老师，遇到一些问题了

package com.imooc.plan;

import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.OutputStreamWriter;
import java.io.Writer;
import java.util.List;

import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;

public class Plan {

	public void planReader() {
		String file = "G:\\workspace\\Xml\\bin\\plan.xml";

		SAXReader reader = new SAXReader();
		try {
			Document document = reader.read(file);
			Element root = document.getRootElement();
			List<Element> courses = root.elements("course");
			for(Element course:courses) {
				System.out.println(course.elementText("course-name"));
				System.out.println(course.elementText("class-hour"));
				System.out.println(course.elementText("exam-form"));
			}
		} catch (DocumentException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

	public void planWriter() {
		String file = "G:\\workspace\\Xml\\bin\\plan.xml";

		    SAXReader reader = new SAXReader();
			Document document;
			try {
				document = reader.read(file);
				Element root = document.getRootElement();
				Element course=	root.addElement("course");
				course.addAttribute("course-num", "003");
				course.addElement("course-name").setText("哲学");
				course.addElement("class-hour").setText("52");
				course.addElement("exam-form").setText("考试");
				Writer writer= new FileWriter(file);

				document.write(writer);
				writer.close();
			} catch (DocumentException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (Exception e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}


	}


	public static void main(String[] args) {
		Plan plan=new Plan();
		plan.planReader();
	    plan.planWriter();
	}

}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE teaching-plan SYSTEM "plan.dtd">
<teaching-plan>
	<course course-num="001">
		<course-name>大学英语</course-name>
		<class-hour>36</class-hour>
		<exam-form>考试</exam-form>
	</course>
	<course course-num="002">
		<course-name>高等数学</course-name>
		<class-hour>70</class-hour>
		<exam-form>考试</exam-form>
	</course>
	<course course-num="003">
		<course-name>计算机应用基础</course-name>
		<class-hour>108</class-hour>
		<exam-form>上机考试</exam-form>
	</course>
</teaching-plan>

1---我写好读取方法后，就马上测试了，也能读取所有的信息了

2--我写好写入的方法后，写入发现文件里面的内容都没了，我哪里写错了？

3--发现文件里面没有后，因为eclipse还没有更新，里面还有内容，我又把里面的内容，复制到了文件里，然后就报错了？​