Overloading constructors isn&#39;t special for case classes:

    case class Foo(bar: Int, baz: Int) {
      def this(bar: Int) = this(bar, 0)
    }
    
    new Foo(1, 2)
    new Foo(1)

However, you may like to also overload the `apply` method in the companion object, which is called when you omit `new`.
    
    object Foo {
      def apply(bar: Int) = new Foo(bar)
    }
    
    Foo(1, 2)
    Foo(1)

In Scala 2.8, named and default parameters can often be used instead of overloading.

    case class Baz(bar: Int, baz: Int = 0)
    new Baz(1)
    Baz(1)

You can define an overloaded constructor the usual way, but to invoke it you have to use the &quot;new&quot; keyword.

    scala&gt; case class A(i: Int) { def this(s: String) = this(s.toInt) }
    defined class A
    
    scala&gt; A(1)
    res0: A = A(1)
    
    scala&gt; A(&quot;2&quot;)
    &lt;console&gt;:8: error: type mismatch;
     found   : java.lang.String(&quot;2&quot;)
     required: Int
           A(&quot;2&quot;)
             ^
    
    scala&gt; new A(&quot;2&quot;)
    res2: A = A(2)



Is it possible to create an *inner class* within an interface?  
If it is possible why would we want to create an *inner class* like that since
we are not going to create any interface objects?

Do these *inner classes* help in any development process?


Inner class within Interface

My django application has become painfully slow on the production. Probably it is due to some complex or unindexed queries. 

Is there any django-ish way to profile my application?

Profiling Django

In Scala 2.8 is there a way to overload constructors of a case class?

If yes, please put a snippet to explain, if not, please explain why?



Overload constructor for Scala&#39;s Case Classes?

<p>In Scala 2.8 is there a way to overload constructors of a case class?</p>
<p>If yes, please put a snippet to explain, if not, please explain why?</p>


How to convert Scala&#39;s `scala.List` into Java&#39;s `java.util.List`?


How to convert a scala.List to a java.util.List?

Let&#39;s say I have a `MyObject` instance which is not initialized:

    var a:MyObject = null

is this the proper way to initialize it to null?

Is this the proper way to initialize null references in Scala?

I almost always have a Scala REPL session or two open, which makes it very easy to give Java or Scala classes a quick test.  But if I change a class and recompile it, the REPL continues with the old one loaded.  Is there a way to get it to reload the class, rather than having to restart the REPL?

Just to give a concrete example, suppose we have the file Test.scala:

    object Test { def hello = &quot;Hello World&quot; }

We compile it and start the REPL:

    ~/pkg/scala-2.8.0.Beta1-prerelease$ bin/scala
    Welcome to Scala version 2.8.0.Beta1-prerelease
    (Java HotSpot(TM) Server VM, Java 1.6.0_16).
    Type in expressions to have them evaluated.
    Type :help for more information.

    scala&gt; Test.hello
    res0: java.lang.String = Hello World

Then we change the source file to

    object Test {
      def hello = &quot;Hello World&quot;
      def goodbye = &quot;Goodbye, Cruel World&quot;
    }

but we can&#39;t use it:

    scala&gt; Test.goodbye
    &lt;console&gt;:5: error: value goodbye is not a member of object Test
           Test.goodbye
                ^
    
    scala&gt; import Test;
    &lt;console&gt;:1: error: &#39;.&#39; expected but &#39;;&#39; found.
           import Test;



How to reload a class or package in Scala REPL?

In Scala one might define methods inside other methods. This limits their scope of use to inside of definition block. I use them to improve readability of code that uses several higher-order functions. In contrast to anonymous function literals, this allows me to give them meaningful names before passing them on.

For example:

    class AggregatedPerson extends HashSet[PersonRecord] {
      def mostFrequentName: String = {
        type NameCount = (String, Int)
        def moreFirst(a: NameCount, b: NameCount) = a._2 &gt; b._2
        def countOccurrences(nameGroup: (String, List[PersonRecord])) =
          (nameGroup._1, nameGroup._2.size) 
 
        iterator.toList.groupBy(_.fullName).
          map(countOccurrences).iterator.toList.
          sortWith(moreFirst).head._1
      }
    }

Is there any runtime cost because of the nested method definition I should be aware of?

Does the answer differ for closures?

The cost of nested methods

Why does [Jorge Ortiz advise to avoid method overloading?][1]


  [1]: http://scalatips.tumblr.com/

Why &quot;avoid method overloading&quot;?

I would like to pass some variables in the constructor of my ListActivity  

I start activity via this code:

    startActivity(new Intent (this, viewContacts.class));

I would like to use similar code, but to pass two strings to the constructor. How is possible?

How to start an Intent by passing some parameters to it?

How could I call the constructor of a class with call_user_func_array

It is not possible to do :

    $obj = new $class();
    call_user_func_array(array($obj, &#39;__construct&#39;), $args); 

because if the constructor has parameters, the *new* will fail.

Constraint : I do not control the classes that I have to instantiate, nor can I modify them.

Don&#39;t ask me why I want to do this crazy thing, this is a crazy test.

