源碼：

/*** Returns a string whose value is this string, with any leading and trailing* whitespace removed.* <p>* If this {@code String} object represents an empty character* sequence, or the first and last characters of character sequence* represented by this {@code String} object both have codes* greater than {@code '\u005Cu0020'} (the space character), then a* reference to this {@code String} object is returned.* <p>* Otherwise, if there is no character with a code greater than* {@code '\u005Cu0020'} in the string, then a* {@code String} object representing an empty string is* returned.* <p>* Otherwise, let <i>k</i> be the index of the first character in the* string whose code is greater than {@code '\u005Cu0020'}, and let* <i>m</i> be the index of the last character in the string whose code* is greater than {@code '\u005Cu0020'}. A {@code String}* object is returned, representing the substring of this string that* begins with the character at index <i>k</i> and ends with the* character at index <i>m</i>-that is, the result of* {@code this.substring(k, m + 1)}.* <p>* This method may be used to trim whitespace (as defined above) from* the beginning and end of a string.** @return A string whose value is this string, with any leading and trailing white* space removed, or this string if it has no leading or* trailing white space.*/public String trim() {int len = value.length;int st = 0;char[] val = value; /* avoid getfield opcode */while ((st < len) && (val[st] <= ' ')) {st++;}while ((st < len) && (val[len - 1] <= ' ')) {len--;}return ((st > 0) || (len < value.length)) ? substring(st, len) : this;}

trim函數功能是去除首位的空格，但是需要注意的是，如果單單自身使用了trim函數，并沒有賦值給實例，則，該自身沒有去除空格。

例：

String s = "aaaa "; s.trim(); System.out.println(s+"*"); s = s.trim(); System.out.println(s+"*");

在socket編程的時候，我接收服務端傳過來的數據，用了直接trim函數，并沒有賦值給自身，因為它已經轉成去除空餓的字符串了，導致后端接收的數據一直有多余的空格。

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