String str="zheshigekendiedetimu";
StringBuffer buff=new StringBuffer(str);
char[] arr=str.toCharArray();
Arrays.sort(arr);

for(int i=0,j=0;i<arr.length;i++){
if(String.valueOf(arr[i]).matches("[a-zA-Z]")){
while(true){
if(String.valueOf(buff.charAt(j)).matches("[a-zA-Z]")){
buff.setCharAt(j, arr[i]);
j++;
break;
}
j++;
}
}
}
//截取
System.out.println(buff.toString());
int s =5;
String str1 = buff.toString().substring(buff.toString().length()-s,buff.toString().length());
System.out.println(str1);//输出