For (non-nullable) object types, Kotlin uses the `null` value to mark that a `lateinit` property has not been initialized and to throw the appropriate exception when the property is accessed. 

For primitive types, there is no such value, so there is no way to mark a property as non-initialized and to provide the diagnostics that `lateinit` needs to provide. (We could try to use a separate marker of some kind, but that marker would not be updated when initializing the field through reflection, which is a major use case of `lateinit`).

Therefore, `lateinit` is supported for properties of object types only.

A short answer is that with primitives you can always use `0` as the default, and with nullable types `null` as a default. Only non-nullable non-primitive types may need `lateinit` to work around the type safety system.

Actually, there is no need for initializing a variable in Kotlin as long as it has a value **before** the first access and it can be **statically** proved. Which means this code is perfectly valid:

    fun main(args: Array&lt;String&gt;) {
        var x: Int
        val y: Double

        x = 0
        y = x + 0.1

        println(&quot;$x, $y&quot;) 
    }

But there are (rare) cases when the initialisation cannot be statically proved. The most common case is a class field which uses any form of dependency injection:

    class Window {
        @Inject lateinit parent: Parent
    }

I think that in case of primitives it&#39;s less resources taking to simply initialise it to let me say 0 and hold the simple value in memory rather than store extra information about the object nullability which is used by lateinit mechanism.

Correct me if it&#39;s not the case.

Would anyone know what I am missing, why those asp-controller and asp-action tags are not working for me. I am implementing a project in ASP.NET MVC Core. 

This does not fire:

    &lt;a asp-controller=&quot;App&quot; asp-action=&quot;Trips&quot; class=&quot;btn btn-lg btn-success&quot;&gt;Go to Trips&lt;/a&gt;

Razor works fine:

    @Html.ActionLink(&quot;Go to Trips&quot;, &quot;Trips&quot;, &quot;App&quot;, new object { }, new { @class = &quot;btn btn-lg btn-success&quot; })

Do I need to configure some service for that to work. And also, which way is preferred? Razor is pretty popular with MVC, are those asp- tags a new, better way?

asp-controller and asp-action attributes not working

I have a vertical `RecyclerView` using a `GridLayoutManager`. I want each column to be centered, but the columns begin all the way on the left. In the following picture you can see what I&#39;m talking about. I used the ugly color scheme to illustrate the columns and background. The green is the background for each item in the `RecyclerView`, the red is the background of the `RecyclerView` itself:

http://imgur.com/a/J3HtF

I&#39;m setting it up with:

    mRecyclerView.setLayoutManager(new GridLayoutManager(this, 2));

Here&#39;s the column_item.xml:

    &lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;
    &lt;LinearLayout xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
                  android:layout_width=&quot;120dp&quot;
                  android:layout_height=&quot;180dp&quot;
                  android:orientation=&quot;vertical&quot;
                  android:padding=&quot;4dp&quot;&gt;
    
        &lt;ImageView
            android:id=&quot;@+id/movie_column_photo&quot;
            android:layout_width=&quot;80dp&quot;
            android:layout_height=&quot;120dp&quot;/&gt;
    
        &lt;TextView
            android:id=&quot;@+id/movie_column_title&quot;
            android:layout_width=&quot;wrap_content&quot;
            android:layout_height=&quot;wrap_content&quot;/&gt;
    &lt;/LinearLayout&gt;


Here&#39;s the recyclerview xml:

    &lt;LinearLayout
        android:layout_width=&quot;match_parent&quot;
        android:layout_height=&quot;wrap_content&quot;&gt;

        &lt;android.support.v7.widget.RecyclerView
            android:id=&quot;@+id/company_details_recyclerview&quot;
            android:layout_width=&quot;match_parent&quot;
            android:layout_height=&quot;match_parent&quot;
            android:scrollbars=&quot;vertical&quot;&gt;
        &lt;/android.support.v7.widget.RecyclerView&gt;
    &lt;/LinearLayout&gt;


How to center RecyclerView items horizontally with vertical GridLayoutManager

In the Kotlin language we, by default, have to initialize each variable when it is introduced. To avoid this, the `lateinit` keyword can be used. Referring to a `lateinit` variable before it has been initialized results in a runtime exception.

