声明方式：

```

class mainClass{

}

```

唯一构造器(默认初始化):

```

class Person(name: String, surname: String)

```

唯一构造器(自定义初始化)

```

class Person(name: String, surname: String) {

    init {

    }

}

```

继承

```

abstract class Animal(name: String){

}

class Person(name: String, surname: String) : Animal(name){

    init {

    }

}

```

只能继承 open 或者 abstract 限定的类