Does instanceof return true if instance of a parent?

问题

I have a class Child that extends Parent.

Parent child = new Child();

if (child instanceof Parent){
    // Do something
}

Does this returns true or false, and why?

回答1:

Yes, it would. And why should it not?

Because child is in fact an instance of Parent. If, you want to perform an operation only for a child you should check

if (child instanceof Child){
}

However you should remember the following statement from Effective C++, by Scott Meyers :

"Anytime you find yourself writing code of the form "if the object is of type T1, then do something, but if it's of type T2, then do something else," slap yourself.

which I think applies in this case too. If you want to doSomething based on what type of class the referenced object belongs to, the following code structure should help you with it.

NOTE: I have not compiled it.

class Parent {
    public void doSomething() {
        System.out.println("I am the Parent, and I do as I like");
    }
}

class ChildA extends Parent {
    public void doSomething() {
        System.out.println("I am a child named A, but I have my own ways, different from Parent");
    }
}

class ChildB extends Parent {
    public void doSomething() {
        System.out.println("I am a child named B, but I have my own ways, different from my Parent and my siblings");
    }
}

public class Polymorphism101 {

    public static void main(String[] args) {

        Parent p = new Parent();
        p.doSomething();

        p = new ChildA();
        p.doSomething();

        p = new ChildB();
        p.doSomething();

    }

}

EDIT: A better example

You could be developing a drawing application. An application that draws shapes of any kind. In that case, you should have an abstract type Shape.

For purpose(s) like; drawing all shapes; list all shapes; find a shape or delete a shape, you need to have a list of Shapes. Since the list is of a parent type, it can store any shapes.

The Shape interface/abstract class/virtual class should have an abstract/pure virtual function Draw(). So, in your DrawToDeviceLoop, you just call Draw() for each shape, you never need to check what shape it is.

The Shape interface can have an abstract implementation AbstractShape, which can have shape name or id as data members and GetName, Cleanup and other functions with functionality common to all shapes.

Remember an abstract type cannot be instantiated, so Shape itself cannot be instantiated, as it cannot be drawn either.

回答2:

instanceof will return true if it's a subclass...

instanceof Documentation

回答3:

Yes. instanceof will be true whenever the reference(left side of the instanceof expression) can be cast to the ReferenceType(the type on the right side of the instanceof expression). This will be true for subclasses in relation to their parent:

Child child = new Child();
Parent parent = (Parent) child; //works!
assert child instanceof Parent; //true

From The Java Language Specification, Java SE 9 Edition:

15.20. Relational Operators
...
RelationalExpression instanceof ReferenceType

15.20.2. Type Comparison Operator instanceof
...
At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.

回答4:

ofcourse it returns true because child is an instance of the parent

来源：https://stackoverflow.com/questions/6304056/does-instanceof-return-true-if-instance-of-a-parent