Conversion problem between parent class and subclass in java

Example 1

The parent class forces the child class to rotate

Father f = new Father();
Son s = (Father)f;//出错 ClassCastException

Analysis:

Create an instance of the parent class and want to force Converting parent class objects into subclasses is not possible! In layman's terms, a real father can never pretend to be a son.

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Example 2

"Fake" parent class forced to rotate subclass

Father f = new Son();
Son s = (Son)f;//可以

Analysis:

The parent class object refers to a subclass instance.

The properties unique to the Son class cannot be operated by f for the time being, because the Father class does not have the unique properties of the Son class (subclass).

Then create the subclass object s, which refers to the object that was forcibly converted from the parent class object f (actually it is a pretending Son, who was forcibly converted back to Son). At this time, you can pass s To operate the unique attributes of the subclass.

In layman's terms, a son is pretending to be his father. He is still a son after all, and his essence has not changed. He can still be forced to turn back into a son.

Example 3

Forced subclass conversion to parent class

Son s = new Son();
Father f = (Father)s;//可以

Analysis:

The subclass is converted to the parent class, but the unique attributes of the subclass object cannot be Using the f operation, f can operate on its non-specific properties (properties inherited from the parent class).

In layman’s terms, what a son and his father have in common is that they are both human beings. A son is a human being and inherits it from his father. They both have basic human behaviors, but a son can never be the same generation as his father. (Subclass type is converted to parent class type).

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