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Java Swing does not display Unicode character

I am working on a Java Swing project and I would like to add a "play/pause" button.
I looked for its Unicode symbol, and got its Java representation which is \u23EF.
So I tried the following:
JButton button = new JButton("\u23EF");
The problem is that the button is blank, the character is not displayed.
In theory, Java with Swing should be able to display Unicode characters exactly like this and without the help of any conversion or anything. I did some research and haven't found any solution.
Why isn't my program behaving correctly? Is it simply a UI problem or did is my code missing something?

Be sure that your font support this unicode symbol and declare it like :
Font font = new Font("HereIsTheNameOfYourFont", Font.PLAIN, 24);
Then initialize the button
JButton button = new JButton("\u23EF");
Finally set the font to button
button.setFont(font);
Ok, you are ready to add it to your JFrame then.

you need to change the encoding of your java project. if you are using eclipse IDE then you can write click on project folder and go to properties and in resource section find the Text file encoding and here you have to change it to UTF-8:
change to this:
System.out.println('\u23EF');
output: ⏯
if you are trying to run your file from cmd:
then this link will be helpful for you:
Setting the default Java character encoding
java -Dfile.encoding=UTF-8 … com.x.Main
you can change the encoding of workspaces in eclipse:
change workspace files encoding
and if you are using IntelliJ IDE then follow this link:
configuring-individual-file-encoding

Arabic in java error

I'm working currently on java project that uses Arabic Language, I found difficulty in writing in Arabic as shown in the image:
I wrote Arabic without any edit.
I added a reverse() method, it worked good but the letters aren't attached to each other, they're separate.
StringBuilder input = new StringBuilder();
input.append(jTextField2.getText());
input = input.reverse();
jTextField1.setText(input.toString());
I use site the flip the text, it didn't work as well.
I use the same site, but with jLabel it worked.
other method I use, but didn't work:
Try Orientation jTextField1.applyComponentOrientation(ComponentOrientation.RIGHT_TO_LEFT);
Change the IDE encoding to URT-8 (I'm using Netbeans-JDK8).
Can anyone help me how to write & print Arabic in java correctly?

Please refer to this question -
Forcing RTL order in a JTextArea
Here is a sugestion to start the string with the character \u202e to force the text to be RTL.
Also i think it is not good approach to reverse the string, as it is not good user experience when the user do "copy paste", as he will copy reversed string...

A string entirely composed of characters from the Arabic block should render with correct RTL presentation without any directionality control characters. If it does not, it is likely that you have a problem with your operating system configuration, not with your Java code. Reversing the string is a terrible idea. Trying for visual-order rendering is going to get all messed up.

Is it possible to write lambda letter (λ) in IntelliJ IDEA?

I would like to use this interface:
http://javaslang.com/javadoc/2.0.0-RC2/javaslang/%CE%BB.html
Fortunately auto completion works, if I start from package name.
But is it possible to write lamda greek letter in IntelliJ IDEA or generally in any editor?

This is all about your keyboard and OS, and not specific to Intellij IDEA. You can create one by copy and pasting, which is pretty easy. If you want to insert it directly then use the way of typing special characters specific to your platform (Windows, Mac, Linux, ...).
So for example on Mac you could:
Do Edit > Special Characters and click on the gear wheel at the top left, select Customize and check the box for Greek. Double click or drag drop to input.
Go to system prefs/language & text/input sources and check the box for Greek, plus the box for Show Input Menu in Finder. Then select Greek from the "flag" menu at the top right of the screen and type.
There are also some suggestions on the Wikipedia article on entering special characters, though IntelliJ may intercept some of these.
I have to admit, I always just copy and paste.

You can express any Unicode character anywhere in Java source code via the \udddd form. It is not just for Strings and chars. (For characters outside the BMP, you need two of those, forming a surrogate pair.) Inasmuch as there sometimes are multiple characters with similar glyphs, Unicode escapes have the advantage of being totally unambiguous.
Presumably the character wanted in this case is the one Unicode names "Greek small letter lamda", U+03BB, which you can express anywhere in Java source code as \u03bb.

Eclipse console printing words incorrectly

I was doing some pdf text extractions.
I have attached screenshot of a scenario where i faced the problem.
Why the eclipse console failed to print the word "specification"?
Instead it is printed as "speci?cation".
I can see the characters overlapped.
But during debugging the code, the same text is shown without a "question mark".
Is there any way to print the same text to the console?
Please help.

The problem is the "fi" ligature ("overlapping letters") that is a single character in Unicode. In the debugging view the Windows methods for drawing text are used; these know about Unicode and can render the ligature correctly.
The console view uses a certain encoding. When used with Windows the default is "cp1252", Codepage 1252, or ISO 8859. These encodings do not know this specific letter and cannot print it, so the question mark is used as substitute.
You can set the encoding for Eclipse in general via Window > Preferences, General > Workspace, Text file encoding. While I think it is a good idea to use UTF-8 everywhere it may lead to problems with existing files.
You can set the encoding per project in the project properties, category Resource.
If you just want to set the encoding for the console view, the least immersive solution, it is not exactly intuitive. The console view encoding is a property of the runtime configuration you use for running your project. Run > Run Configurations..., your run configuration, Common.
When you use one of these methods to set the encoding to UTF-8 then the ligature will be printed correctly to the console view.
Of course the more general settings only have effect if not overwritten by more specific ones (Workspace, Project, Run Configuration).

I'm surely getting all the terminology wrong here, but the PDF is probably using a glyph for the "fi" combination that isn't part of the ASCII character set. Thus it renders in the console as "?". Notice in the middle part of the window that the "i" in "fi" is closer to the "f" than it would be if it were the ASCII sequence "f" followed by "i" and that the "i" is also missing the dot.

Is it safe to use Unicode characters in Java GUI?

For a play button in a Java GUI I currently use a button with the label set to ' ▻ ' (found this symbol in a Unicode symbol table). As I understand, it is better to not use such symbols directly in source code but rather use the explicit unicode representation like \u25BB in this example, because some tools (editor, ...) might not be able to handle files with non-ASCII content (is that correct?).
Assuming the compiled class contains the correct character, under which circumstances would the GUI not show the intended symbol on a current PC operating system? Linux, Windows, Mac should all support UTF-16, right? Do available fonts or font settings cause problems to this approach?
(Of course I could add an icon, but why add extra resources if a symbol should already be available... given that this is a portable solution)

Do available fonts or font settings cause problems to this approach?
Unfortunately they do. You can use unicode in the source code of course, but the problem is that currently unicode has 246,943 code points assigned so obviously no font has even a fraction of those defined. You'll get squares or some other weird rendering when the glyph isn't available. I've had cases where relatively simple symbols such as ³ render fine on one Windows computer and show up as squares in the next, almost identical computer. All sort of language and locale settings and minor version changes affect this, so it's quite fragile.
AFAIK there are few, if any, characters guaranteed to be always available. Java's Font class has some methods such as canDisplay and canDisplayUpTo, which can be useful to check this at runtime.
Instead of using icons, you could bundle some good TrueType font that has the special characters you need, and then use that font everywhere in your app.

I currently use a button with the label set to ' ▻ '
rather than I always use JButton(String text, Icon icon), and Icon doesn't matter if is there this Font or another Font, UTF-16 or Unicode

Most of editors have support for unicode, so go on.
Look at this post: Eclipse French support
If you are using simple editor like notepad then when you save type name and below it choose UTF encoding ( http://www.sevenforums.com/software/72727-how-make-notepad-save-txt-files-unicode.html )