一个简单的小程序，想尝试使用jna的Structure，我是Java初学者，还请各位大大给看看怎么搞定啊

package com;

import com.sun.jna.*;



public class wp_demo

{		

	public static class AAA extends Structure{

		public static class ByReference extends AAA implements Structure.ByReference {}

		int a;

		int b;

	}



	public static void main(String[] args)

	{

		AAA etherDevice = new AAA();

		//

	}

}

运行会报下面的错误：

Exception in thread "main" java.lang.IllegalArgumentException: Structure class com.wp_demo$AAA has unknown size (ensure all fields are public)
at com.sun.jna.Structure.calculateSize(Structure.java:1013)
at com.sun.jna.Structure.allocateMemory(Structure.java:247)
at com.sun.jna.Structure.<init>(Structure.java:150)
at com.sun.jna.Structure.<init>(Structure.java:140)
at com.sun.jna.Structure.<init>(Structure.java:136)
at com.sun.jna.Structure.<init>(Structure.java:127)
at com.wp_demo$AAA.<init>(wp_demo.java:6)
at com.wp_demo.main(wp_demo.java:14)