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 a is a subtype of the type of b. And the type of c is a subtype of the type of d.
Using covariance, wherever List<? extends Number> appears you can pass a List<Integer>, therefore List<Integer> is a subtype of List<? extends Number>.
Contravariance produce a similar effect and wherever the type List<? super Number> appears, you could pass a List<Object>, which makes of List<Object> a subtype of List<? super Number>.
And also boxing and unboxing are subject to some casting rules (yet again this is also some form of coercion in my opinion).