Only one upper bound can be specified inside the angle brackets.

Kotlin offers different syntax for generic constraints when there is more than one constraint:

    class Foo&lt;T&gt;(val t: T) where T : Bar, T : Baz { ... }

and for functions:

    fun &lt;T&gt; f(): Foo where T : Bar, T : Baz { ... }

It is documented [here][1].


  [1]: https://kotlinlang.org/docs/reference/generics.html#upper-bounds

I&#39;m currently developing a website with the framework Django (I&#39;m very beginner), but I have a problem with Python: since I have created my templates, I can&#39;t run server anymore for this reason (the stack trace points to a line in file *urls.py*):

```
&lt;stacktrace&gt;
...
path(&#39;apppath/&#39;, include(&#39;myapp.urls&#39;)),
NameError: name &#39;include&#39; is not defined
```

Where can I import `include` from?





Python NameError: name &#39;include&#39; is not defined

What is the best way for me to do this? Should I use regex or is there another in-built PHP function I can use?

For example, I&#39;d want: `12 months` to become `12`. `Every 6 months` to become `6`, `1M` to become `1`, etc.


PHP - remove all non-numeric characters from a string

Say I want a type variable, T, that extends a certain class and implements an interface. Something like:

    class Foo &lt;T : Bar implements Baz&gt; { ... }

What is the syntax for this in Kotlin?

Generic extending class AND implements interface in Kotlin

Say I want a type variable, T, that extends a certain class and implements an interface. Something like:
<pre><code class="hljs language-csharp">class Foo &#x3C;T : Bar implements Baz> { ... }
</code></pre>
What is the syntax for this in Kotlin?

I want this protocol:

    protocol AddsMoreCommands {
         /* ... */
    }

only to be adopted by classes that inherit from the class `UIViewController`. [This page][1] tells me I can specify that it is only adopted by a class (as opposed to a struct) by writing

    protocol AddsMoreCommands: class {
    }
but I cannot see how to require that it is only adopted by a particular class. [That page later][1] talks about adding `where` clauses to protocol extensions to check conformance but I cannot see how to adapt that either.

    extension AddsMoreCommands where /* what */ {
    }

Is there a way to do this?
Thanks!

  [1]: https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html#//apple_ref/doc/uid/TP40014097-CH25-ID267

How to require that a protocol can only be adopted by a specific class

I have a protocol RequestType and it has associatedType Model as below.

    public protocol RequestType: class {
    
        associatedtype Model
        var path: String { get set }
           
    }

    public extension RequestType {
        
        public func executeRequest(completionHandler: Result&lt;Model, NSError&gt; -&gt; Void) {
            request.response(rootKeyPath: rootKeyPath) { [weak self] (response: Response&lt;Model, NSError&gt;) -&gt; Void in
                completionHandler(response.result)
                guard let weakSelf = self else { return }
                if weakSelf.logging { debugPrint(response) }
            }
        }
        
    }

Now I am trying to make a queue of all failed requests.

    public class RequestEventuallyQueue {
        
        static let requestEventuallyQueue = RequestEventuallyQueue()
        let queue = [RequestType]()
        
    }

But I get the error on line `let queue = [RequestType]()` that Protocol RequestType can only be used as a generic constraint because it has Self or associatedType requirements.


Protocol can only be used as a generic constraint because it has Self or associatedType requirements

*This is a real-world example from a 3rd party library API, but simplified.*

*Compiled with Oracle JDK 8u72*

Consider these two methods:

    &lt;X extends CharSequence&gt; X getCharSequence() {
        return (X) &quot;hello&quot;;
    }

    &lt;X extends String&gt; X getString() {
        return (X) &quot;hello&quot;;
    }

Both report an &quot;unchecked cast&quot; warning - I get why. The thing that baffles me is why can I call

    Integer x = getCharSequence();

and it compiles? The compiler should know that `Integer` does not implement `CharSequence`. The call to

    Integer y = getString();

gives an error (as expected)

&gt; 
    incompatible types: inference variable X has incompatible upper bounds java.lang.Integer,java.lang.String

Can someone explain why would this behaviour be considered valid? How would it be useful?