`lateinit` can not, however, be used with the primitive types. Why is it so?

Why doesn&#39;t Kotlin allow to use lateinit with primitive types?

<p>In the Kotlin language we, by default, have to initialize each variable when it is introduced. To avoid this, the <code>lateinit</code> keyword can be used. Referring to a <code>lateinit</code> variable before it has been initialized results in a runtime exception.</p>
<p><code>lateinit</code> can not, however, be used with the primitive types. Why is it so?</p>


It is well known that missing initializers for an array of scalars are defaulted to zero.

    int A[5]; // Entries remain uninitialized
    int B[5]= { 0 }; // All entries set to zero

But is this (below) guaranteed ?

    int C[5]= { }; // All entries set to zero


Initializing an array of zeroes

This is the code I have:

    public class StupidClass {
        static {
            System.out.println(&quot;Stupid class loaded!&quot;);
        }
    }

And the tests I have, which I run **separately**.

    import org.junit.Test;
    
    public class StupidTest {
        @Test
        public void foo() throws ClassNotFoundException {
            final Class&lt;?&gt; stupidClass = Class.forName(&quot;StupidClass&quot;);
            System.out.println(stupidClass.getSimpleName());
        }
    
        @Test
        public void bar() throws ClassNotFoundException {
            final Class&lt;StupidClass&gt; stupidClassClass = StupidClass.class;
            System.out.println(stupidClassClass.getSimpleName());
        }
    }

When I run test **foo** I will see:

    Stupid class loaded!
    StupidClass

But when I run the test **bar** all I see is:

    StupidClass

Quoting from [this][1] page..

&gt; Class objects are constructed automatically by the Java Virtual
&gt; Machine as classes are loaded and by calls to the defineClass method
&gt; in the class loader.

So my understanding is, in test bar, Stupid class is loaded, otherwise I would have seen a null I guess? So Class object is created because class itself is loaded..

And now quoting from [this][2] page  

&gt; Static initialization blocks are run when the JVM (class loader - to
&gt; be specific) loads StaticClass (which occurs the first time it is
&gt; referenced in code).

So I am expecting to see the &quot;Stupid class loaded!&quot; text in test bar as well, but I am not.

Also quoting from [Thinking in Java][3]

&gt; Each of the classes Candy, Gum, and Cookie has a static clause that is
&gt; executed as the class is loaded for the first time.

which is not very accurate it seems..

What am I missing?


  [1]: http://docs.oracle.com/javase/8/docs/api/java/lang/Class.html
  [2]: https://stackoverflow.com/questions/9130461/when-is-the-static-block-of-a-class-executed
  [3]: https://www.amazon.com/Thinking-Java-4th-Bruce-Eckel/dp/0131872486

Why does .class not invoke the static block in a Class?

I am having troubles to understand the difference between both, or the purpose of the `convenience init`.

What is the difference between convenience init vs init in swift, explicit examples better

**UPDATE:**
Use structs and not classes. Struct is better in many ways has got an initializer of its own.

This is my model class. Is it possible to create the `init` method automatically? Everytime I have to initialize all the variables one by one and it costs a lot of time. 
    
    class Profile {
            
        var id: String
        var name: String
        var image: String
            
        init(id: String, name: String, image: String) {
            self.id = id
            self.name = name
            self.image = image
        }
    }

I want `self.id = id` and other variables to initialize automatically.

How to automatically create an initializer for a Swift class?

I had been writing things like 

    char *x=NULL;

on the assumption that 

     char *x=2;
would create a `char` pointer to address 2.  

But, in &lt;a href=&quot;http://www.crasseux.com/books/ctutorial/Pointers-and-initialization.html#Pointers%20and%20initialization&quot;&gt;The GNU C Programming Tutorial&lt;/a&gt; it says that `int *my_int_ptr = 2;` stores the integer value `2` to whatever random address is in `my_int_ptr` when it is allocated.

