java - 局部变量初始化和开关选择

Question

本人初学java，本例尝试模拟掷骰子游戏。

当我没有初始化myPoint 变量时出现编译器错误，所以我在switch 构造之前将它设置为零。

我想明白为什么我需要初始化 myPoint 变量，当它的值设置为 sumOfDice 声明后在 switch 中的值时，不像 gameStatus 变量？

  /***** roll dice game ****/

 //some code here..

   private enum Status { CONTINUE, WON, LOST };

   // plays one game of craps
   public static void main( String[] args )
   {
      // point if no win or loss on first roll
      int myPoint = 0; 
      Status gameStatus; // can contain CONTINUE, WON or LOST

      int sumOfDice = rollDice(); // first roll of the dice

      // determine game status and point based on first roll 
      switch ( sumOfDice ) 
      {
         case SEVEN: // win with 7 on first roll
         case YO_LEVEN: // win with 11 on first roll           
            gameStatus = Status.WON;
            break;
         case SNAKE_EYES: // lose with 2 on first roll
         case TREY: // lose with 3 on first roll
         case BOX_CARS: // lose with 12 on first roll
            gameStatus = Status.LOST;
            break;
         default: // did not win or lose, so remember point         
            gameStatus = Status.CONTINUE; // game is not over
            myPoint = sumOfDice; // remember the point
            System.out.printf( "Point is %d\n", myPoint );
            break; // optional at end of switch
      } // end switch 

// rest of the code here ..

      // display won or lost message

   // roll dice, calculate sum and display results
   public static int rollDice()
   {
       // return sum of dice
    } 
} // end class Craps

Answer 1

根据 Java specification

A local variable (§14.4, §14.14) must be explicitly given a value before it is used, by either initialization (§14.4) or assignment (§15.26), in a way that can be verified using the rules for definite assignment (§16).

rules for switch 语句有效地强制您在 switch 语句的每个可能执行路径中初始化变量。

所以答案是:

• gameStatus 变量通过 switch 语句在每一个可能的执行路径中被初始化，因此编译器是满意的。
• myPoint 不会被初始化，例如，如果 sumOfDice 是 SEVEN，因此编译器会强制您在 switch block 之外初始化变量