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POWhere am I not Threadsafe?
 

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    <p>I'm using an object, which serves roughly as a wrapper for a map of Strings:</p> <pre><code>public class Wrapper { private Map&lt;String, String&gt; values; private Formatter formatter; public BigDecimal getSpecialValue() { String result = values.get("Special"); return formatter.formatNumber(result); } } </code></pre> <p>The abovementioned formatter serves roughly as a mapper for a SimpleDateFormat</p> <pre><code> public class Formatter { private static final NumberFormat NUMBER_FORMAT; public BigDecimal formatNumber(String s) { Number num = NUMBER_FORMAT.parse(s); if (num instanceof Integer) { return new BigDecimal((Integer) num); } else if (num instanceof Double) { return new BigDecimal((Double) num); } ... } } </code></pre> <p>When I access the <code>getSpecialValue()</code> method by several threads at once, some behaviour accurs, which can only be explained by the concurrent access, for example there may be a <code>NumberFormatException</code> or a <code>ParseException</code> where the parsed string is .430.430 instead of .430 and so on.</p> <p>There are two aspects which arouse my interest: 1.) The wrapper is only accessed read-only. Although access to the collection is not synchronized, I was under the impression that this should always work. 2.) In one of the first attempts to find the problem I changed the constructor of the <code>Wrapper</code> class to execute the <code>formatNumber</code> method (obviously single threaded), which eliminated all Exceptions down the execution.</p> <p>Can someone explain?</p> <p>Edit: The map in the <code>Wrapper</code> class is filled in the constructor, which most definitly happens single threaded. Afterwards the wrapper is designed such that the map is immutable.</p>
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