Generally each thread will have its own resource or data to action. In this case there is no possibility for data inconsistency. Somtimes two threads may act on one resource of data and in this case data inconsistency may occur. To overcome this problem java introduces synchronization concept. In synchronization only one thread allowed to operate on the resource when a thread access a synchronized data it calls wait method it is a indication to the thread schedulor not to allow any thread to access the same resource until the first thread job is completed. "synchronized" is a keyword which can be used to tell through method. We can make a method as synchronized and block as synchronized. We can use synchronized keyword to avoid inconsistency.

There are two types of variables.
1. local variables
2. static or instance variables.

local valiables like method parameters are stored in stacks and instance are stored in Heap. You are not bothered to take care for any local things for synchronizations as that remains in thread's address. So two threads can never spoil the state that is in stack. So only thing that is in heap should be considered for synchronization. When you need some code to be atomic keep that in synchronized block. Like if you want to set r g b colors in a method keep them in asynchronized block as the possibility is another thread will get partialy written data.