變量初始化並聲明後可能尚未初始化

Question

編譯它時說變量secondNumberString可能沒有被初始化。但是我已經將它聲明爲一個字符串並在secondNumber開關中初始化它。我在這裏做錯了什麼？我只是想將secondNumber轉換爲一個名爲seondNumberString的字符串，並將其輸入爲5，然後顯示它。它必須使用開關。

/* 
Programmer: 
Date: Wednesday, October 8, 2014 
Description: A simple calculator 
*/ 

import javax.swing.JOptionPane;  // Imports the JOptionPane class 

public class CalculatorStevenHasaka 
{ 
    public static void main(String[] args) 
    { 
     String input;     // To temporarily hold the input 
     String firstNumberString;  // To hold the string name of the first number 
     String secondNumberString; // To hold the string name of the second number 
     String operatorString;  // To hold the string name of the operator 
     int firstNumber;    // To hold the first number 
     int secondNumber;    // To hold the second number 
     int answer;     // To hold the answer 
     char operator;    // To hold the operator 

     // Ask the user for a number from 0-9 (The first number) 
     input = JOptionPane.showInputDialog(null, "Please enter the first number. \nIt must be from 0 to 9. \nNo decimals, please.", "Calculator v1.0", JOptionPane.QUESTION_MESSAGE); 
     // Convert the input to an integer 
     firstNumber = Integer.parseInt(input); 


     // Validate the input of firstNumber 
     while (firstNumber < 0 || firstNumber > 9) 
     { 
     // Ask the user for a number from 0-9 
     input = JOptionPane.showInputDialog(null, "Invalid number! \nIt must be a number from 0 to 9. \nNo decimals, please.", "Invalid Number", JOptionPane.WARNING_MESSAGE); 
     // Convert the number to an integer 
     firstNumber = Integer.parseInt(input); 
     } // End of firstNumber validation 


     // Ask the user for an operator 
     input = JOptionPane.showInputDialog(null, "Please input an operator. \nYou can use +, -, *, /, or ^", "Calculator v1.0", JOptionPane.QUESTION_MESSAGE); 
     // Convert the input to a character 
     operator = input.charAt(0); 


     // Validate the input of the operator 
     while ((operator != '+') && (operator != '-') && (operator != '*') && (operator != '/') && (operator != '^')) 
     { 
     // Ask the user for an operator 
     input = JOptionPane.showInputDialog(null, "Invalid operator! \nYou can only use +, -, *, /, or ^", "Invalid Operator", JOptionPane.WARNING_MESSAGE); 
     // Convert the input to a character 
     operator = input.charAt(0); 
     } // End of operator validation 


     // Ask the user for a number from 0-9 (The second number) 
     input = JOptionPane.showInputDialog(null, "Please enter the second number. \nIt must be from 0 to 9. \nNo decimals, please.", "Calculator v1.0", JOptionPane.QUESTION_MESSAGE); 
     // Convert the number to an integer 
     secondNumber = Integer.parseInt(input); 


     // Validate the input of secondNumber 
     while (secondNumber < 0 || secondNumber > 9) 
     { 
     //Ask the user for a number from 0-9 
     input = JOptionPane.showInputDialog(null, "Invalid number! \nIt must be a number from 0 to 9. \nNo decimals, please.", "Invalid Number", JOptionPane.WARNING_MESSAGE); 
     // Convert the number to an integer 
     secondNumber = Integer.parseInt(input); 
     } // End of secondNumber validation 


     // Convert firstNumber to a string 
     switch (firstNumber) 
     { 
     case 0: 
      firstNumberString = "Zero"; 
      break; 
     case 1: 
      firstNumberString = "One"; 
      break; 
     case 2: 
      firstNumberString = "Two"; 
      break; 
     case 3: 
      firstNumberString = "Three"; 
      break; 
     case 4: 
      firstNumberString = "Four"; 
      break; 
     case 5: 
      firstNumberString = "Five"; 
      break; 
     case 6: 
      firstNumberString = "Six"; 
      break; 
     case 7: 
      firstNumberString = "Seven"; 
      break; 
     case 8: 
      firstNumberString = "Eight"; 
      break; 
     case 9: 
      firstNumberString = "Nine"; 
      break; 
     default: 
      JOptionPane.showMessageDialog(null, "Invalid input!", "Error", JOptionPane.ERROR_MESSAGE); 
     } // End of firstNumber switch 


