在 java 中声明和初始化变量

Question

此代码用于掷骰子游戏（详细说明 java 如何编程）

为什么myPoint变量需要初始化，否则会出现编译错误？？？

我的意思是，如果第一卷有赢或输，则无需初始化 mypoint（因为它不再被使用）

如果第一次掷骰没有赢或输，myPoint 将在开关默认标签中获取值

import java.util.Random;

public class Craps 
{
   // create random number generator for use in method rollDice
   private static final Random randomNumbers = new Random(); 

   // enumeration with constants that represent the game status
   private enum Status { CONTINUE, WON, LOST };

   // constants that represent common rolls of the dice
   private static final int SNAKE_EYES = 2;
   private static final int TREY = 3;
   private static final int SEVEN = 7;
   private static final int YO_LEVEN = 11;
   private static final int BOX_CARS = 12;

   // plays one game of craps
   public static void main( String[] args )
   {
      int myPoint = 0; // point if no win or loss on first roll
      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 

      // while game is not complete
      while ( gameStatus == Status.CONTINUE ) // not WON or LOST
      { 
         sumOfDice = rollDice(); // roll dice again

         // determine game status
         if ( sumOfDice == myPoint ) // win by making point
            gameStatus = Status.WON;
         else 
            if ( sumOfDice == SEVEN ) // lose by rolling 7 before point
               gameStatus = Status.LOST;
      } // end while 

      // display won or lost message
      if ( gameStatus == Status.WON )
         System.out.println( "Player wins" );
      else
         System.out.println( "Player loses" );
   } // end main

   // roll dice, calculate sum and display results
   public static int rollDice()
   {
      // pick random die values
      int die1 = 1 + randomNumbers.nextInt( 6 ); // first die roll
      int die2 = 1 + randomNumbers.nextInt( 6 ); // second die roll

      int sum = die1 + die2; // sum of die values

      // display results of this roll
      System.out.printf( "Player rolled %d + %d = %d\n", 
         die1, die2, sum );

      return sum; // return sum of dice
   } // end method rollDice
} // end class Craps

Answer 1

您需要初始化 myPoint，因为并非通过代码的所有路径都会导致变量的首次使用（即 if 中的 sumOfDice == myPoint）赋值给myPoint.

具体来说，如果 sumOfDice 等于超出 switch 语句的 default 情况的任何值，就会发生这种情况。

与默认初始化的成员变量不同，局部变量需要显式初始化。如果编译器发现使用未分配变量的路径，则会产生错误。

Answer 2

Java 中的局部变量是在堆栈上创建的。在运行时，它不知道那个内存位置会是什么，所以这被认为是 "garbage"。编译器/运行时 不初始化 局部变量，只有实例（和静态）变量。

这就是为什么您总是需要在Java中初始化您的局部变量。如果不是，编译器会报错

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