Q: What characters allowed in file names on Android? What special characters are allowed for file names on Android?
~!@#$%^&*()_+/\.,

Also, can I save file with Unicode name?

A: final String[] ReservedChars = {"|", "\\", "?", "*", "<", "\"", ":", ">"};

for(String c :ReservedChars){
    System.out.println(dd.indexOf(c));
    dd.indexOf(c);
}


A: This is correct InputFilter for File Names in Android:
    InputFilter filter = new InputFilter()
    {
        public CharSequence filter(CharSequence source, int start, int end, Spanned dest, int dstart, int dend) 
        { 
            if (source.length() < 1) return null;
            char last = source.charAt(source.length() - 1);
            String reservedChars = "?:\"*|/\\<>";
            if(reservedChars.indexOf(last) > -1) return source.subSequence(0, source.length() - 1);
            return null;
        }  
    };


A: *

*On Android (at least by default) the file names encoded as UTF-8.

*Looks like reserved file name characters depend on filesystem mounted (http://en.wikipedia.org/wiki/Filename).
I considered as reserved:
private static final String ReservedChars = "|\\?*<\":>+[]/'";


A: This is clearly filesystem and Android operating system dependent. On my oneplus/oxygenOS, the only characters in the accepted answer
private static final String ReservedChars = "|\\?*<\":>+[]/'";

that I could not use to rename a file were / and *
However, Android wide, the list above would seem to be sensible.

A: I tested this quickly on my Galaxy Note 8 on Android 4.4.2. The default My Files app helpfully greys out invalid characters which are as follows:
? : " * | / \ < >
I put all the other special chars available into a filename and it saved. This may not be consistent across all Android versions so maybe it's best to be conservative and replace them with similarly meaningful characters.

A: On Android as suggested there you can use an input filter to prevent user entering invalid characters, here is a better implementation of it:
/**
 * An input filter which can be attached to an EditText widget to filter out invalid filename characters
 */
class FileNameInputFilter: InputFilter
{
override fun filter(source: CharSequence?, start: Int, end: Int, dest: Spanned?, dstart: Int, dend: Int): CharSequence? {
    if (source.isNullOrBlank()) {
        return null
    }

    val reservedChars = "?:\"*|/\\<>\u0000"
    // Extract actual source
    val actualSource = source.subSequence(start, end)
    // Filter out unsupported characters
    val filtered = actualSource.filter { c -> reservedChars.indexOf(c) == -1 }
    // Check if something was filtered out
    return if (actualSource.length != filtered.length) {
        // Something was caught by our filter, provide visual feedback
            if (actualSource.length - filtered.length == 1) {
                // A single character was removed
                BrowserApp.instance.applicationContext.toast(R.string.invalid_character_removed)
            } else {
                // Multiple characters were removed                    
     BrowserApp.instance.applicationContext.toast(R.string.invalid_characters_removed)
                }
            // Provide filtered results then
            filtered
        } else {
            // Nothing was caught in our filter
            null
        }
    }
}


A: From android.os.FileUtils
    private static boolean isValidFatFilenameChar(char c) {
        if ((0x00 <= c && c <= 0x1f)) {
            return false;
        }
        switch (c) {
            case '"':
            case '*':
            case '/':
            case ':':
            case '<':
            case '>':
            case '?':
            case '\\':
            case '|':
            case 0x7F:
                return false;
            default:
                return true;
        }
    }

    private static boolean isValidExtFilenameChar(char c) {
        switch (c) {
            case '\0':
            case '/':
                return false;
            default:
                return true;
        }
    }

Note: FileUtils are hidden APIs (not for apps; for AOSP usage). Use as a reference (or by reflection at one's own risk)

A: According to wiki and assuming that you are using external data storage which has FAT32.

Allowable characters in directory entries

are

Any byte except for values 0-31, 127 (DEL) and: " * / : < > ? \ | + , . ; = [] (lowcase a-z are stored as A-Z). With VFAT LFN any Unicode except NUL

