compile-time constant expression

 

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:

 

• Literals of primitive type and literals of type String (§3.10.5)
• 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) .

Compile-time constant expressions are used in case labels in switch statements (§14.11) and have a special significance for assignment conversion (§5.2) . Compile-time constants of type String are always "interned" so as to share unique instances, using the method String.intern.

A compile-time constant expression is always treated as FP-strict (§15.4) , even if it occurs in a context where a non-constant expression would not be considered to be FP-strict.

Examples of constant expressions:

true
(short)(1*2*3*4*5*6)
Integer.MAX_VALUE / 2
2.0 * Math.PI
"The integer " + Long.MAX_VALUE + " is mighty big."

 

其中complete abruptly就是指突然结束，比如抛出一个异常之类