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POLearning Java, use of synchronized keyword
 

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  1. POLearning Java, use of synchronized keyword
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    <p>so i was testing with <code>synchronized</code> keyword. Here is an example that I tried: </p> <pre><code>public class MyTest { static int i = 0; public static void main(String[] args) { new Thread(t1).start(); new Thread(t2).start(); } private static void countMe(String name){ i++; System.out.println("Current Counter is: " + i + ", updated by: " + name); } private static Runnable t1 = new Runnable() { public void run() { try{ for(int i=0; i&lt;5; i++){ countMe("t1"); } } catch (Exception e){} } }; private static Runnable t2 = new Runnable() { public void run() { try{ for(int i=0; i&lt;5; i++){ countMe("t2"); } } catch (Exception e){} } }; } </code></pre> <p>When I run it, the output of calling <code>countMe()</code> method from two threads generates this output: </p> <pre><code>Current Counter is: 1 Current Counter is: 2 Current Counter is: 4 Current Counter is: 5 Current Counter is: 6 Current Counter is: 7 Current Counter is: 3 Current Counter is: 8 Current Counter is: 9 Current Counter is: 10 </code></pre> <p>And when I change the method <code>countMe()</code> to: </p> <pre><code>private synchronized static void countMe(){ i++; System.out.println("Current Counter is: " + i); } </code></pre> <p>I get this output: </p> <pre><code>Current Counter is: 1 Current Counter is: 2 Current Counter is: 3 Current Counter is: 4 Current Counter is: 5 Current Counter is: 6 Current Counter is: 7 Current Counter is: 8 Current Counter is: 9 Current Counter is: 10 </code></pre> <p>Although this gives me clear understanding the purpose of <code>synchronized</code>, I want to know is there any other reason as well, that we can use <code>synchronized</code>. Or what I have done here, is the only eason why we need the use of this <code>synchronized</code> keyword? </p> <p>Thanks. </p> <p><strong>EDIT:</strong> Another thing that I am confused with is that in first output why the counter went to 3 after 7. It seems a bit impossible to me, but similar results do happen every time I try, is this normal?</p>
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    1. COThere is no guarantee the threads will alternate. The thread which counted 3 may not print until after the other thread has counted to 7.
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      1. POLearning Java, use of synchronized keyword
      1. USPeter Lawrey
    2. COthanks, but like after 7 how can the next call print 3. I mean it could print 6 or even5, but this seems like the new method call did not read i after it was 3, this sounds very strange. So you think it should be wise to always use synchronized whenever i have to use same block of code from 2 or more threads??
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    3. COI suggest you print out the thread's name and it will become more obvious as to where the confusion is. Concurrency is a complex topic and there are no simple answers. My preference is to use actor patterns and share as little data between threads as possible. ideally, none. One thread performing code very efficiently is usually more than enough.
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