java - How does a thread know that there is a join method ahead -

below sample code, when a.start() called should create thread , print "run" immediately. why called after printing "begin" 20 times.

how thread "a" decide doesn't have call run() immediately.

public class jointest implements runnable {      public static void main(string[] args) throws interruptedexception {         thread = new thread(new jointest());         a.start();          (int = 0; < 20; i++) {             system.out.print("begin");         }         thread.sleep(1000);          a.join();         system.out.print("\nend");     }      public void run() {         system.out.print("\nrun");     } } 

output:

beginbeginbeginbeginbeginbeginbeginbeginbeginbeginbeginbeginbeginbeginbeginbeginbeginbeginbeginbegin run end 

i little confused behavior of thread.

in opinion "run" should printed before "begin" because printed before join() method called, , @ time of join method called thread "a" must have finished execution , calling join must useless @ point.

calling start() on thread doesn't triggers execution of run() method immediately. thread marked started main thread pursues execution for loop. jvm switching thread once main thread reaches sleep() statement.