TreeMap的一个错误 ...cannot be cast to java.lang.Comparable

今天在页面使用jstl的<c:froEach>遍历HashMap

		<c:forEach items="${ydMap }" var="m">
			<div class="mbody">
				<div class="title cv">${m.key.stateName }<a href="${pageContext.request.contextPath }/pl/deleteBlackStatesByState.do?id=${m.key.id }&flag=0"><img src="${pageContext.request.contextPath }/images/jh.png" /></a>:</div>
				<div class="text">
					<c:forEach items="${m.value }" var="l">
						<span class="cv">${l.states }<a href="${pageContext.request.contextPath }/pl/removeBlackStates.do?id=${l.id }"><img src="${pageContext.request.contextPath }/images/jh.png" /></a></span>
					</c:forEach>
				</div>
			</div>
		</c:forEach>

但是每次刷新时顺序都会变化，原来HashMap是遍历时是无序的！如是就想到使用TreeMap

Map<BaseState,List<BlackChannelStates>> map=new TreeMap<BaseState,List<BlackChannelStates>>();

BaseState的代码 如下：

public class BaseState implements Comparable<BaseState>{

	private Integer id;

	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}
}

但是使用后就一直报, ...cannot be cast to java.lang.Comparable
查看了TreeMap的原码发现HashMap有如下的构造方法：

    public TreeMap(Comparator<? super K> comparator) {
        this.comparator = comparator;
    }

原来TreeMap是有序的，有序就说明TreeMap中的每个Key元素都需要能比较，只有这样才能排序。如是我就在BaseState这个Bean改成了如下：

public class BaseState implements Comparable<BaseState>{

	private Integer id;

	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}


	@Override
	public int compareTo(BaseState o) {
		// TODO Auto-generated method stub
		return o.getId()-this.getId();
	}
}

此时再调用这个错误解决了！