To take advantage of these reference classes, you must break the strong reference to the MyObject object by setting obj to null.

要利用这些引用类，您必须取消对MyObject对象的强引用，方法是将ob j设置为null。

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Before the reference classes were introduced, only strong references were available. For example, the following line of code shows a strong reference, obj.

在引入引用类之前，我们只能使用强引用(strong reference)。

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