How to call the constructor with call_user_func_array in PHP

I&#39;m having a brain cramp... how do I initialize an array of objects properly in C++?

non-array example:

    struct Foo { Foo(int x) { /* ... */  } };

    struct Bar { 
         Foo foo;

         Bar() : foo(4) {}
    };

array example:

    struct Foo { Foo(int x) { /* ... */  } };

    struct Baz { 
         Foo foo[3];

         // ??? I know the following syntax is wrong, but what&#39;s correct?
         Baz() : foo[0](4), foo[1](5), foo[2](6) {}
    };

**edit:** Wild &amp; crazy workaround ideas are appreciated, but they won&#39;t help me in my case. I&#39;m working on an embedded processor where std::vector and other STL constructs are not available, and the obvious workaround is to make a default constructor and have an explicit `init()` method that can be called after construction-time, so that I don&#39;t have to use initializers at all. (This is one of those cases where I&#39;ve gotten spoiled by Java&#39;s `final` keyword + flexibility with constructors.)

C++: constructor initializer for arrays

Does the default constructor (created by the compiler) initialize built-in-types?

Does the default constructor initialize built-in types?

Can I call constructor explicitly, without using `new`, if I already have a memory for object?

    class Object1{
        char *str;
    public:
        Object1(char*str1){
            str=strdup(str1);
            puts(&quot;ctor&quot;);
            puts(str);
        }
        ~Object1(){
            puts(&quot;dtor&quot;);
            puts(str);
            free(str);
        }
    };

    Object1 ooo[2] = {
         Object1(&quot;I&#39;m the first object&quot;), Object1(&quot;I&#39;m the 2nd&quot;)
    };

    do_smth_useful(ooo);
    ooo[0].~Object1(); // call destructor
    ooo[0].Object1(&quot;I&#39;m the 3rd object in place of first&quot;); // ???? - reuse memory



     


C++, is it possible to call a constructor directly, without new?

In C# you can do this:  

    foo = string.Format(&quot;{0} {1} {2} {3} ...&quot;, &quot;aa&quot;, &quot;bb&quot;, &quot;cc&quot; ...);

This method `Format()` accepts infinite parameters, being the first one how the string should be formatted and the rest are values to be put in the string.

Today I&#39;ve come to a situation where I had to get a set of strings and test them, then I remembered this language functionality, but I had no clue. After a few unsuccessful web searches, I&#39;ve realised it would be more prudent to just get an array, which didn&#39;t make me quite satisfied.

**Q:** How do I make a function that accepts infinite parameters? And how do I use it ?

Creating methods with infinite parameters?

Why is it not possible to overload a function just by changing the return type?  Will that change in a future version of Java?

By the way, just for reference, is this possible in C++?

Overload with different return type in Java?

I&#39;d like to know:

1. Why can&#39;t static methods be overridden in Java?
2. Can static methods be overloaded in Java?


Can I override and overload static methods in Java?

How does the following code work?

    typedef char (&amp;yes)[1];
    typedef char (&amp;no)[2];
    
    template &lt;typename B, typename D&gt;
    struct Host
    {
      operator B*() const;
      operator D*();
    };
    
    template &lt;typename B, typename D&gt;
    struct is_base_of
    {
      template &lt;typename T&gt; 
      static yes check(D*, T);
      static no check(B*, int);
    
      static const bool value = sizeof(check(Host&lt;B,D&gt;(), int())) == sizeof(yes);
    };
    
    //Test sample
    class Base {};
    class Derived : private Base {};
    
    //Expression is true.
    int test[is_base_of&lt;Base,Derived&gt;::value &amp;&amp; !is_base_of&lt;Derived,Base&gt;::value];

1. Note that `B` is private base. How does this work? 

2. Note that `operator B*()` is const. Why is it important?

3. Why is `template&lt;typename T&gt; static yes check(D*, T);` better than `static yes check(B*, int);` ? 

**Note**: It is reduced version (macros are removed) of `boost::is_base_of`. And this works on wide range of compilers.

 


How does `is_base_of` work?

Following watching [Nick Partidge&#39;s presentation][1] on deriving [scalaz][2], I got to looking at this example, which is just awesome:

    import scalaz._
    import Scalaz._
    def even(x: Int) : Validation[NonEmptyList[String], Int] 
        = if (x % 2 ==0) x.success else &quot;not even: %d&quot;.format(x).wrapNel.fail

    println( even(3) &lt;|*|&gt; even(5) ) //prints: Failure(NonEmptyList(not even: 3, not even: 5))

I was trying to understand what the `&lt;|*|&gt;` method was doing, here is the source code:

    def &lt;|*|&gt;[B](b: M[B])(implicit t: Functor[M], a: Apply[M]): M[(A, B)] 
        = &lt;**&gt;(b, (_: A, _: B))

OK, that is fairly confusing (!) - but it references the `&lt;**&gt;` method, which is declared thus:

    def &lt;**&gt;[B, C](b: M[B], z: (A, B) =&gt; C)(implicit t: Functor[M], a: Apply[M]): M[C] 
        = a(t.fmap(value, z.curried), b)

So I have a few questions:

 1. How come the method appears to take a *higher-kinded type* of one type parameter (`M[B]`) but can get passed a `Validation` (which has two type paremeters)?
 2. The syntax `(_: A, _: B)` defines the function `(A, B) =&gt; Pair[A,B]` which the 2nd method expects: *what is happening to the Tuple2/Pair in the failure case? There&#39;s no tuple in sight!*

  [1]: http://vimeo.com/10482466
  [2]: http://github.com/scalaz/scalaz

Function syntax puzzler in scalaz

Or does it just clutter up the code for something the JIT would take care of automatically anyway?

