java - Why do I need to synchronize this variable -

i have 4 threads each trying find max value in linked list.

this thread class:

public class mythread extends thread {      linkedlist<integer> list;     int max = integer.min_value;      public mythread(linkedlist<integer> list) {         this.list = list;     }      public void run() {         synchronized (list) {       /* if don't synchronize list, nosuchelementexception @ list.remove() */             while (!list.isempty()) {                 int num = list.remove();                  if (num > max) {                     max = num;                 }             }         }     } } 

and here class main method:

public class application {      public static void main(string args[]) throws interruptedexception {         linkedlist<integer> list = new linkedlist<integer>();          (int = 0; < 10; i++) {             list.add(i);         }          mythread t1 = new mythread(list);         mythread t2 = new mythread(list);         mythread t3 = new mythread(list);         mythread t4 = new mythread(list);          t1.start();         t2.start();         t3.start();         t4.start();          t1.join();         t2.join();         t3.join();         t4.join();          system.out.println(t1.max);         system.out.println(t2.max);         system.out.println(t3.max);         system.out.println(t4.max);     } } 

in above code, have synchronize list variable within run method or else i'll nosuchelementexception @ list.remove(). why case?

doesn't each thread have it's own list there no thread interference?

thanks

i address different part of question @rishi addressed:

doesn't each thread have it's own list there no thread interference?

the simple answer is: no, not. in java, when pass object of class type constructor or method, aren't passing obejct rather pointer it. if want pass separate copy of linked list each thread, need use linkedlist#clone.

if use clone, when thread removes 1 integer linked list, not removed other linked lists. paralellize this, should use standard array of numbers , assign segment of array each thread (ie. thread 1 0-9, thread 2 10-19, thread 3 20-29, etc.). array's contents visible threads created after contents deposited in array.

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should note should not extend thread. instead, extend runnable , pass thread. furthermore, array(list) better 4 separate variables allows change number of threads later.