Notice that the invariant and other tests refer to an implied balance field, even though an interface can't define fields.

注意，虽然接口不能定义字段，但不变条件测试和其他测试引用了一个隐含的余额字段。

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For example, the default invariant condition for a field requires the field to be non-null.

例如，字段的默认不变条件要求字段不能为空。

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(The class invariant above was effectively a field-invariant specification!)

(上面的类不变条件实际上就是一个字段不变条件的规范！)

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