     // Convert secondNumber to a string 
     switch (secondNumber) 
     { 
     case 0: 
      secondNumberString = "zero"; 
      break; 
     case 1: 
      secondNumberString = "one"; 
      break; 
     case 2: 
      secondNumberString = "two"; 
      break; 
     case 3: 
      secondNumberString = "three"; 
      break; 
     case 4: 
      secondNumberString = "four"; 
      break; 
     case 5: 
      secondNumberString = "five"; 
      break; 
     case 6: 
      secondNumberString = "six"; 
      break; 
     case 7: 
      secondNumberString = "seven"; 
      break; 
     case 8: 
      secondNumberString = "eight"; 
      break; 
     case 9: 
      secondNumberString = "nine"; 
      break; 
     default: 
      JOptionPane.showMessageDialog(null, "Invalid input!", "Error", JOptionPane.ERROR_MESSAGE); 
     } // End of secondNumber switch 


     // Convert operator to a string and perform the calculations 
     if (operator == '+') 
     { 
     operatorString = "plus"; 
     answer = firstNumber + secondNumber; 
     JOptionPane.showMessageDialog(null, "Blah: " + secondNumberString); 
     } 
     else if (operator == '-') 
     { 
     operatorString = "minus"; 
     } 
     else if (operator == '*') 
     { 
     operatorString = "multiplied by"; 
     } 
     else if (operator == '/') 
     { 
     operatorString = "divided by"; 
     } 
     else if (operator == '^') 
     { 
     operatorString = "to the power of"; 
     } 
     else 
     { 
     JOptionPane.showMessageDialog(null, "Invalid input!", "Error", JOptionPane.ERROR_MESSAGE); 
     } // End of operator if/else if/else      
    } // End of main 
} // End of public class        

Answer 1

編譯器是不是足夠聰明，制定出你所投保的secondNumber每一個可能的值，你while循環的方式（這確保了secondNumber必須在0和9之間）。

你需要給secondNumberString默認值大switch/case之前如null或""防止這種情況發生，或者在default情況。

Answer 2

但我已經聲明它爲一個字符串並在secondNumber開關中初始化它。

那麼，你有，如果你有任何指定的情況下。但是，你的default情況就是：

default: 
    JOptionPane.showMessageDialog(null, "Invalid input!", "Error", JOptionPane.ERROR_MESSAGE); 

你有什麼期望的secondNumberString值是之後呢？你應該在那裏分配一個值 - 或者在那個點退出。鑑於您希望在切換之前驗證secondNumber，我會拋出某種RuntimeException而不是顯示消息對話框。編譯器會知道在那時你不會繼續嘗試使用該變量，因此在switch語句後肯定會分配將。

注意，即使我們可以告訴你永遠不會真正打在switch語句中default情況下，definite assignment和Java中reachability規則沒有覆蓋的想法，編譯器應該有理由相信secondNumber有成爲0..9之一。我們需要告訴它，我們真的，真的不希望到達這裏 - 這就是爲什麼例外是這裏最好的選擇。

順便說一句，如果你想將你的一個巨大的方法分解成許多不同的方法，那麼你的代碼將會是更清晰的。除了其他任何東西，你都不需要那麼多的局部變量。我還建議只在首次使用時聲明變量，而不是在頂部聲明所有變量。你也應該學會使用陣列 - 這將擺脫完全的switch語句...

Answer 3

編譯器看到的問題是在這裏：

// Convert secondNumber to a string 
    switch (secondNumber) 
    { 
    case 0: 
     secondNumberString = "zero"; 
     break; 
    case 1: 
     secondNumberString = "one"; 
     break; 
    case 2: 
     secondNumberString = "two"; 
     break; 
    case 3: 
     secondNumberString = "three"; 
     break; 
    case 4: 
     secondNumberString = "four"; 
     break; 
    case 5: 
     secondNumberString = "five"; 
     break; 
    case 6: 
     secondNumberString = "six"; 
     break; 
    case 7: 
     secondNumberString = "seven"; 
     break; 
    case 8: 
     secondNumberString = "eight"; 
     break; 
    case 9: 
     secondNumberString = "nine"; 
     break; 
    default: 
     JOptionPane.showMessageDialog(null, "Invalid input!", "Error", JOptionPane.ERROR_MESSAGE); 
    } // End of secondNumber switch 

secondNumber可能不是0之間8，因此造成secondNumberString不會得到一個值。

您可以在默認部分設置一個值以刪除此警告。

你的邏輯似乎是，它必須是8之間0只，所以我認爲你可以設定在默認

secondNumberString = null or secondNumberString = "" 

Answer 4

Java的嚴格要求變量沿着任何可能的路徑通過代碼進行初始化，下至指出使用變量的地方。

在上文中，secondNumberString未在default情況下switch語句的初始化。