Consider a scenario when two thread (Thread1 and Thread2) are accessing same variable 'mObject' with value 1.

when a Thread1 runs, it doesn't expect other threads to modify the variable 'mObject'. In this scenario the Thread1 caches the variable 'mObject' with value 1.

And if the Thread2 modify the value of 'mObject' to 2, still the Thread1 would be refering the mObject value as 1 since it did caching. To avoid this caching we should to declare the variable as

private volatile int mObject;

in this scenarion the Thread1 will be getting updated value of mObject