表达式节点支持包含算术和布尔(真或假逻辑)运算符的简单表达式。
Expression nodes support simple expressions involving arithmetic and Boolean (true or false logic) operators.
表达式可以简单到就是一个变量（如清单 2 所示）或者是一个方法调用（例如 getList()），亦或是包含布尔逻辑或三目运算符的复杂表达式。
This expression can be as simple as a variable (as shown in Listing 2) or a method call (like getList()), or a complex expression that involves boolean logic or a ternary operator.
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