The client does not know that this call is unsafe - the client&#39;s code compiles without warning. Why wouldn&#39;t the compile warn about that / issue an error?

Also, how is it different from this example:

    &lt;X extends CharSequence&gt; void doCharSequence(List&lt;X&gt; l) {
    }

    List&lt;CharSequence&gt; chsL = new ArrayList&lt;&gt;();
    doCharSequence(chsL); // compiles

    List&lt;Integer&gt; intL = new ArrayList&lt;&gt;();
    doCharSequence(intL); // error

Trying to pass `List&lt;Integer&gt;` gives an error, as expected:

&gt; 
    method doCharSequence in class generic.GenericTest cannot be applied to given types;
      required: java.util.List&lt;X&gt;
      found: java.util.List&lt;java.lang.Integer&gt;
      reason: inference variable X has incompatible bounds
        equality constraints: java.lang.Integer
        upper bounds: java.lang.CharSequence

If that is reported as an error, why `Integer x = getCharSequence();` isn&#39;t?

Generic return type upper bound - interface vs. class - surprisingly valid code

I want to write a function that returns every item in a `List` that is not the first or the last item (a via point). The function gets a generic `List&lt;*&gt;` as input. A result should only be returned if the elements of the list are of the type `Waypoint`:

    fun getViaPoints(list: List&lt;*&gt;): List&lt;Waypoint&gt;? {
        
        list.forEach { if(it !is Waypoint ) return null }
        
        val waypointList = list as? List&lt;Waypoint&gt; ?: return null
        
        return waypointList.filter{ waypointList.indexOf(it) != 0 &amp;&amp; waypointList.indexOf(it) != waypointList.lastIndex}
    }

When casting the `List&lt;*&gt;` to `List&lt;Waypoint&gt;`, I get the warning: 

&gt; Unchecked Cast: kotlin.collections.List&lt;Kotlin.Any?&gt;
&gt; to kotlin.colletions.List&lt;Waypoint&gt;

I can&#39;t figure out a way to implement it otherwise. What&#39;s the right way to implement this function without this warning?

Kotlin: How to work with List casts: Unchecked Cast: kotlin.collections.List&lt;Kotlin.Any?&gt; to kotlin.colletions.List&lt;Waypoint&gt;

Say you have an arraylist defined as follows:

    ArrayList&lt;String&gt; someData = new ArrayList&lt;&gt;();

Later on in your code, because of generics you can say this:

    String someLine = someData.get(0);

And the compiler knows outright that it will be getting a string. Yay generics! However, this will fail:

    String[] arrayOfData = someData.toArray();

`toArray()` will always return an array of Objects, not of the generic that was defined. Why does the `get(x)` method know what it is returning, but `toArray()` defaults to Objects?

Java generics in ArrayList.toArray()

If I have a nullable type `Xyz?`, I want to reference it or convert it to a non-nullable type `Xyz`.  What is the idiomatic way of doing so in Kotlin?

For example, this code is in error:

&lt;!-- language-all: kotlin --&gt;

    val something: Xyz? = createPossiblyNullXyz()
    something.foo() // Error: &quot;Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type Xyz?&quot;

But if I check null first it is allowed, why?

    val something: Xyz? = createPossiblyNullXyz()
    if (something != null) {
        something.foo() 
    }

How do I change or treat a value as not `null` without requiring the `if` check, assuming I know for sure it is truly never `null`?  For example, here I am retrieving a value from a map that I can guarantee exists and the result of `get()` is not `null`.  But I have an error:

    val map = mapOf(&quot;a&quot; to 65,&quot;b&quot; to 66,&quot;c&quot; to 67)
    val something = map.get(&quot;a&quot;)
    something.toLong() // Error: &quot;Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type Int?&quot;
    
The method `get()` thinks it is possible that the item is missing and returns type `Int?`.  Therefore, what is the best way to force the type of the value to be not nullable?

