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    <h2>tl;dr</h2> <ul> <li><code>class C</code> defines a class, just as in Java or C++.</li> <li><code>object O</code> creates a <a href="http://en.wikipedia.org/wiki/Singleton_pattern" rel="noreferrer">singleton</a> object <code>O</code> as instance of some anonymous class; it can be used to hold static members that are not associated with instances of some class.</li> <li><code>object O extends T</code> makes the object <code>O</code> an instance of <code>trait T</code>; you can then pass <code>O</code> anywhere, a <code>T</code> is expected.</li> <li>if there is a <code>class C</code>, then <code>object C</code> is the <a href="https://docs.scala-lang.org/tour/singleton-objects.html#companions" rel="noreferrer">companion object</a> of class <code>C</code>; note that the companion object is <strong>not</strong> automatically an instance of <code>C</code>.</li> </ul> <p>Also see Scala documentation for <a href="https://docs.scala-lang.org/tour/singleton-objects.html" rel="noreferrer">object</a> and <a href="https://docs.scala-lang.org/tour/classes.html" rel="noreferrer">class</a>.</p> <h3>Usage as host of static members</h3> <p>Most often, you need an <code>object</code> to hold methods and values/variables that shall be available without having to first instantiate an instance of some class. This is use is closely related to <code>static</code> members in Java.</p> <pre><code>object A { def twice(i: Int): Int = 2*i } </code></pre> <p>You can then call above method using <code>A.twice(2)</code>.</p> <p>If <code>twice</code> were a member of some class <code>A</code>, then you would need to make an instance first:</p> <pre><code>class A() { def twice(i: Int): Int = 2 * i } val a = new A() a.twice(2) </code></pre> <p>You can see how this is redundant, as <code>twice</code> does not require any instance-specific data.</p> <h3>Usage as a special named instance</h3> <p>You can also use the <code>object</code> itself as some special instance of a class or trait. When you do this, your object needs to extend some <code>trait</code> in order to become an instance of a subclass of it.</p> <p>Consider the following code:</p> <pre><code>object A extends B with C { ... } </code></pre> <p>This declaration first declares an anonymous (inaccessible) class that extends both <code>B</code> and <code>C</code>, and instantiates a single instance of this class named <code>A</code>.</p> <p>This means <code>A</code> can be passed to functions expecting objects of type <code>B</code> or <code>C</code>, or <code>B with C</code>.</p> <h3>Additional Features of <code>object</code></h3> <p>There also exist some special features of objects in Scala. I recommend to read the <a href="https://docs.scala-lang.org/tour/singleton-objects.html" rel="noreferrer">official documentation</a>.</p> <ul> <li><code>def apply(...)</code> enables the usual method name-less syntax of <code>A(...)</code></li> <li><code>def unapply(...)</code> allows to create custom pattern matching <a href="http://docs.scala-lang.org/tour/extractor-objects.html" rel="noreferrer">extractors</a></li> <li>if accompanying a class of the same name, the object assumes a special role when resolving <a href="http://docs.scala-lang.org/tour/implicit-parameters.html" rel="noreferrer">implicit parameters</a></li> </ul>
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    1. COIt will also define class A, and create all of the methods in object A as static methods on class A (for interfacing with Java). (Modulo a bug in Scala 2.7 that's been fixed in Scala 2.8)
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      1. USKen Bloom
    2. CO@KenBloom really? I tried and doesn't work: scala> Commerce res8: Commerce.type = Commerce$@6eb2756 scala> classOf[Commerce] <console>:23: error: not found: type Commerce classOf[Commerce] ^ scala> new Commerce <console>:23: error: not found: type Commerce new Commerce ^
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      1. USHendy Irawan
    3. CO@Hendy: Scala won't recognize the `Commerce` class, but the JVM and the Java language will. (That's how you can do `object Foo{ def main(args:Seq[String]) }` and expect the program to run.)
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