Java回文字符串检测程序判定结果异常的修正方案

Hey there! Let's sort out why your palindrome detection program is giving wrong results, even though it compiles just fine. The issue boils down to a super common Java gotcha with string comparison.

The Root Cause #

Your code's critical mistake is using == to compare the lowercase versions of a and rev. In Java:

• == checks if two string variables point to the exact same object in memory, not whether their content matches.
• Even if a and rev have identical characters (like with "level"), they're separate string objects (one's your input, the other's built by your loop), so == will always return false here.

The Fix #

Replace the == check with the equals() method, which compares the actual content of the strings:

if (a.equals(rev)) {
    System.out.println("It is a palindrome");
} else {
    System.out.println("It is not a palindrome");
}

Optional Improvements (For Cleaner, More Efficient Code) #

While we're polishing things up, here are a few tweaks to make your code better:

• Use StringBuilder for reversed strings: String concatenation in a loop creates unnecessary intermediate string objects. StringBuilder is more efficient:

  StringBuilder revBuilder = new StringBuilder();
  for (int i = b - 1; i >= 0; i--) {
      revBuilder.append(a.charAt(i));
  }
  String rev = revBuilder.toString();
  

• Skip the loop entirely: Java's StringBuilder has a built-in reverse() method that does the work for you:

  String rev = new StringBuilder(a).reverse().toString();
  

• Fix the main method signature: The standard main method should accept a String[] args parameter. Some Java environments might throw errors without it, even if the code compiles:

  public static void main(String[] args) {
      // Your code here
  }
  

Full Corrected Code #

Here's the complete, working version of your program with all core fixes applied:

import java.util.Scanner;
class Palindrome_string {
    public static void main(String[] args) {
        System.out.println("\f");
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter a string");
        String a = sc.nextLine();
        int b = a.length();
        String rev = "";
        for (int i = b - 1; i >= 0; i--) {
            char c = a.charAt(i);
            rev = rev + c;
        }
        System.out.println("Original word "+a);
        System.out.println("Reversed word "+rev);
        a = a.toLowerCase();
        rev = rev.toLowerCase();
        if (a.equals(rev)) {
            System.out.println("It is a palindrome");
        } else {
            System.out.println("It is not a palindrome");
        }
        sc.close();
    }
}

内容的提问来源于stack exchange，提问作者Mr Gubbo