java 检查父类是否是子类的实例
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Check if parent class is instance of child class
提问by user3339562
I have one class defined like this:
我有一个这样定义的类：
class Car {
}
And many other defined like this:
还有许多其他定义如下：
class Audi extends Car {
}
class Seat extends Car {
}
class Mercedes extends Car {
}
class Opel extends Car {
}
...
I have a situation where I receive a list of all these cars which is defined like this:
我有一种情况，我收到了所有这些汽车的列表，其定义如下：
List<Car> cars = new ArrayList<Car>();
In that list there are many different cars so how can I find out which one is Audi, which one is Seat etc?
在该列表中有许多不同的汽车，那么我如何才能找出哪一款是奥迪，哪一款是 Seat 等？
I've tried to do this for cars for which I know they are of type Audi:
我已经尝试为我知道它们属于奥迪类型的汽车执行此操作：
Audi audi = (Audi) cars.get(0);
This throws ClassCastException. How to handle this situation?
这抛出ClassCastException. 如何处理这种情况？
回答by Jesper
You can use instanceof:
您可以使用instanceof：
Car car = cars.get(0);
if (car instanceof Audi) {
// It's an Audi, now you can safely cast it
Audi audi = (Audi) car;
// ...do whatever needs to be done with the Audi
}
However, in practice you should use instanceofsparingly - it goes against object oriented programming principles. See, for example: Is This Use of the "instanceof" Operator Considered Bad Design?
然而，在实践中你应该instanceof谨慎使用——它违背了面向对象的编程原则。例如，请参阅：“instanceof”运算符的这种使用是否被认为是糟糕的设计？
回答by Tobias
Obvious or not, this will do the trick:
无论是否明显，这都可以解决问题：
Car car = cars.get(0);
if(car instanceof Audi) {
Audi audi = (Audi) car;
}
回答by Peter Pei Guo
Check with instanceof, for example:
使用 instanceof 检查，例如：
car instanceof Audi
This returns true if variable car is an instance of Audi, otherwise returns false.
如果变量 car 是 Audi 的实例，则返回 true，否则返回 false。
回答by Dmytro
You can use instanceofbefore casting. For example:
您可以instanceof在铸造前使用。例如：
Car someCar = cars.get(0);
Audi audi = null;
if(someCar instanceof Audi){
audi = (Audi) someCar;
}
if(audi != null){
//...
But likely it's a bad idea, because generics was introduced to avoid using casting and instanceof.
但这可能是一个坏主意，因为引入泛型是为了避免使用强制转换和instanceof.
