字符串Java谜题的原因是什么？

Question

我写了以下代码

class String { 

    private final java.lang.String s; 

    public String(java.lang.String s){ 
        this.s = s; 
    } 

    public java.lang.String toString(){ 
        return s; 
    } 

    public static void main(String[] args) { 
        String s = new String("Hello world"); 
        System.out.println(s); 
    } 
}

当我执行它时，得到以下错误

The program compiled successfully, but main class was not found.
  Main class should contain method: public static void main (String[] args).

为什么会这样？。。。虽然定义了主要方法，但为什么系统不读取/识别它