Does the @inline annotation in Scala really help performance?

[Language FAQ][1] says 

    import scala.collection.mutable.{_, Map =&gt; _, Set =&gt; _}

should import all classes from package `scala.collection.mutable`, except `Map` and `Set`. But it gives me this error: 

    error: &#39;}&#39; expected but &#39;,&#39; found.
           import scala.collection.mutable.{_, Map =&gt; _, Set =&gt; _}

Is there still a way to do this?

  [1]: http://scala.sygneca.com/faqs/language#how-do-i-import


How do I exclude/rename some classes from import in Scala?

One of the new features of Scala 2.8 are context bounds. What is a context bound and where is it useful?

Of course I searched first (and found for example [this][1]) but I couldn&#39;t find any really clear and detailed information.


  [1]: https://stackoverflow.com/questions/2961986/how-do-you-write-an-idiomatic-scala-quicksort-function/2962799#2962799

What is a &quot;context bound&quot; in Scala?

What are package objects, not so much the concept but their usage?

I&#39;ve tried to get an example working and the only form I got to work was as follows:

    package object investigations {
        val PackageObjectVal = &quot;A package object val&quot;
    }

    package investigations {

        object PackageObjectTest {
            def main(args: Array[String]) {
                println(&quot;Referencing a package object val: &quot; + PackageObjectVal)
            }
        }
    }

Observations I&#39;ve made so far are:

    package object _root_ { ... }

is disallowed (which is reasonable),

    package object x.y { ... }

is also disallowed.

It seems that a package object must be declared in the immediate parent package and, if written as above, the brace delimited package declaration form is required.

Are they in common use? If so, how?


Package objects

Say I have got following two `case class`es:

    case class Address(street: String, city: String, state: String, zipCode: Int)
    case class Person(firstName: String, lastName: String, address: Address)

and the following instance of `Person` class:

    val raj = Person(&quot;Raj&quot;, &quot;Shekhar&quot;, Address(&quot;M Gandhi Marg&quot;, 
                                               &quot;Mumbai&quot;, 
                                               &quot;Maharashtra&quot;, 
                                               411342))

Now if I want to update `zipCode` of `raj` then I will have to do:

    val updatedRaj = raj.copy(address = raj.address.copy(zipCode = raj.address.zipCode + 1))

With more levels of nesting this gets even more uglier. Is there a cleaner way (something like Clojure&#39;s `update-in`) to update such nested structures?

Cleaner way to update nested structures

If you&#39;re writing code that&#39;s using lots of beautiful, immutable data structures, case classes appear to be a godsend, giving you all of the following for free with just one keyword:

 - Everything immutable by default
 - Getters automatically defined
 - Decent toString() implementation
 - Compliant equals() and hashCode()
 - Companion object with unapply() method for matching

But what are the disadvantages of defining an immutable data structure as a case class?

What restrictions does it place on the class or its clients?

Are there situations where you should prefer a non-case class?

What are the disadvantages to declaring Scala case classes?

So here&#39;s the situation.  I want to define a case class like so:

    case class A(val s: String)

and I want to define an object to ensure that when I create instances of the class, the value for &#39;s&#39; is always uppercase, like so:

    object A {
      def apply(s: String) = new A(s.toUpperCase)
    }

However, this doesn&#39;t work since Scala is complaining that the apply(s: String) method is defined twice.  I understand that the case class syntax will automatically define it for me, but isn&#39;t there another way I can achieve this?  I&#39;d like to stick with the case class since I want to use it for pattern matching.



How to override apply in a case class companion

I&#39;ve read that Scala&#39;a `case class` construct automatically generates a fitting `equals` and `hashCode` implementation. What does exactly the generated code look like?

hashCode in case classes in Scala

Is there an easy way to convert a case class into a tuple?

I can, of course, easily write boilerplate code to do this, but I mean without the boilerplate.

What I&#39;m really after is a way to easily make a case class lexicographically Ordered. I can achieve the goal for tuples by importing scala.math.Ordering.Implicits._, and voila, my tuples have an Ordering defined for them. But the implicits in scala.math.Ordering don&#39;t work for case classes in general.


Content Type	Original Author	Original Content on Stackoverflow
Question	Felix	View Question on Stackoverflow
Solution 1 - Scala	retronym	View Answer on Stackoverflow
Solution 2 - Scala	Lukas Rytz	View Answer on Stackoverflow

Overload constructor for Scala's Case Classes?

Scala Problem Overview

Scala Solutions

Solution 1 - Scala

Solution 2 - Scala

Profiling Django

Inner class within Interface

Attributions