in short y'don't
once the last reference to that object is broken the garbage collector will, when it needs to or can do will clear up after you.
so providing you dereference the object as soon as that portion of code has ceased using the object garbage collection should happen
you requet the JVM to do a garbage collection through Runtime.gc() but all that does is suggest to the JVM that it should do a garbage collection.
if you find you NEED to run the collector then it may be a design issue with your code, especially if you are doing a lot of string manipulation