Java - 带括号的开关 Label

Question

I found that parentheses can be used in switch label, eg:

switch(id) {
  case (CONSTANT):
  case (1):
     // Do action
     break;
}

But why Java allow parentheses in this case, is there a use case? because I can't use || or , to use multiple, eg

  case (CONSTANT||1):
  case (CONSTANT,1):

So why allow this syntax, I didn't find in JLS :

 SwitchLabel: case ConstantExpression: case EnumConstantName: default: EnumConstantName: Identifier

Answer 1

Well, a ConstantExpression can contain parentheses:

A 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:

• ...

• Parenthesized expressions (§15.8.5) whose contained expression is a constant expression.

• ...

Hence, since any constant expression (whose type is char, byte, short, int, Character, Byte, Short, Integer, String, or an enum type) is allowed after case , parentheses are allowed.

Answer 2

The case just needs to be a constant expression. Something in parenthesis may be a constant expression.

private static final int TWO = 2;

public static void main(String[] args) {
    foo(3);
    foo(9);
}

private static void foo(int i) {
    switch (i) {
        case (TWO + 1):
            System.out.println("a");
            break;
        case (TWO + 1) * 3:
            System.out.println("b");
            break;
    }
}

CONSTANT || 1 CONSTANT || 1 is not allowed because integers are not valid operands of || .

The comma syntax is not a thing.