What is a compile constant expression?

A compile time constant expression is an expression whose value can be determined at compile time and can not include any value that could possibly change after compilation.

A compile-time constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the following (JLS3: 15.28 constant expression):

• Literals of primitive type and literals of type String
• Casts to primitive types and casts to type String
• The unary operators +, -, ~, and ! (but not ++ or --)
• The multiplicative operators *, /, and %
• The additive operators + and -
• The shift operators <<, >>, and >>>
• The relational operators <, <=, >, and >= (but not instanceof)
• The equality operators == and !=
• The bitwise and logical operators &, ^, and |
• The conditional-and operator && and the conditional-or operator ||
• The ternary conditional operator ? :
• Parenthesized expressions whose contained expression is a constant expression.
• Simple names that refer to constant variables (?4.12.4).
• Qualified names of the form TypeName . Identifier that refer to constant variables (?4.12.4).