编写一个方法，输出在一个字符串中，指定字符串出现的次数。

public class Test {
    public static void main(String args[]) {
        String s = "I love java and I want to learn java!";
        String findString = "java";
        int count = Test.countString(s,findString);
        System.out.print("字符串个数为： " + count);
    }
    
    public static int countString(String s1, String s2) {
        int count = 0; 
        int findIndex = 0;
        int findLen = s2.length();
        findIndex = s1.indexOf(s2);
        while(findIndex != -1){
            s1 = s1.substring(findIndex + findLen);
            findIndex = s1.indexOf(s2);
            count++;
        }
        return count;
    }
    
}