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According to Android Reference Document of Bitmap.recycle():
Free the native object associated with this bitmap, and clear the reference to the pixel data. This will not free the pixel data synchronously; it simply allows it to be garbage collected if there are no other references. The bitmap is marked as "dead", meaning it will throw an exception if getPixels() or setPixels() is called, and will draw nothing. This operation cannot be reversed, so it should only be called if you are sure there are no further uses for the bitmap. This is an advanced call, and normally need not be called, since the normal GC process will free up this memory when there are no more references to this bitmap.
But, many books I read suggest to free memory by calling Bitmap.recycle() once make sure no longer need it.
It make me confused: Is it needed to call Bitmap.recycle() after used?
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The first bitmap is not garbage collected when you decode the second one. Garbage Collector will do it later whenever it decides. If you want to free memory ASAP you should call recycle() just before decoding the second bitmap.
Choose the most appropriate decode method based on your image data source. These methods attempt to allocate memory for the constructed bitmap and therefore can easily result in an OutOfMemory exception. Each type of decode method has additional signatures that let you specify decoding options via the BitmapFactory.
Bitmap pooling is a simple technique (though fairly complex to implement), that aims to reuse bitmaps instead of creating new ones every time. To put it simply, when you need a bitmap, you check a bitmap stack to see if there are any bitmaps available.
It depends.
If you run your app on Android 3.0 and above, it's not needed as the GC will take care of it perfectly.
However, if you run your app on older versions, since bitmaps don't get monitored well by the GC (it thinks they are the size of a reference), you could get OOM, as shown on Google IO lecture here.
In any case, it's still recommended to call recycle as soon as you are sure you don't need the bitmap anymore. It's good even for new android versions, since it lowers the work needed for automatic memory management...
In fact, I remember I've asked a similar question here.
Also, if you need extra control of bitmaps using JNI, check out this post.
So, in short, the answer is that it's not needed anymore, but still recommended.
EDIT: Ever since Android 8.0, Bitmaps are stored in native memory, so it's harder to reach OOM. In fact, it's technically impossible, as you will get into other issues instead. More information about this can be found here.
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In my experience, we run a heavy Bitmap compression in production code, and without calling recycle() we run into many OOM Exceptions in old Lollypop devices, and after adding it to the code, the number of OOM reduced significantly.
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