Possible to Divert/Forward Functions in Android?

Assalam wa alaikum! Greetings!

I just wanted to know if there is any way to divert/forward a system class's methods. Means to change it's function while in app.

Example: One of my Jar libraries is using java.io.File 's method length() several times. I want to edit/code the length() (like some kind of inheritance but not really) and make it cache the length so speeds up the process.

Note: It's just an example

Maybe some kind of reflection am expecting. Thanks in advance :-D