String str = "abcd";
String newStr = str.substring(0, 2).concat(":").concat(str.substring(2));
System.out.println(newStr); // 输出 ab:cd
String str = "abcd";
StringBuilder sb = new StringBuilder(str);
for (int i = 2; i < sb.length(); i += 3) {
sb.insert(i, ":");
}
String newStr = sb.toString();
System.out.println(newStr); // 输出 ab:cd