This would seem to imply that my own `char *x=NULL` is assigning whatever the value of `NULL` cast to a `char` is to some random address in memory.

While

    #include &lt;stdlib.h&gt;
    #include &lt;stdio.h&gt;
    
    int main()
    {
        char *x=NULL;

        if (x==NULL)
            printf(&quot;is NULL\n&quot;);
    
        return EXIT_SUCCESS;
    }

does, in fact, print 

&gt; is NULL

when I compile and run it, I am concerned that I am relying on undefined behavior, or at least under-specified behavior, and that I should write

    char *x;
    x=NULL;

instead.

Is it possible to initialize a C pointer to NULL?

1. This code **compiles with a warning** (*insignificant performance impact*):

        inline fun test(noinline f: () -&gt; Unit) {
            thread(block = f)
        }

2. This code **does not compile** (*illegal usage of inline-parameter*):

        inline fun test(crossinline f: () -&gt; Unit) {
            thread(block = f)
        }

3. This code **compiles with a warning** (*insignificant performance impact*):

        inline fun test(noinline f: () -&gt; Unit) {
            thread { f() }
        }

4. This code **compiles with no warning or error**:

        inline fun test(crossinline f: () -&gt; Unit) {
            thread { f() }
        }


Here are my questions:

* How come (2) does not compile but (4) does?
* What exactly is the difference between `noinline` and `crossinline`?
* If (3) does not generates a no performance improvements, why would (4) do?

What is the difference between crossinline and noinline in Kotlin?

I want to convert/map some &quot;data&quot; class objects to similar &quot;data&quot; class objects. For example, classes for web form to classes for database records.

    data class PersonForm(
        val firstName: String,
        val lastName: String,
        val age: Int,
        // maybe many fields exist here like address, card number, etc.
        val tel: String
    )
    // maps to ...
    data class PersonRecord(
        val name: String, // &quot;${firstName} ${lastName}&quot;
        val age: Int, // copy of age
        // maybe many fields exist here like address, card number, etc.
        val tel: String // copy of tel
    )

I use ModelMapper for such works in Java, but it can&#39;t be used because data classes are final (ModelMapper creates CGLib proxies to read mapping definitions). We can use ModelMapper when we make these classes/fields open, but we must implement features of &quot;data&quot; class manually.
(cf. ModelMapper examples: https://github.com/jhalterman/modelmapper/blob/master/examples/src/main/java/org/modelmapper/gettingstarted/GettingStartedExample.java)

How to map such &quot;data&quot; objects in Kotlin?

**Update:**
ModelMapper automatically maps fields that have same name (like tel -&gt; tel) without mapping declarations. I want to do it with data class of Kotlin.

**Update:**
The purpose of each classes depends on what kind of application, but these are probably placed in the different layer of an application.

For example:

 * data from database (Database Entity) to data for HTML form (Model/View Model)
 * REST API result to data for database

These classes are similar, but are not the same.

I want to avoid normal function calls for these reasons:

 * It depends on the order of arguments. A function for a class with many fields that have same type (like String) will be broken easily.
 * Many declarations are nesessary though most mappings can be resolved with naming convention.

Of course, a library that has similar feature is intended, but information of the Kotlin feature is also welcome (like spreading in ECMAScript).


