Inheritance and test result "Instanceof"
I am learning "instanceof" java, but I could not understand "instanceof", I thought below answer would be true and false, but the result is correct. Could you please explain why this result is happening? As I know when A is a child of B (parent) and instance B is "false", but the result is different from what I thought.
class Car{
String color;
int door;
}
class FireEngine extends Car{
void water(){
System.out.println("water");
}
}
public class Operator {
public static void main(String[] args) {
Car car = new FireEngine();
FireEngine fireCar = new FireEngine();
System.out.println(car instanceof FireEngine);
System.out.println(fireCar instanceof Car);
}
}
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Declaration! = Value
You advertise car
as a car, but a value FireEngine
.
instanceof
works based on values, not on their variable declarations !!!
An abbreviation can help you understand:
System.out.println(new FireEngine() instanceof FireEngine); // true
System.out.println(new FireEngine() instanceof Car); // true
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The output instanceof
depends on the runtime type of the variable you are testing. The compile-time type of the variable does not matter (as long as it is possible for x instanceof
Y to return true for some value x
, otherwise the expression will fail compilation).
Both car
and fireCar
include copies FireEngine
in your code. And as FireEngine
a kind of a car
, then car
, and fireCar
it is instanceof
like car
, and FireEngine
, therefore, your code prints true
and true
.
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Instanceof operator implementation. Returns a Boolean value if the Object parameter (which can be an expression) is an instance of the class type.
Input 1: an object or expression that returns an object.
Input 2: a class or expression that returns a class Returns: A Boolean value that is the result of testing an object against a class.
For more information go to javadocs @ http://docs.oracle.com/cd/E13155_01/wlp/docs103/javadoc/com/bea/p13n/expression/operator/Instanceof.html
For a more detailed explanation with examples, visit the following web page: http://mindprod.com/jgloss/instanceof.html
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There are two types of bindings in Java: Static (reference type) and dynamic (object type).
In your case:
Car car = new FireEngine();
Car is a static type and FireEngine is a dynamic type. This means that you are actually working with a FireEngine (type object ). You can imagine the Car as a special sign with the car shape pointing to the real object, which is your amazing Fire Earth. If you are reading "instanceof" you can understand this, this method tells you that an object is an instance of a class and not a reference. So the compiler will see: FireEngine (car) instanceOf FireEngine? Of course, let the truth come back!
You can also have a look at this post: What is the 'instanceof' operator used for?
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