title: caching of java.lang.Double
body: <p>I have lots of Doubles in my program (notice the capital D) and I was thinking of a possible caching mechanism.</p>
  
  <p>By looking at the Double constructors / factories, none is actually implementing a "cache" (like the way Integer, works, for example when using <code>valueOf</code>).</p>
  
  <p>I was thinking to implement such a cache by using a <code>HashMap&lt;String,Double&gt;</code> where the key is the string representation. </p>
  
  <p>Any visible drawbacks / cons of such implementation?</p>
  
  <p>I wonder why this wasn't done or provided yet by Java.</p>