话不多说，直接上代码...........
public static void main(String[] args) {
String str="I'm go to swimming";
Set<String> set=new HashSet<>();
for (int i = 0; i < str.length(); i++) {
String s = str.substring(i, i+1);
set.add(s);
}
Iterator<String> it = set.iterator();
while(it.hasNext()){
String sr=(String)it.next();
int k=0;
for (int j = 0; j < str.length(); j++) {
if(sr.equals(str.substring(j,j+1))){
k++;
}
}
System.out.println(sr+"有"+k+"个");
}
}