运行时出现找不到类的异常，怎么办

public class text06{
    public static void main(String[] args){
        String s1 = "dsafa12a12werfa123fa12";
        String s2 = "fa12";
        int count = 0;
        int o = s1.length();
        String s3 = s1.replace(s2,"");
        int n = s3.length();
        count = (o-n)/4;
        System.out.println(count);
    }
}

运行时怎么会有找不到类的异常，编译时才会有吧

1.项目是否配置本地jdk
2.项目是否有中文路径
3.本地环境变量classpath是否正确