freemarker中 The only legal comparisons are between two .. 问题解决

最近在用freemarker的时候碰到一个问题，在<#list>中的元素用<#if>进行比较

<#if category.categoryId==categoryId>

结果出现问题如下：

freemarker.template.TemplateException: The only legal comparisons are between two numbers, two strings, or two dates.
Left  hand operand is a freemarker.template.SimpleNumber
Right hand operand is a freemarker.template.SimpleScalar

其原因为左边与右边的类型不一样，将它们类型转换成一样就OK了

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