Should you synchronize the run method? Why or why not?

Question

I have always thought that synchronizing the run method in a java class which implements Runnable is redundant. I am trying to figure out why people do this:

Answer 1

actually it is really easy to justify "sync or not to sync"

if your method invocation can mutate the internal state of your object, then "sync" otherwise no need

simple example

public class Counter {

  private int count = 0; 

  public void incr() {
    count++;
  }

  public int getCount() {
    return count;
  }
}

in the example above, incr() needs to be synced, since it will change the val of count, while getCount() synchronization is not necessary

however there is another corner case, if the count is java.lang.Long, Double, Object then you need to declare as

private volatile long count = 0;

to make sure the ref update is atomic

basically that's what you need to think about 99% of time when dealing with multithreading