why is CATEGORY_OPENABLE used for in android -

here have written code image picker gallery, can tell me roll of setaction() , addcategory() ?

there lots of "static final string" available in intent class file, totally unaware of using these action , category parameters in program

public class imagepicker extends baseactivity implements view.onclicklistener {    private final int pick_from_gallery_request = 1;   private imageview pickedimage;    @override   protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     super.onstart();     setcontentview(r.layout.activity_image_picker);     pickedimage= (imageview) findviewbyid(r.id.image);     button camerabutton= (button) findviewbyid(r.id.pick_from_camera);     button gallerybutton= (button) findviewbyid(r.id.pick_from_gallery);     camerabutton.setonclicklistener(this);     gallerybutton.setonclicklistener(this);     setviewheight(pickedimage);   }    private void setviewheight(imageview pickedimage) {     displaymetrics displaymetrics=getresources().getdisplaymetrics();     pickedimage.getlayoutparams().height=displaymetrics.heightpixels/2;   }     @override   protected void onactivityresult(int requestcode, int resultcode, intent data) {     super.onactivityresult(requestcode, resultcode, data);     if(requestcode==pick_from_gallery_request && resultcode==result_ok && data!=null){         inputstream stream = null;         try {             stream = getcontentresolver().openinputstream(                     data.getdata());             bitmap bitmap = bitmapfactory.decodestream(stream);             stream.close();             pickedimage.setimagebitmap(bitmap);         } catch (filenotfoundexception e) {             e.printstacktrace();         } catch (ioexception e) {             e.printstacktrace();         }     }   }    @override   public void onclick(view v) {     switch (v.getid()){         case r.id.pick_from_gallery:             intent intent = new intent();             intent.settype("image/*");             intent.setaction(intent.action_get_content);             intent.addcategory(intent.category_openable);             startactivityforresult(intent, pick_from_gallery_request);             break;         case r.id.pick_from_camera:              break;     }   }