unicode - What are "connecting characters" in Java identifiers?

Question

I am reading for SCJP and I have a question regarding this line:

Identifiers must start with a letter, a currency character ($), or a connecting character such as the underscore ( _ ). Identifiers cannot start with a number!

It states that a valid identifier name can start with a connecting character such as underscore. I thought underscores were the only valid option? What other connecting characters are there?

Answer 1

Here is a list of connecting characters. These are characters used to connect words.

http://www.fileformat.info/info/unicode/category/Pc/list.htm

U+005F _ LOW LINE
U+203F ? UNDERTIE
U+2040 ? CHARACTER TIE
U+2054 ? INVERTED UNDERTIE
U+FE33 ︳ PRESENTATION FORM FOR VERTICAL LOW LINE
U+FE34 ︴ PRESENTATION FORM FOR VERTICAL WAVY LOW LINE
U+FE4D ﹍ DASHED LOW LINE
U+FE4E ﹎ CENTRELINE LOW LINE
U+FE4F ﹏ WAVY LOW LINE
U+FF3F ＿ FULLWIDTH LOW LINE

This compiles on Java 7.

int _, ?, ?, ?, ︳, ︴, ﹍, ﹎, ﹏, ＿;

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An example. In this case tp is the name of a column and the value for a given row.

Column<Double> ︴tp︴ = table.getColumn("tp", double.class);

double tp = row.getDouble(︴tp︴);

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The following

for (int i = Character.MIN_CODE_POINT; i <= Character.MAX_CODE_POINT; i++)
    if (Character.isJavaIdentifierStart(i) && !Character.isAlphabetic(i))
        System.out.print((char) i + " ");
}

prints

$ _ ￠ ￡ ¤ ￥ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? € ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ︳ ︴ ﹍ ﹎ ﹏ ﹩ ＄ ＿ ￠ ￡ ￥ ?