is the getClass() method of the current object.
Using methods or properties of the current object within the current object can be ignored this，除非有参数名重复，才需要特别指定 this

class Demo {
    private String id;
    private String name;
    
    public Demo(String id, String name){
       this.id = id; // 参数与字段名重复，所以需要指定 this
       this.name = name;
    }
    
    public Demo(String username) {
       name = username; // 这里的name 就是当前对象的name
       id = getClass().toString(); //  这里的 getClass() 也是当前对象的getClass() 方法
    }   
}

Every time a member method is called, it will be passed in implicitlythis对象，编译器会去给getClass() 加上this