为什么只在App.js中实现本例子好用，新建其他的JS文件中不能用？

import React from 'react';

import './SubApp.css'

import CommentBox from './components/CommentBox';

import CommentList from './components/CommentList';

class SubApp extends React.Component{

constructor(props){

super(props);

this.state = {

comments:['This is my first replay']

};

this.addComment = this.addComment.bind(this);

}

addComment(msg){

this.setState({

comments:[...this.comments,msg]

});

}

render(){

const {comments} = this.state;

return(

<div className="SubApp">

<span>TEST</span>

<CommentList messages={comments}/>

<CommentBox  

commentsLength={comments.length}

onAddComment={this.addComment}

/>

</div>

);

}

}

export default SubApp;