Do private methods increase Dex Count in Android?

Question

I was doing code review and told someone to remove a private method that was only used once. They said that it didn't matter since dex count wouldn't get increased by private method references. Is this true? I wasn't able to find an answer with a simple google search.

Answer 1

The 64k limit is a limit on the number of unique method references in a dex file. A method reference consists of a specific class name, the method name and the method prototype, and is created when you either invoke a method or declare/define/override a method.

So yes, defining a new private method will add a method reference to the dex file.

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For more information, see: https://source.android.com/devices/tech/dalvik/dex-format.html and https://source.android.com/devices/tech/dalvik/dalvik-bytecode.html, which are the main references for the dex format.

The "method reference list" is a sorted list of method_id_items in the dex file. e.g. look for "method_ids" in the "File layout" section of dex-format.html. And further down the page, method_id_item is defined as consisting of a class reference, a method name and a method prototype.

The class_data_item section is used to define the methods and fields that are defined by the class. The "direct_methods" and "virtual_methods" lists are lists of indexes into the method_ids list - which requires that a reference for that method exists in the method_ids list.

And in dalvik-bytecode.html, the invoke-* instructions use a method index to refer the method to invoke.

Interestingly, the method reference list itself is defined with a 32-bit size value (search for "method_ids_size" in dex-format.html). So the list of method references itself can be as large as 4294967296 entries.

However, the problem comes when you need to reference any of these methods. The invoke-* instructions only use 16 bits to encode the method index.

Additionally, the method references in the class_data item can be up to the full 32 bits. So you could theoretically have definitions of methods past the 64k limit in a dex file, as long as you never actually tried to invoke them from within that dex file. But they could still be invoked from another dex file.