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copied!<pre><code>public DM(Euro a){ </code></pre> <p><code>a</code> isn't a "new" object. When the rest of the program uses this constructor, it will need to have an <em>existing</em> <code>Euro</code> object, like this:</p> <pre><code>Euro someEuroVariable; // for example ... DM d = new DM(someEuroVariable); </code></pre> <p>and the constructor will then use <code>someEuroVariable</code> as its parameter, which it refers to as <code>a</code>. But no new <code>Euro</code> object is created by this.</p> <p>Hope this helps.</p> <p><b>EDIT:</b> To answer the question in the comments: You can't say </p> <pre><code>DM d = new DM(); </code></pre> <p>because <code>DM</code> doesn't have a constructor that you can use with no parameters. But let's say you used a legal constructor like</p> <pre><code>DM d = new DM(1.234); </code></pre> <p>and now you say</p> <pre><code>Euro a = d; </code></pre> <p><code>a</code> and <code>d</code> will be references to the <em>same</em> object. This object has a <em>run-time</em> type of <code>DM</code>. When you declare a variable <code>a</code> to have type <code>Euro</code>, that means that <code>a</code> can, during its lifetime, be an object of type <code>Euro</code> <em>or any of its subclasses</em>. However, since the compiler can't tell what the actual type will be at runtime, you can only use a method <code>a.method()</code> if the method was defined for <code>Euro</code>. If <code>DM</code> has a new method <code>method2</code> that <code>Euro</code> doesn't have, you can't call it directly with <code>a.method2()</code>. It also means that later on, you could reassign <code>a</code> to an object with some other subclass:</p> <pre><code>a = new Lira(100000000000.0); </code></pre> <p>while you can't do that with <code>d</code> because it can only be of type <code>DM</code> or a subclass of <code>DM</code>:</p> <pre><code>d = new Lira(100000000000.0); // Error at compile time </code></pre> <p>This distinction between compile-time type and run-time type can be difficult to grasp until you get used to it. It's also totally unrelated to what happens when you pass a <code>Euro</code> as a parameter to a <code>DM</code> constructor; the object that you pass as a parameter is a separate object and must exist before the constructor is called.</p>

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