查找一个该字符串中有多少个af

public static void main(String[] args) {
String str="asdfjvjadsffvaadfkfasaffdsasdffadsafafsafdadsfaafd";
int count=0;
for(int i = 0;i<str.length();i++)
{
String num=str.substring(i, i+2);
if(num.equals("af"))
{
count++;
}
}
System.out.println(count);
}

string s="asdfjvjadsffvaadfkfasaffdsasdffadsafafsafdadsfaafd";
int i=0;
for(;;){
if(s.indexof("af")!==-1){
s=s.substring(0,s.indexof("af")+1);
i++
}
else
break;
}