要循環，但變量未初始化

Question

我的問題始於第一個do-while循環。 我要問“請輸入一個介於0到90之間的角度：”，如果用戶確實輸入了一個介於0到90之間的數字，他們必須輸入槍粉的數量。 他們的輸入將經過物理計算，如果結果不是“這很成功！”。 然后我想讓他們返回並再次輸入角度和火葯。 但是事情在

while (distance - distance2 != 0 || distance - distance2 != 1 || distance2 - distance != 1); System.out.println("It's a hit!");

該錯誤顯示：變量distance2可能尚未初始化。

---

    import java.util.*;
    import java.text.*;
    public class BombsAway {
    private static double ZERO_THOUSAND = 1000;
    private static double KG_TO_VELOCITY = 50;
    private static double GRAVITY = -9.81;
    public static void main(String[] args) {

      Scanner input = new Scanner(System.in);
      DecimalFormat threeDec = new DecimalFormat("#.##");
      Random gen = new Random();
      BombsAway distanceAn = new BombsAway();
      boolean invalidInput = true;
      double distance, distance2;

      System.out.println("Please enter a positive integer seed value: ");
      while(invalidInput) {
         try {
            int seedValue = Integer.valueOf(input.nextLine());
            if (seedValue <= 0) {
               System.out.println("Please enter a positive integer seed value:"
                + " ");
            } 
            else {
               distance = gen.nextDouble() * ZERO_THOUSAND;
               System.out.println("That target is " + threeDec.format(distance) 
                + "m away.");  
               do {
               System.out.println("Please enter an angle between 0 and 90 " +
                "degrees: ");
               while (invalidInput) {
                  try {
                     double angle = Double.valueOf(input.nextLine());
                     if (angle < 0 || angle > 90) {
                        System.out.println("Please enter an angle between " +
                         "0 and 90 degrees: ");
                     } 
                     else {
                        System.out.println("Please enter the amount of " + 
                         "gunpowder in kilograms: ");
                        try {
                           double gunpowder = 
                            Double.valueOf(input.nextLine());
                           //physics part
                           double initialV = gunpowder * KG_TO_VELOCITY;
                           double vX = initialV * Math.cos(angle);
                           double vY = initialV * Math.sin(angle);
                           double airBorn = (-vY - vY) / GRAVITY;
                           distance2 = (vX * airBorn);

                           if (distance > distance2) {
                              double finalDist = distance - distance2;
                              System.out.println("It's " +
                               threeDec.format(finalDist) + "m short.");
                           }
                           else if (distance < distance2) {
                              double finalDist = distance2 - distance;
                              System.out.println("It's " + 
                               threeDec.format(finalDist) + "m long.");
                           }
                        }
                        catch(NumberFormatException e) {
                           System.out.println("Please enter the amount of " +
                            "gunpowder in kilograms: ");
                        }
                     }
                  }
                  catch(NumberFormatException e) {
                      System.out.println("Please enter an angle between " +
                       "0 and 90 degrees: ");
                  }
               }
            }while (distance - distance2 != 0 || distance - distance2 != 1
              || distance2 - distance != 1);
                System.out.println("It's a hit!");
            }
         }
         catch(NumberFormatException e) {
            System.out.println("Please enter a positive integer seed value: ");
         }
      }
   }
}

Answer 1

在你的代碼- distance2內部條件塊中的一個初始化（在else塊內while循環）可能存在這樣的情況，當你while還是else條件將不會被滿足，在這種情況下， distance2保留未初始化和局部變量不能無需初始化即可使用，因此只需使用任何有意義的或所需的默認值將該distance2初始化為：

double distance2 = 0.0; 或具有您的代碼邏輯可以使用的某些特定值

Answer 2

distance2變量在try / catch塊中初始化。 可能會引發異常，並在初始化distance2之前將控制流放入catch塊中。 在catch塊之后，distance2在while循環的末尾用於計算中，因此是的：它可能未在此行初始化：

while (distance - distance2 == 0 || distance - distance2 == 1
      || distance2 - distance == 1)

如果在讀取input的gunpowder或angle值時拋出了NumberFormatException 。 還有一個if條件（對angle邊界檢查）可能會使distance2保持不變。