**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.  Also to clarify some really old answers written for alphas of Kotlin that are not accurate for current-day Kotlin.*

In Kotlin, what is the idiomatic way to deal with nullable values, referencing or converting them

How to use Kotlin Android Extensions with `Fragment`s?
If I use them inside `onCreateView()`, I get this `NullPointerException` exception: 

&gt; Caused by: java.lang.NullPointerException: Attempt to invoke virtual
&gt; method &#39;android.view.View android.view.View.findViewById(int)&#39; on a
&gt; null object reference

Here is the fragment code: 

    package com.obaied.testrun.Fragment
    
    import android.os.Bundle
    import android.support.v4.app.Fragment
    import android.util.Log
    import android.view.LayoutInflater
    import android.view.View
    import android.view.ViewGroup
    import com.obaied.acaan.R
    import kotlinx.android.synthetic.main.fragment_card_selector.*
    
    public class CardSelectorFragment : Fragment() {
        val TAG = javaClass.canonicalName
    
        companion object {
            fun newInstance(): CardSelectorFragment {
                return CardSelectorFragment()
            }
        }
    
        override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
            var rootView = inflater?.inflate(R.layout.fragment_card_selector, container, false)
            btn_K.setOnClickListener { Log.d(TAG, &quot;onViewCreated(): hello world&quot;); }
    
            return rootView
        }
    }
    `

NullPointerException when trying to access views in a Kotlin fragment

In my Kotlin project, I have some parts of Java code that I want to convert to Kotlin. The menu item that converts the Java file to Kotlin is disabled because it&#39;s not a whole file I want to convert. 

How can I convert Java code to Kotlin?

How can I convert a part of Java source file to Kotlin?

I have a list:

&lt;!-- language-all: kotlin --&gt;

    val someList = listOf(1, 20, 10, 55, 30, 22, 11, 0, 99)

And I want to iterate it while modifying some of the values.  I know I can do it with `map` but that makes a copy of the list.  

    val copyOfList = someList.map { if (it &lt;= 20) it + 20 else it }

How do I do this without a copy?

**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.  Also to clarify some really old answers written for alphas of Kotlin that are not accurate for current-day Kotlin.*

In Kotlin, how do you modify the contents of a list while iterating

In Java 8, there is [`Stream.collect`](https://docs.oracle.com/javase/tutorial/collections/streams/reduction.html#collect) which allows aggregations on collections.  In Kotlin, this does not exist in the same way, other than maybe as a collection of extension functions in the stdlib.  But it isn&#39;t clear what the equivalences are for different use cases.  

For example, at the [top of the JavaDoc for `Collectors`](https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html) are examples written for Java 8, and when porting them to Kolin you can&#39;t use the Java 8 classes when on a different JDK version, so likely they should be written differently.  

In terms of resources online showing examples of Kotlin collections, they are typically trivial and don&#39;t really compare to the same use cases.  What are good examples that really match the cases such as documented for Java 8 `Stream.collect`?  The list there is:

 - Accumulate names into a List
 - Accumulate names into a TreeSet
 - Convert elements to strings and concatenate them, separated by commas
 - Compute sum of salaries of employee
 - Group employees by department
 - Compute sum of salaries by department
 - Partition students into passing and failing

With details in the JavaDoc linked above.

***Note:*** *this question is intentionally written and answered by the author ([Self-Answered Questions](https://stackoverflow.blog/2011/07/01/its-ok-to-ask-and-answer-your-own-questions/)), so that the idiomatic answers to commonly asked Kotlin topics are present in SO. Also to clarify some really old answers written for alphas of Kotlin that are not accurate for current-day Kotlin.*

Content Type	Original Author	Original Content on Stackoverflow
Question	frenchdonuts	View Question on Stackoverflow
Solution 1 - Generics	hotkey	View Answer on Stackoverflow

Generic extending class AND implements interface in Kotlin

Generics Problem Overview

Generics Solutions

Solution 1 - Generics

PHP - remove all non-numeric characters from a string

Python NameError: name 'include' is not defined

Attributions