import java.util.*;
class Palindrome
{
   public static void main(String args[])
   {
      String original, reverse = "";
      Scanner in = new Scanner(System.in);
      System.out.print("Enter a string : ");
      original = in.nextLine();
      int length = original.length();
      for ( int i = length - 1; i >= 0; i-- )
         reverse = reverse + original.charAt(i);
      if (original.equals(reverse))
         System.out.println("Entered string is a palindrome.");
      else
         System.out.println("Entered string is not a palindrome.");
    }
}
/*
OUTPUT:
Enter a string : MADAM
Entered string is a palindrome.
Enter a string : 15351
Entered string is a palindrome.
* /

你在这里使用了错误的逻辑。如果你想检查回文，你不应该使用double。希望这段代码有帮助！

double answer = input.nextDouble();
double x = input.nextDouble();
double y = input.nextDouble();
double z = input.nextDouble();

您的代码正在等待4个不同的输入。如果你全部输入4，它会运行-但是你的逻辑显然有问题。

正如其他人提到的，你是a）与双打和b）试图读取太多的数字：

import java.util.Scanner;
public class ChapterThreeQuiz {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter a three-digit integer: ");
         // Read an int
        int answer = 0;
        try {
            answer = input.nextInt();
        }
        catch (InputMismatchException ex) {
            // Handle error
        }
        // Make sure it's 3 digits
        if (answer < 100 || answer >= 1000) {
           // Do something with bad input
        }
        else {
            // Just need to check if first and third digits are equal.
            // Get those values using integer math
            int digit1 = answer / 100;
            int digit3 = answer % 10;
            if (digit1 == digit3) {
                System.out.println(answer + " is a palindrome! ");
            }
            else {
                System.out.println(answer + " is not a palindrome");
            }
        }
    }
}