JAVA报Exception in thread "main" java.lang.NoClassDefFoundError的错误_"exception in thread \"main\" java.lang.noclassdef

package Test;
 
public class Test{
 
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		student student1 = new student("李明",18,"本科");
		student1.show();
		undergraduate unstudent1 = new undergraduate("计算机");
		unstudent1.name = "李丽";
		unstudent1.age = 20;
		unstudent1.show();
		unstudent1.showb();
		graduate grastudent1 = new graduate("机器学习");
		grastudent1.name = "刘明";
		grastudent1.age = 24;
		grastudent1.show();
		grastudent1.showm();
 
	}
 
}
 
class student {
	String name;
	int age;
	String degree;
	student(){
	}
	student(String name,int age){
		this.name = name;
		this.age = age;
	}
	student(String name,int age,String degree){
		this.name = name;
		this.age = age;
		this.degree = degree;
	}
	void show(){
		System.out.println("学生信息为：");
		System.out.println("姓名："+name);
		System.out.println("年龄："+age);
		System.out.println("学位为："+degree);
	}
}
 
class undergraduate extends student{
	
	String major;
	undergraduate(){		
		degree = "bachelor";
	}
	undergraduate(String major){
		degree = "bachelor";
		this.major = major;
	}
	void showb(){
		System.out.println("专业为："+major);
	}	
}
 
class graduate extends student{
	
	String  Research_Area;
	graduate(){		
		degree = "master";
	}
	graduate(String Research_Area){
		degree = "master";
		this.Research_Area = Research_Area;
	}
	void showm(){
		System.out.println("研究方向为："+Research_Area);
	}	
}

public class Student {
 
 
		/**
		 * @param args
		 */
		public static void main(String[] args) {
			// TODO Auto-generated method stub
			student student1 = new student("李明",18,"本科");
			student1.show();
			undergraduate unstudent1 = new undergraduate("计算机");
			unstudent1.name = "李丽";
			unstudent1.age = 20;
			unstudent1.show();
			unstudent1.showb();
			graduate grastudent1 = new graduate("机器学习");
			grastudent1.name = "刘明";
			grastudent1.age = 24;
			grastudent1.show();
			grastudent1.showm();
		}
 
	}
 
	class student {
		String name;
		int age;
		String degree;
		student(){
		}
		student(String name,int age){
			this.name = name;
			this.age = age;
		}
		student(String name,int age,String degree){
			this.name = name;
			this.age = age;
			this.degree = degree;
		}
		void show(){
			System.out.println("学生信息为：");
			System.out.println("姓名："+name);
			System.out.println("年龄："+age);
			System.out.println("学位为："+degree);
		}
	}
 
	class undergraduate extends student{
		
		String major;
		undergraduate(){		
			degree = "bachelor";
		}
		undergraduate(String major){
			degree = "bachelor";
			this.major = major;
		}
		void showb(){
			System.out.println("专业为："+major);
		}	
	}
 
	class graduate extends student{
		
		String  Research_Area;
		graduate(){		
			degree = "master";
		}
		graduate(String Research_Area){
			degree = "master";
			this.Research_Area = Research_Area;
		}
		void showm(){
			System.out.println("研究方向为："+Research_Area);
		}	
}

前一阵写过一个java小程序，可以正常运行。但是今天再去运行时，发现报出如下错误：

Exception in thread "main" java.lang.NoClassDefFoundError: Student (wrong name: student)

at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)

at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)

尝试了很多发方法，终于找到一个方法：就是在该工程下新建一个包，修改类名Student为Test就能正常运行。