save images in specific folders(Android)

Good evening,

I made a program for downloading images, the problem is I can not save them in an orderly way. when I have an image is no problem with the order, but my idea is to deal with over 300 different images and I want a quick program.

I want to create folders with different categories.

However although I get to do what I want in java, I fail in android.

What I can do?

this code work:

PImage a;
void setup(){ a = loadImage("http://img110.xooimage.com/files/4/5/b/mini-robot-4a0c8b8.png");
if(a != null){a.loadPixels();
a.save("aadfg.png");
}}
void draw(){if(a != null){a.loadPixels();image(a,20,20);}}

but, this code don´t work:

PImage a;
void setup(){ a = loadImage("http://img110.xooimage.com/files/4/5/b/mini-robot-4a0c8b8.png");
if(a != null){a.loadPixels();
a.save("folder/aadfg.png");
}}
void draw(){if(a != null){a.loadPixels();image(a,20,20);}}

this is the difference:

a.save("aadfg.png");
**||**
a.save("folder/aadfg.png");

regards and thanks in advance.