Explicit type casting example in Java
I have come across this example on http://www.javabeginner.com/learn-java/java-object-typecasting and in the part where it talks about explicit type casting there is one example which confuses me.
The example:
class Vehicle {
String name;
Vehicle() {
name = "Vehicle";
}
}
class HeavyVehicle extends Vehicle {
HeavyVehicle() {
name = "HeavyVehicle";
}
}
class Truck extends HeavyVehicle {
Truck() {
name = "Truck";
}
}
class LightVehicle extends Vehicle {
LightVehicle() {
name = "LightVehicle";
}
}
public class InstanceOfExample {
static boolean result;
static HeavyVehicle hV = new HeavyVehicle();
static Truck T = new Truck();
static HeavyVehicle hv2 = null;
public static void main(String[] args) {
result = hV instanceof HeavyVehicle;
System.out.print("hV is an HeavyVehicle: " + result + "\n");
result = T instanceof HeavyVehicle;
System.out.print("T is an HeavyVehicle: " + result + "\n");
result = hV instanceof Truck;
System.out.print("hV is a Truck: " + result + "\n");
result = hv2 instanceof HeavyVehicle;
System.out.print("hv2 is an HeavyVehicle: " + result + "\n");
hV = T; //Sucessful Cast form child to parent
T = (Truck) hV; //Sucessful Explicit Cast form parent to child
}
}
In the last line where T is assigned the reference hV and typecast as (Truck), why does it say in the comment that this is a Successful Explicit Cast from parent to child? As I understand casting (implicit or explicit) will only change the declared type of object, not the actual type (which shouldn't ever change, unless you actually assign a new class instance to that object's field reference). If hv was already assigned an instance of a HeavyVehicle class which is a super class of the Truck class, how can then this field be type cast into a more specific subclass called Truck which extends from the HeavyVehicle class?
The way I understand it is that casting serves the purpose of limiting access to certain methods of an object (class instance). Therefore you can't cast an object as a more specific class which has more methods then the object's actual assigned class. That means that the object can only be cast as a superclass or the same class as the class from which it was actually instantiated. Is this correct or am I wrong here? I am still learning so I am not sure if this is the correct way of looking at things.
I also understand that this should be an example of downcasting, but I am not sure how this actually works if the actual type doesn't have the methods of the class to which this object is being downcasted. Does explicit casting somehow change the actual type of object (not just the declared type), so that this object is no longer an instance of HeavyVehicle class but now becomes an instance of Truck class?
Answer
Reference vs Object vs Types
The key, for me, is understanding the difference between an object and its references, or put in other words the difference between an object and its types.
When we create an object in Java, we declare its true nature, which will never change (e.g. new Truck()). But any given object in Java is likely to have multiple types. Some of these types are obviously given by the class hierarchy, others are not so obvious (i.e. generics, arrays).
Specifically for reference types, the class hierarchy dictates the subtyping rules. For instance in your example all trucks are heavy vehicles, and all heavy vehicles are vehicles. Therefore, this hierarchy of is-a relationships dictates that a truck has multiple compatible types.
When we create a Truck, we define a "reference" to get access to it. This reference must have one of those compatible types.
Truck t = new Truck(); //or
HeavyVehicle hv = new Truck(); //or
Vehicle h = new Truck() //or
Object o = new Truck();
So the key point here is the realization that the reference to the object is not the object itself. The nature of the object being created is never going to change. But we can use different kinds of compatible references to gain access to the object. This is one of the features of polymorphism here. The same object may be accessed through references of different "compatible" types.
When we do any kind of casting, we are simply assuming the nature of this compatibility between different types of references.
Upcasting or Widening Reference Conversion
Now, having a reference of type Truck, we can easily conclude that it's always compatible with a reference of type Vehicle, because all Trucks are Vehicles. Therefore, we could upcast the reference, without using an explicit cast.
Truck t = new Truck();
Vehicle v = t;
It is also called a widening reference conversion, basically because as you go up in the type hierarchy, the type gets more general.
You could use an explicit cast here if you wanted, but it would be unnecessary. We can see that the actual object being referenced by t and v is the same. It is, and will always be a Truck.
Downcasting or Narrowing Reference Conversion
Now, having a reference of type Vechicle we cannot "safely" conclude that it actually references a Truck. After all it may also reference some other form of Vehicle. For instance
Vehicle v = new Sedan(); //a light vehicle
If you find the v reference somewhere in your code without knowing to which specific object it is referencing, you cannot "safely" argument whether it points to a Truck or to a Sedan or any other kind of vehicle.
The compiler knows well that it cannot give any guarantees about the true nature of the object being referenced. But the programmer, by reading the code, may be sure of what s/he is doing. Like in the case above, you can clearly see that Vehicle v is referencing a Sedan.
In those cases, we can do a downcast. We call it that way because we are going down the type hierarchy. We also call this a narrowing reference conversion. We could say
Sedan s = (Sedan) v;
This always requires an explicit cast, because the compiler cannot be sure this is safe and that's why this is like asking the programmer, "are you sure of what you are doing?". If you lie to the compiler you will throw you a ClassCastException at run time, when this code is executed.
Other Kinds of Subtyping Rules
There are other rules of subtyping in Java. For instance, there is also a concept called numeric promotion that automatically coerce numbers in expressions. Like in
double d = 5 + 6.0;
In this case an expression composed of two different types, integer and double, upcasts/coerces the integer to a double before evaluating the expression, resulting in a double value.
You may also do primitive upcasting and downcasting. As in
int a = 10;
double b = a; //upcasting
int c = (int) b; //downcasting
In these cases, an explicit cast is required when information can be lost.
Some subtyping rules may not be so evident, like in the cases of arrays. For instance, all reference arrays are subtypes of Object[], but primitive arrays are not.
And in the case of generics, particularly with the use of wildcards like super and extends, things get even more complicated. Like in
List<Integer> a = new ArrayList<>();
List<? extends Number> b = a;
List<Object> c = new ArrayList<>();
List<? super Number> d = c;
Where the type of b is a subtype of the type of a. And the type of d is a subtype of the type of c.
And also boxing and unboxing are subject to some casting rules (yet again this is also some form of coercion in my opinion).