java - Why is 'create' asynchronous? -

i told, creating new instance async message; don't understand why.

e.g.:

foo myfoo = new foo(); 

here have wait until constructor finishes , returns new object. doesn't asynchronous mean, go on independently (no waiting) - starting thread?

i told, creating new instance async message;

sorry, have either heard wrong or told wrong. first off, should terminology straight. term "async" or "asynchronous" means invocation returns immediately caller. can demonstrate not true constructor, simple experiment [1]. in other words, constructor must return caller make progress.

starting thread indeed asynchronous. call thread.start() returns , @ later point in time thread starts running , executing run() method.

1 experiment

consider class (for illustration only) below:

foo.java

class foo {     foo() throws interruptedexception {         while (true) {             system.out.println("not returning yet ...");             thread.sleep(2000);         }     }     public static void main(string[] args) throws interruptedexception {          foo foo = new foo();     } } 

if compiled , run class (i used java 8 on mac, not requirement). expected, class runs forever producing output every 2 seconds:

not returning yet ... not returning yet ... not returning yet ... not returning yet ... 

note sleep call added make bearable. try experiment without it, program overwhelm 1 of cpu's stressing 100%.

if, while running, took thread dump (for example, using command jstack), see below (curtailed brevity):

"main" #1 prio=5 os_prio=31 tid=0x00007f9522803000 nid=0xf07  waiting on condition [0x000000010408f000]    java.lang.thread.state: timed_waiting (sleeping)     @ java.lang.thread.sleep(native method)     @ foo.<init>(foo.java:5)     @ foo.main(foo.java:9) 

regardless of state of thread (runnable, blocked, waiting, timed_waiting), see (take various thread dumps see means) see these 2 lines:

    @ foo.<init>(foo.java:5)     @ foo.main(foo.java:9) 

which means caller (in case, main thread) never make progress. , since constructor never returns, no progress happens.