Java 字符串操作方法封装：提取数字、反转、回文判断、字符计数、最长子字符串

以下是对应问题的解决方法：\n\n1. 获取给定字符串中的所有数字：\njava\npublic String getDigits(String str) {\n StringBuilder sb = new StringBuilder();\n for (int i = 0; i < str.length(); i++) {\n char ch = str.charAt(i);\n if (Character.isDigit(ch)) {\n sb.append(ch);\n }\n }\n return sb.toString();\n}\n\n\n2. 获取给定字符串的反序字符串：\njava\npublic String reverseString(String str) {\n StringBuilder sb = new StringBuilder(str);\n return sb.reverse().toString();\n}\n\n\n3. 判断给定字符串是否是对称回文：\njava\npublic boolean isPalindrome(String str) {\n StringBuilder sb = new StringBuilder(str);\n return sb.reverse().toString().equals(str);\n}\n\n\n4. 统计给定字符串中特定字符出现的次数：\njava\npublic int countChar(String str, char ch) {\n int count = 0;\n for (int i = 0; i < str.length(); i++) {\n if (str.charAt(i) == ch) {\n count++;\n }\n }\n return count;\n}\n\n\n5. 统计给定字符串中最长的子字符串：\njava\npublic String longestSubstring(String str) {\n String[] substrings = str.split(",");\n String longest = "";\n for (String substring : substrings) {\n if (substring.length() > longest.length()) {\n longest = substring;\n }\n }\n return longest;\n}\n