Re: Reuse of argument objects

Eric Sosman <esosman@comcast-dot-net.invalid>
Thu, 24 Apr 2014 09:50:29 -0400
On 4/24/2014 8:30 AM, Stefan Ram wrote:

   For example, in Android, there is this interface specification:

public void setLayoutParams (ViewGroup.LayoutParams params)
   Set the layout parameters associated with this view. ...
     params The layout parameters for this view, ...

   . An example usage:

final android.widget.LinearLayout.LayoutParams parameters
= new android.widget.LinearLayout.LayoutParams
( android.view.ViewGroup.LayoutParams.MATCH_PARENT,
   android.view.ViewGroup.LayoutParams.MATCH_PARENT, 1f );

Main.this.increment.setLayoutParams( parameters );

   I think there is something missing in the documentation:

   Can the ?parameters? object be reused to supply the same
   parameter values to other calls of ?setLayoutParams? with
   other target objects? Or even with other parameter values?

   Has setLayoutParams made its own copy of the parameter values,
   or has it just stored a reference to the parameters object?

   This is like a kind of ownership: Who owns ?parameters?
   after the call to ?setLayoutParams?? The caller or the callee?

     If LayoutParams is immutable the question needn't be answered,
otherwise it ought to be. If it's mutable and the documentation
doesn't provide an answer, I think you've got two choices:

     1) Look at the code for the "consumers" of LayoutParams and
        see what they do with it. Risky, of course, because the
        next release might change things.

     2) Use a fresh LayoutParams instance for every call (a copy
        constructor or copying factory or clone() would be helpful).
        Possibly wasteful, but safe. (Make your copy *before* giving
        the original to a consumer.)

   Or is there a common standard in the Java world for this?

     I resolutely and nobly resist the impulse to remark snarkily
upon your respect for "common standards" in coding style. ;-)

Eric Sosman

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