我的if（choice.isEmpty（））{}不起作用

Question

如果用戶沒有輸入值，我想顯示一條錯誤消息，所以我使用了這個。如果你可以幫助我在用戶沒有輸入一個很好的值時顯示我的錯誤。

while (!choice.equalsIgnoreCase("y") && !choice.equalsIgnoreCase("n")) 
    { //start of while 

     if (choice.isEmpty()) { 
      System.out.println("Error! This entry is required. Try again."); //display error message when the user enters invalid response 


     } 


     else { 
      System.out.println("Error! Entry must be 'y' or 'n'. Try again."); // display error message when the user enters no response 
     } 

     System.out.print ("Continue (y/n): "); //promt users for input 
     choice = sc.next(); 
} // End of while 

else部分工作，但不是if。它只是顯示空白行，當我按下輸入，我什麼都沒有輸入。這是整個程序的完整代碼。這是一款擊球平均程序。

import java.util.*; 
import java.text.NumberFormat; 

public class BattingAverage 

{ 



public static void main(String[] args) 
{ 

System.out.println("Welcome to the Batting Average Calculator."); 
System.out.println(); 

Scanner sc = new Scanner(System.in); // scanner for input 
String choice = "y"; // initialize string 
while (choice.equalsIgnoreCase("y")) //start of while to continue while the user answers y to the continue prompt below 

{ 

int numberOfTimesAtBat; 


System.out.print("Enter Number of times at bat: "); 
numberOfTimesAtBat = sc.nextInt(); 

System.out.println("\n" + "0 = out, 1 = single, 2 = double, 3 = triple, 4 = home run"); 

    int[] atBats = new int[numberOfTimesAtBat]; 

    for(int counter = 0; counter < numberOfTimesAtBat; counter++){ 
    System.out.print("Result for at-bat "+(counter+1)+": "); 
    atBats[counter] = sc.nextInt(); 

} //End of for 



int sum = 0; 


for (int i=0; i < atBats.length; i++) { 

sum += atBats[i]; 

} 
double Avg = sum/atBats.length; 



// System.out.print("Batting average:"); 
System.out.print("Slugging percent: " + Avg); 


System.out.print ("\n" + "Continue (y/n): "); //promt users for input 
    choice = sc.next(); //make choice the next input 


    while (!choice.equalsIgnoreCase("y") && !choice.equalsIgnoreCase("n")) 
    { //start of while 

     if (choice.isEmpty()) { 
      System.out.println("Error! This entry is required. Try again."); //display error message when the user enters invalid response 


     } 


     else { 
      System.out.println("Error! Entry must be 'y' or 'n'. Try again."); // display error message when the user enters no response 
     } 

     System.out.print ("Continue (y/n): "); //promt users for input 
     choice = sc.next(); 
} // End of while 




} // end of while 




} //end of main 
} // end of class 

Answer 1

你只是有While循環之前更改sc.next()到sc.nextLine()，

choice = sc.nextLine(); //make choice the next input 


    while (!choice.equalsIgnoreCase("y") && !choice.equalsIgnoreCase("n")) 
    { //start of while 

     if (choice.isEmpty()) { 
      System.out.println("Error! This entry is required. Try again."); //display error message when the user enters invalid response 


     } 


     else { 
      System.out.println("Error! Entry must be 'y' or 'n'. Try again."); // display error message when the user enters no response 
     } 

     System.out.print ("Continue (y/n): "); //promt users for input 
     choice = sc.next(); 
} 

Answer 2

這只是表明空白行，當我按下回車鍵，我不輸入任何內容。

scanner.next()不讀取新的生產線，所以你需要更換choice = sc.next();到choice = sc.nextLine();

Answer 3

假設你修改這樣的代碼。

System.out.print ("\n" + "Continue (y/n): "); //promt users for input 
     sc.useDelimiter("[;\r\n]+"); 
     choice = sc.next(); //make choice the next input 


      while (!choice.equalsIgnoreCase("y") && !choice.equalsIgnoreCase("n")) 
      { //start of while 


       if (choice.trim().isEmpty()) { 
        System.out.println("Error! This entry is required. Try again."); //display error message when the user enters invalid response 


       }else { 
        System.out.println("Error! Entry must be 'y' or 'n'. Try again."); // display error message when the user enters no response 
       } 

       System.out.print ("Continue (y/n): "); //promt users for input 
       choice = sc.next(); 
     }/