java - Set non-primitive types when returning from thread -

my question is: why setting non-primitive type work when returning threads ?

the following works:

final int[] newtask = new int[1];     try{         thread thread = new thread(new runnable(){              @override             public void run(){                 newtask[0] = somemethod();                 return;             }         });         thread.start();         thread.sleep(3000);         if(thread.isalive()){             thread.interrupt();             newtask[0] = null;         }     }catch(interruptedexception ie){         log.error("timeout", ie);     } 

the following doesn't:

final int newtask;     try{         thread thread = new thread(new runnable(){              @override             public void run(){                 newtask = somemethod();                 return;             }         });         thread.start();         thread.sleep(3000);         if(thread.isalive()){             thread.interrupt();             newtask = null;         }     }catch(interruptedexception ie){         log.error("timeout", ie);     } 

non-primitives variables work when returning thread primitives don't. why ?

in first case, newtask reference array. reference cannot changed, because final. content of array can modified, because array not immutable.

in second case, newtask primitive value. can't changed, because final.

a simpler example of same behavior this:

final stringbuilder buf = new stringbuilder(); buf.append('x'); /* modified mutable object; no problem. */ buf = new stringbuilder(); /* compiler error: can't reassign final var */ 

it has nothing threads, or differences between primitive values , reference types.