Arrays.sort如果先将字符放入数组中，则可以使用进行此操作。（为了使用不区分大小写的自定义比较器，它必须是Character对象数组，而不是char基元。）
// put the characters into an array
Character[] chars = new Character[str.length()];
for (int i = 0; i < chars.length; i++)
chars[i] = str.charAt(i);
// sort the array
Arrays.sort(chars, new Comparator<Character>() {
public int compare(Character c1, Character c2) {
int cmp = Character.compare(
Character.toLowerCase(c1.charValue()),
Character.toLowerCase(c2.charValue())
);
if (cmp != 0) return cmp;
return Character.compare(c1.charValue(), c2.charValue());
}
});
// rebuild the string
StringBuilder sb = new StringBuilder(chars.length);
for (char c : chars) sb.append(c);
str = sb.toString();
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