A class invariant must hold before and after all method calls (once the object is constructed).

类在方法调用之前和之后必须保持不变(一旦对象已经构建)。

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Similarly, field-invariant tests are never evaluated during object construction, but they are evaluated after construction completes.

类似地，字段的不变条件测试从不在对象构造期间进行，但是它们会在构造完成之后进行。

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This paper presents a novel object tracking method based on SIFT(scale invariant feature transform) feature matching.

提出一种基于尺度不变特征变换（SIFT）特征匹配的目标跟踪方法。

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