The DecimalFormat specification about the pattern syntax says that

 "The negative subpattern is optional; if absent, then the positive subpattern prefixed with the localized minus sign ('-' in most locales) is used as the negative subpattern."
 
Use of term "localized minus sign" seems incorrect, because when a pattern is passed in the DecimalFormat constructor "new DecimalFormat(pattern)" the special characters in the pattern are always expected to be in non-localized (ASCII) form e.g. "." (decimal separator), "-" (minus sign) etc. This description in the specification doesn't seem right. Also, in the implementation, if an explicit negative subpattern is absent, the positive subpattern prefixed with non-localized minus sign is used as negative subpattern.
 "negPrefixPattern = "'-" + posPrefixPattern;".

The same is mentioned in LDML specification

http://unicode.org/reports/tr35/tr35-numbers.html#Special_Pattern_Characters

"A pattern contains a positive subpattern and may contain a negative subpattern, for example, "#,##0.00;(#,##0.00)". Each subpattern has a prefix, a numeric part, and a suffix. If there is no explicit negative subpattern, the implicit negative subpattern is the ASCII minus sign (-) prefixed to the positive subpattern. That is, "0.00" alone is equivalent to "0.00;-0.00"."