How do I override an Android Java class in Unity?

I’m trying to use Picasso (Picasso) to load images.

To get Picasso to return a bitmap, instead of using a passed in view, this must be done in native code:

    .into(new Target() {
        public void onBitmapLoaded (final Bitmap bitmap, Picasso.LoadedFrom from){
            /* Save the bitmap or do something with it here */

I’m trying to call it from Unity so I can simply include the Picasso JAR file. Can it be done somehow?

I don’t believe you can. Unity only supports C# and Javascript (and it’s own built-in language). I don’t think you can import or use Java libraries with Unity. You would need to find a C# equivalent or write your own in C#. Luckily, the syntax differences between C# and Java are pretty minor, so if you know one, learning the other is pretty easy.