with item 52 of Effective Java suggesting that objects must be refered to by their interface, so I am wondering what was the reason behind this API decision.

Is ArrayList perhaps the preferred list implementation in Android?

Best Answer

Just a guess: Maybe Bundele-content must be be serializable and not every Lists implementation is serializable.

From sdk-doc public Bundle.Bundle (ClassLoader)

  Constructs a new, empty Bundle that uses a specific 
  ClassLoader for instantiating Parcelable and Serializable objects.

TL;DR

In Java, the reason of public static void main(String[] args) is that

Gosling wanted
the code written by someone experienced in C (not in Java)
to be executed by someone used to running PostScript on NeWS

For C#, the reasoning is transitively similar so to speak. Language designers kept the program entry point syntax familiar for programmers coming from Java. As C# architect Anders Hejlsberg puts it,

...our approach with C# has simply been to offer an alternative... to Java programmers...

Long version

expanding above and backed up with boring references.

java Terminator Hasta la vista Baby!

VM Spec, 2.17.1 Virtual Machine Start-up

...The manner in which the initial class is specified to the Java virtual machine is beyond the scope of this specification, but it is typical, in host environments that use command lines, for the fully qualified name of the class to be specified as a command-line argument and for subsequent command-line arguments to be used as strings to be provided as the argument to the method main. For example, using Sun's Java 2 SDK for Solaris, the command line

java Terminator Hasta la vista Baby!

will start a Java virtual machine by invoking the method main of class Terminator (a class in an unnamed package) and passing it an array containing the four strings "Hasta", "la", "vista", and "Baby!"...

...see also: Appendix: I need your clothes, your boots and your motorcycle

My interpretation:
execution targeted for use like typical scripts in command line interface.

important sidestep

...that helps avoid a couple of false traces in our investigation.

VM Spec, 1.2 The Java Virtual Machine

The Java virtual machine knows nothing of the Java programming language...

^{I noticed above when studying prior chapter - 1.1 History which I thought could be helpful (but turned out useless).}

My interpretation:
execution is governed by VM spec alone, which
explicitly declares that it has nothing to do with Java language
=> OK to ignore JLS and anything Java language related at all

Gosling: a compromise between C and scripting language...

Based on above, I began searching the web for JVM history. Didn't help, too much garbage in results.

Then, I recalled legends about Gosling and narrowed down my search to Gosling JVM history.

Eureka! How The JVM Spec Came To Be

In this keynote from the JVM Languages Summit 2008, James Gosling discusses... Java's creation,... a compromise between C and scripting language...

My interpretation:
explicit declaration that at the moment of creation,
C and scripting have been considered most important influences.

Already seen nod to scripting in VM Spec 2.17.1,
command line arguments sufficiently explain String[] args
but static and main aren't there yet, need to dig further...

^{Note while typing this - connecting C, scripting and VM Spec 1.2 with its nothing-of-Java - I feel like something familiar, something... object oriented is slowly passing away. Take my hand and keep movin' Don't slow down we're nearly there now}

Keynote slides are available online: 20_Gosling_keynote.pdf, quite convenient for copying key points.

    page 3
    
        The Prehistory of Java
        * What shaped my thinking
    
    page 9
    
        NeWS
        * Networked Extensible Window System
        * A window system based on scripting....
          PostScript (!!)
    
    page 16
    
        A Big (but quiet) Goal:
          How close could I get to a
          "scripting" feel...
    
    page 19
    
        The original concept
        * Was all about building
          networks of things,
          orchestrated by a scripting
          language
        * (Unix shells, AppleScript, ...)
    
    page 20
    
        A Wolf in Sheeps Clothing
        * C syntax to make developers
          comfortable

A-ha! Let's look closer at C syntax.

The "hello, world" example...
main()
{
    printf("hello, world\n");
}
...a function named main is being defined. The main function serves a special purpose in C programs; the run-time environment calls the main function to begin program execution.

...The main function actually has two arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments...

Are we getting closer? you bet. It is also worth following "main" link from above quote:

the main function is where a program starts execution. It is responsible for the high-level organization of the program's functionality, and typically has access to the command arguments given to the program when it was executed.

My interpretation:
To be comfortable for C developer, program entry point has to be main.
Also, since Java requires any method to be in class, Class.main is
as close as it gets: static invocation, just class name and dot,
no constructors please - C knows nothing like that.

This also transitively applies to C#, taking into account
the idea of easy migration to it from Java.

^{Readers thinking that familiar program entry point doesn't matter are kindly invited to search and check Stack Overflow questions where guys coming from Java SE are trying to write Hello World for Java ME MIDP. Note MIDP entry point has no main nor static.}

Conclusion

Based on above I would say that static, main and String[] args were at the moments of Java and C# creation most reasonable choices to define program entry point.

Appendix: I need your clothes, your boots and your motorcycle

Have to admit, reading VM Spec 2.17.1 was enormous fun.

...the command line

java Terminator Hasta la vista Baby!

will start a Java virtual machine by invoking the method main of class Terminator (a class in an unnamed package) and passing it an array containing the four strings "Hasta", "la", "vista", and "Baby!".

We now outline the steps the virtual machine may take to execute Terminator, as an example of the loading, linking, and initialization processes that are described further in later sections.

The initial attempt... discovers that the class Terminator is not loaded...

After Terminator is loaded, it must be initialized before main can be invoked, and a type (class or interface) must always be linked before it is initialized. Linking (§2.17.3) involves verification, preparation, and (optionally) resolution...

Verification (§2.17.3) checks that the loaded representation of Terminator is well formed...

Resolution (§2.17.3) is the process of checking symbolic references from class Terminator...

Symbolic references from Terminator oh yeah.

Java Design – Why Use Interface Instead of Implementation for Java Objects

The reason is that the implementation of these interfaces is usually not relevant when handling them, therefore if you oblige the caller to pass a HashMap to a method, then you're essentially obliging which implementation to use. So as a general rule, you're supposed to handle its interface rather than the actual implementation and avoid the pain and suffering which might result in having to change all method signatures using HashMap when you decide you need to use LinkedHashMap instead.

It should be said that there are exceptions to this when implementation is relevant. If you need a map when order is important, then you can require a TreeMap or a LinkedHashMap to be passed, or better still SortedMap which doesn't specify a specific implementation. This obliges the caller to necessarily pass a certain type of implementation of Map and strongly hints that order is important. That said, could you override SortedMap and pass an unsorted one? Yes, of course, however expect bad things to happen as a result.

However best practice still dictates that if it isn't important, you shouldn't use specific implementations. This is true in general. If you're dealing with Dog and Cat which derive from Animal, in order to make best use of inheritance, you should generally avoid having methods specific to Dog or Cat. Rather all methods in Dog or Cat should override methods in Animal and it will save you trouble in the long run.