To understand this, lets consider what the compiler does at each step for both possibilities.  Lets start with:

    ch = &#39;4&#39; + 2;

The compiler converts &#39;4&#39; to an int.  So it becomes
 
    ch = 52 + 2;

Which the compiler then turns into

    ch = 54;

ch is a char, and the compiler is allowed to convert 54 to a char as it can prove that there is no loss in the conversion. 


Now lets consider the second version:

    ch = ch  + 2;

ch has no known value at compile time.  Thus this becomes

    ch = ((int) ch) + 2;

Now the compiler cannot prove that the result of this (an int) is storable within the range of a char.  So it will **not** automatically narrow it, and reports it as an error.


EDIT1:

If the compiler can prove that the variable will never change, and is inlineable.  Then the second form can be turned into the first.   Subir pointed out that adding &#39;final&#39; makes this possible.  Although if a compiler was to perform change analysis then it is technically capable of figuring this out without the final keyword, but final does make it easier for the compiler and readers of the code.

EDIT2: 

Narrowing of int to char is covered in the [Java Language Spec][1], the link was kindly provided by Jon Skeet.


  [1]: http://docs.oracle.com/javase/specs/jls/se8/html/jls-5.html#jls-5.2

I need to concatenate a `String` and `Int` as below:

    let myVariable: Int = 8
    return &quot;first &quot; + myVariable

But it does not compile, with the error:

 &gt; Binary operator &#39;+&#39; cannot be applied to operands of type &#39;String&#39; and &#39;Int&#39;

What is the proper way to concatenate a String + Int?

Concatenate number with string in Swift

The following error occurs when I attempt to inflate a layout within a `ListView`:
    
    requestLayout() improperly called by android.widget.TextView{...} during layout: running second layout pass

I am attempting to inflate a layout within a `ListView` as follows:

    @Override
	public View getView(int position, View convertView, ViewGroup parent) {
        if(convertView == null){
            LayoutInflater inflater = (LayoutInflater) musicActivity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
	        convertView = inflater.inflate(R.layout.list_item, parent, false);
            ...
        }else{...}
    }

The layout being inflated can look as simple as the following, and will still produce the error

    &lt;TextView
		android:id=&quot;@+id/txt&quot;
		android:layout_width=&quot;wrap_content&quot;
		android:layout_height=&quot;wrap_content&quot;
		android:textSize=&quot;@dimen/txt_size&quot;/&gt;

I have looked into similar questions, and no solutions found seem to work [Question 1](https://stackoverflow.com/questions/21249935/requestlayout-improperly-called-by-android-widget-relativelayout-android), [Question 2](https://stackoverflow.com/questions/20717022/trace-requestlayout-improperly-called), [Question 3](https://stackoverflow.com/questions/19170863/requestlayout-improperly-called-by-error-on-android-4-3). 

Does anyone know what causes this type of error?  Any troubleshooting advice?  For more context, this `ListView` is displayed within a `Fragment` within a `ViewPager`


&lt;h1&gt;UPDATE&lt;/h1&gt;
Here is the full XML Layout (minus a bunch of attributes), that still results in the problem

    &lt;RelativeLayout xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
	 android:layout_width=&quot;match_parent&quot;
	 android:layout_height=&quot;match_parent&quot;&gt;
    	
	 &lt;TextView
		android:id=&quot;@+id/txt1&quot;
		android:layout_width=&quot;wrap_content&quot;
		android:layout_height=&quot;wrap_content&quot; /&gt;
    
	 &lt;TextView
		android:id=&quot;@+id/txt2&quot;
		android:layout_width=&quot;wrap_content&quot;
		android:layout_height=&quot;wrap_content&quot; /&gt;
	 
	 &lt;TextView
		android:id=&quot;@+id/txt3&quot;
		android:layout_width=&quot;wrap_content&quot;
		android:layout_height=&quot;wrap_content&quot; /&gt;
	 				
	&lt;TextView
	    android:id=&quot;@+id/txt4&quot;
	    android:layout_width=&quot;wrap_content&quot;
	    android:layout_height=&quot;wrap_content&quot;/&gt;
	 	  
    &lt;/RelativeLayout&gt;

Based on this, I would think the XML itself is not a problem, unless it has to do with the fact that I am using a ViewPager and Fragments

Android: requestLayout() improperly called

Why does

    char ch = &#39;4&#39;;
    ch = &#39;4&#39; + 2;

 work, but

    char ch = &#39;4&#39;;
    ch = ch  + 2;
doesn&#39;t?

Why can&#39;t you add an int and a char in some cases?

Why does
<pre><code class="hljs language-ini">char ch = '4';
ch = '4' + 2;
</code></pre>
work, but
<pre><code class="hljs language-ini">char ch = '4';
ch = ch + 2;
</code></pre>
doesn't?

