Java如何编写正确的if语句校验4位double字面量合法性

原有代码核心问题 #

• 逻辑顺序错误：第一个判断直接判定长度<=4就输出合法，后续所有校验规则完全没有执行
• 字符比较逻辑错误：char类型的数字字符（如'0'）的ASCII值为48，直接写j <= 9是将字符的ASCII码值和数字9比较，永远不会匹配到数字字符，需要改成j >= '0' && j <= '9'
• 缺少核心校验逻辑：没有统计小数点出现次数、没有校验符号只能出现在首位、没有校验所有字符都属于允许的字符集
• 缺少边界保护：如果输入字符串长度不足4，调用charAt会直接抛出数组越界异常
• 存在冗余无效变量：i、n变量没有实际作用，相关逻辑无效

修正后完整实现 #

import java.util.Scanner;

public class DoubleLiteralChecker {
    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        System.out.print("Enter a valid four character double literal: ");
        String doubleliteral = keyboard.nextLine().trim();
        
        // 校验输入长度必须为4位
        if (doubleliteral.length() != 4) {
            System.out.print("\n" + doubleliteral + " is not a valid four character double literal.");
            keyboard.close();
            return;
        }
        
        int dotCount = 0;
        boolean isValid = true;
        
        for (int index = 0; index < doubleliteral.length(); index++) {
            char currentChar = doubleliteral.charAt(index);
            // 首位字符校验：允许为符号、小数点、数字
            if (index == 0) {
                if (currentChar == '+' || currentChar == '-') {
                    continue;
                } else if (currentChar == '.') {
                    dotCount++;
                    continue;
                } else if (currentChar >= '0' && currentChar <= '9') {
                    continue;
                }
                isValid = false;
                break;
            }
            // 非首位字符校验：不允许出现符号，仅允许小数点、数字
            else {
                if (currentChar == '+' || currentChar == '-') {
                    isValid = false;
                    break;
                } else if (currentChar == '.') {
                    dotCount++;
                    if (dotCount > 1) {
                        isValid = false;
                        break;
                    }
                    continue;
                } else if (currentChar >= '0' && currentChar <= '9') {
                    continue;
                }
                isValid = false;
                break;
            }
        }
        
        // 最终校验小数点必须恰好出现1次
        if (isValid && dotCount == 1) {
            System.out.print("\n" + doubleliteral + " is a valid four character double literal.");
        } else {
            System.out.print("\n" + doubleliteral + " is not a valid four character double literal.");
        }
        
        keyboard.close();
    }
}

内容的提问来源于stack exchange，提问作者Fried_Sauer.Kraut