当obj超出范围或被显式地指定为null时，垃圾收集器就认为没有对这个对象的其它引用，也就可以收集它了。
When obj goes out of scope or is explicitly assigned to null, the object is available for collection, assuming there are no other references to it.
当obj超出范围或被显式地指定为null时，垃圾收集器就认为没有对这个对象的其它引用，也就可以收集它了。
When obj goes out of scope or is explicitly assigned to null, the object is available for collection, assuming there are no other references to it.
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