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POHow do you return one instance of a variable that has been edited in 4 different Threads?
 

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  1. POHow do you return one instance of a variable that has been edited in 4 different Threads?
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    <p>OKAY, I'm sorry for the Title. I really didn't know how to phrase this. I'll try better here.</p> <p>So, I have 2 Java classes. We'll call them <strong>FirstClass</strong> and <strong>SecondClass</strong> (which implemts Runnable). In <strong>FirstClass</strong>, I'm doing some stuff and then I'm creating 4 threads. </p> <pre><code>Thread t1 = new Thread (new SecondClass(s1)); Thread t2 = new Thread (new SecondClass(s2)); Thread t3 = new Thread (new SecondClass(s3)); Thread t4 = new Thread (new SecondClass(s4)); </code></pre> <p><strong>s1</strong>, <strong>s2</strong>, <strong>s3</strong> and <strong>s4</strong> are all of type <em>String</em> and hold individual values.</p> <p>Then I start the threads straight away. </p> <pre><code>t1.start(); t2.start(); t3.start(); t4.start(); </code></pre> <p>Then in my <strong>SecondClass</strong>, I am taking these strings in the default constructor as follows</p> <pre><code>HashMap&lt;String, Integer&gt; map; public SearchResults(String s) { map.put(s, 0); } </code></pre> <p>in the run() method I am doing the following</p> <pre><code>public void run() { try { System.out.println(map); } catch (Exception e) { // TODO: handle exception } } </code></pre> <p>So the result of this useless program is that <strong>map</strong> is printed out 4 times with 4 different values. </p> <p>I am wondering how I can return one instance of <strong>map</strong> that has all the values that <strong>t1</strong> put into it and all the values that <strong>t2</strong> put into it etc etc. They are all working off the same variable, <strong>map</strong>, but each Thread does it's own thing it seems. </p> <p>Could I maybe, let the threads execute and then when they are all finished, return the <strong>map</strong> to another class or something? I really don't know much about Threads so this has been confusing me. Any help would be greatly appreciated.</p>
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    1. COResearch the keyword "static" and the type ConcurrentHashMap.
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    2. COHave you think about TreadLocal variable. [In this question](http://stackoverflow.com/questions/1490919/purpose-of-threadlocal) [or this one](http://stackoverflow.com/questions/817856/when-and-how-should-i-use-a-threadlocal-variable)
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