While browsing the code for the Java 8 version of ForkJoinPool(which has a few interesting changes from Java 7) I ran across this construct ([here][1]):


    do {} while (!blocker.isReleasable() &amp;&amp;
                 !blocker.block());

I&#39;m struggling with why you would write it like this instead of just

    while (!blocker.isReleasable() &amp;&amp;
           !blocker.block());

Is it just a semantics/readability choice, since you could read the first construct as `do &quot;nothing&quot; while &quot;conditions&quot;`? Or is there some additional benefit I&#39;m missing?

  [1]: http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/concurrent/ForkJoinPool.java#l3207

Do &quot;nothing&quot; while &quot;condition&quot;

`Collectors.toMap` throws a `NullPointerException` if one of the values is `null`. I don&#39;t understand this behaviour, maps can contain null pointers as value without any problems. Is there a good reason why values cannot be null for `Collectors.toMap`?

Also, is there a nice Java 8 way of fixing this, or should I revert to plain old for loop?

An example of my problem:

    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    
    class Answer {
    	private int id;
    
    	private Boolean answer;
    
    	Answer() {
    	}
    
    	Answer(int id, Boolean answer) {
    		this.id = id;
    		this.answer = answer;
    	}
    
    	public int getId() {
    		return id;
    	}
    
    	public void setId(int id) {
    		this.id = id;
    	}
    
    	public Boolean getAnswer() {
    		return answer;
    	}
    
    	public void setAnswer(Boolean answer) {
    		this.answer = answer;
    	}
    }
    
    public class Main {
    	public static void main(String[] args) {
    		List&lt;Answer&gt; answerList = new ArrayList&lt;&gt;();
    
    		answerList.add(new Answer(1, true));
    		answerList.add(new Answer(2, true));
    		answerList.add(new Answer(3, null));
    
    		Map&lt;Integer, Boolean&gt; answerMap =
    		answerList
    				.stream()
    				.collect(Collectors.toMap(Answer::getId, Answer::getAnswer));
    	}
    }

Stacktrace:

    Exception in thread &quot;main&quot; java.lang.NullPointerException
    	at java.util.HashMap.merge(HashMap.java:1216)
    	at java.util.stream.Collectors.lambda$toMap$168(Collectors.java:1320)
    	at java.util.stream.Collectors$$Lambda$5/1528902577.accept(Unknown Source)
    	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1359)
    	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
    	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
    	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
    	at Main.main(Main.java:48)
    	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    	at java.lang.reflect.Method.invoke(Method.java:483)
    	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

This problem still exists in Java 11.

NullPointerException in Collectors.toMap with null entry values

I know there are questions relating to [java.util.Date][1] and Joda-Time. But after some digging, I couldn&#39;t find a thread about the differences between the [java.time API][2] (new in [Java 8][3], defined by [JSR 310][4]) and [Joda-Time][5].

I have heard that Java 8’s java.time API is much cleaner and can do much more than Joda-Time. But I cannot find examples comparing the two. 

 - What can java.time do that Joda-Time cannot? 
 - What can java.time do better than Joda-Time? 
 - Is the performance better with java.time?


  [1]: http://docs.oracle.com/javase/8/docs/api/java/util/Date.html
  [2]: http://docs.oracle.com/javase/8/docs/api/java/time/package-summary.html
  [3]: http://en.wikipedia.org/wiki/Java_version_history#Java_SE_8_.28March_18.2C_2014.29
  [4]: https://jcp.org/en/jsr/detail?id=310
  [5]: http://www.joda.org/joda-time/

Differences between Java 8 Date Time API (java.time) and Joda-Time

I can think of two ways:

    public static IntStream foo(List&lt;Integer&gt; list)
    {
        return list.stream().mapToInt(Integer::valueOf);
    }
    
    public static IntStream bar(List&lt;Integer&gt; list)
    {
        return list.stream().mapToInt(x -&gt; x);
    }

What is the idiomatic way? Maybe there is already a library function that does exactly what I want?

How do I get an IntStream from a List&lt;Integer&gt;?

I encountered this question in technical test for a job. Given the following code example:

    public class Manager {
    	public static void main (String args[]) {
    		System.out.println((int) (char) (byte) -2);
    	}
    }

It gives the output as 65534.

This behavior shows for negative values only; 0 and positive numbers yield the same value, meaning the one entered in SOP. The byte cast here is insignificant; I have tried without it.

So my question is: what exactly is going on here?

Content Type	Original Author	Original Content on Stackoverflow
Question	Lester	View Question on Stackoverflow
Solution 1 - Java	Chris K	View Answer on Stackoverflow

Why can't you add an int and a char in some cases?

Java Problem Overview

Java Solutions

Solution 1 - Java

Android: requestLayout() improperly called

Concatenate number with string in Swift

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