Better way to map Kotlin data objects to data objects

I want to know what exactly an asterisk does before a variable name in Kotlin.
I saw this (`*args`) in a [Spring boot Kotlin example][1]:

    @SpringBootApplication
    open class Application {
    
    	@Bean
    	open fun init(repository: CustomerRepository) = CommandLineRunner {
    		repository.save(Customer(&quot;Jack&quot;, &quot;Bauer&quot;))
    		repository.save(Customer(&quot;Chloe&quot;, &quot;O&#39;Brian&quot;))
    		repository.save(Customer(&quot;Kim&quot;, &quot;Bauer&quot;))
    		repository.save(Customer(&quot;David&quot;, &quot;Palmer&quot;))
    		repository.save(Customer(&quot;Michelle&quot;, &quot;Dessler&quot;))
    	}
    }
    
    fun main(args: Array&lt;String&gt;) {
    	SpringApplication.run(Application::class.java, *args)
    }

  [1]: https://spring.io/blog/2016/02/15/developing-spring-boot-applications-with-kotlin


Kotlin asterisk operator before variable name or Spread Operator in Kotlin

I recently saw code for reading entire contents of an `InputStream` into a String in Kotlin, such as:

&lt;!-- language-all: kotlin --&gt;

    // input is of type InputStream
    val baos = ByteArrayOutputStream()
    input.use { it.copyTo(baos) }
    val inputAsString = baos.toString()

And also:

    val reader = BufferedReader(InputStreamReader(input))
    try {
        val results = StringBuilder()
        while (true) { 
            val line = reader.readLine()
            if (line == null) break
            results.append(line) 
        }
        val inputAsString = results.toString()
    } finally {
        reader.close()
    }

And even this that looks smoother since it auto-closes the `InputStream`:

    val inputString = BufferedReader(InputStreamReader(input)).useLines { lines -&gt;
        val results = StringBuilder()
        lines.forEach { results.append(it) }
        results.toString()
    }

Or slight variation on that one:

    val results = StringBuilder()
    BufferedReader(InputStreamReader(input)).forEachLine { results.append(it) }
    val resultsAsString = results.toString()   

Then this functional fold thingy:

    val inputString = input.bufferedReader().useLines { lines -&gt;
        lines.fold(StringBuilder()) { buff, line -&gt; buff.append(line) }.toString()
    }

Or a **bad** variation which doesn&#39;t close the `InputStream`:

    val inputString = BufferedReader(InputStreamReader(input))
            .lineSequence()
            .fold(StringBuilder()) { buff, line -&gt; buff.append(line) }
            .toString()

But they are all clunky and I keep finding newer and different versions of the same... and some of them never even close the `InputStream`.  What is a non-clunky (idiomatic) way to read the `InputStream`?

**Note:** *this question is intentionally written and answered by the author ([Self-Answered Questions](https://blog.stackoverflow.com/2011/07/its-ok-to-ask-and-answer-your-own-questions/)), so that the idiomatic answers to commonly asked Kotlin topics are present in SO.*

In Kotlin, how do I read the entire contents of an InputStream into a String?

I am trying to write function which will tell me that string is nice, nice means string has at least one repetition of letters in the string. But I can&#39;t to make return from lambda, it&#39;s always return false, though condition in if statement passed. Can somebody explain me how make return?

I have tried to write return, but IDEA gave me the message **Kotlin: &#39;return&#39; is not allowed here**


    fun main(args: Array&lt;String&gt;) {
        println(&quot;sddfsdf&quot;.isNice())
    }
    
    fun String.isNice(): Boolean {
        val hasRepeat = {
            for (i in 0 .. (length - 2)) {
                if (subSequence(i, i + 2).toSet().size == 1) {
                    true
                    println(subSequence(i, i + 2))
                }
            }
            false
        }
    
        return hasRepeat()
    }


ouput is:

    dd
    false

Content Type	Original Author	Original Content on Stackoverflow
Question	Kallja	View Question on Stackoverflow
Solution 1 - Initialization	yole	View Answer on Stackoverflow
Solution 2 - Initialization	voddan	View Answer on Stackoverflow
Solution 3 - Initialization	Artur Ex	View Answer on Stackoverflow

Why doesn't Kotlin allow to use lateinit with primitive types?

Initialization Problem Overview

Initialization Solutions

Solution 1 - Initialization

Solution 2 - Initialization

Solution 3 - Initialization

How to center RecyclerView items horizontally with vertical GridLayoutManager

asp-controller and asp-action attributes not working

Attributions