Android :: Try To Learn Java For Phone Devices / Update Function Not Working

I try to learn java for android devices. i have to create the update function. But still have one question: How?

in class root

public void update(){maindebug("update"); // This is my debug function}
public void run(){ while(isRunning){ // isRunning is a boolean variable
SystemClock.sleep(100);
update();}}

and inside onCreate run(); but it doesnt work.