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    <p>No new objects are created in the system when you use explicit casting (except in your last case, where you cast a primitive type to an <a href="http://en.wikipedia.org/wiki/Primitive_wrapper_class" rel="nofollow noreferrer">object wrapper</a>, since <code>double</code> is not an object like <code>Double</code> is). Note that this explicit cast isn't necessary due to Java's <a href="http://docs.oracle.com/javase/tutorial/java/data/autoboxing.html" rel="nofollow noreferrer">autoboxing feature</a>.</p> <p>In your <code>(Child) new Object()</code> scenario, you will receive a <a href="http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/ClassCastException.html" rel="nofollow noreferrer">ClassCastException</a> because an <code>Object</code> is not a <code>Child</code> (although the opposite is true).</p> <p>The answer to your first scenario is the most complicated. Essentially the parent class is treated like an interface might be. When you cast the <code>Child</code> to the <code>Parent</code>, only the <code>Parent</code> API is available. However, <a href="http://docs.oracle.com/javase/tutorial/java/IandI/override.html" rel="nofollow noreferrer">the overridden method will still be called</a>. So, if you do:</p> <pre><code>Parent p = (Parent) new Child(); p.a(); </code></pre> <p>... the <code>Child</code>'s <code>public void a()</code> will be called, even though it is being seen through the lens of the <code>Parent</code> class. However if you were to have a second method in the <code>Child</code> that the <code>Parent</code> does not have (let's say <code>public void b()</code> for instance), you would <em>not</em> be able to call that without casting the object back to a <code>Child</code>.</p> <p>"Behind the scenes", as you say, the only new thing that is created is another <a href="https://stackoverflow.com/questions/40480/is-java-pass-by-reference">object reference</a> which points to the same object. You can have as many references as you like to the same, singular object. Consider this example:</p> <pre><code>Parent p = new Parent(); Parent p1 = p; Parent p2 = p; Parent p3 = p2; </code></pre> <p>Here, there are four references (<code>p</code>, <code>p1</code>, <code>p2</code>, and <code>p3</code>) each of which points to the same object you created with the <code>new Parent()</code> declaration.</p> <p>I would probably argue on the philosophical point, though, that this creation of new references is actually explicit rather than behind the scenes when you say <code>Parent p = something</code>.</p> <p><strong>Links:</strong></p> <ul> <li><a href="http://en.wikipedia.org/wiki/Primitive_wrapper_class" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Primitive_wrapper_class</a></li> <li><a href="http://docs.oracle.com/javase/tutorial/java/data/autoboxing.html" rel="nofollow noreferrer">http://docs.oracle.com/javase/tutorial/java/data/autoboxing.html</a></li> <li><a href="http://docs.oracle.com/javase/7/docs/api/java/lang/ClassCastException.html" rel="nofollow noreferrer">http://docs.oracle.com/javase/7/docs/api/java/lang/ClassCastException.html</a></li> <li><a href="http://docs.oracle.com/javase/tutorial/java/IandI/override.html" rel="nofollow noreferrer">http://docs.oracle.com/javase/tutorial/java/IandI/override.html</a></li> <li><a href="https://stackoverflow.com/questions/40480/is-java-pass-by-reference">Is Java &quot;pass-by-reference&quot; or &quot;pass-by-value&quot;?</a></li> </ul>
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