Casting an instance to a type reeks of bad design. Still, there are situations where there is no other choice. The ability to do this has hence been part of Java since day one.
I think Java 8 created a need to slightly improve this ancient technique.
Static Casting
The most common way to cast in Java is as follows:
Static Casting
Object obj; // may be an integer
if (obj instanceof Integer) {
Integer objAsInt = (Integer) obj;
// do something with 'objAsInt'
}This uses the instanceof and cast operators, which are baked into the language. The type to which the instance is cast, in this case Integer, must be statically known at compile time, so let’s call this static casting.
If obj is no Integer, the above test would fail. If we try to cast it anyways, we’d get a ClassCastException. If obj is null, it fails the instanceof test but could be cast because null can be a reference of any type.
Dynamic Casting
A technique I encounter less often uses the methods on Class that correspond to the operators:
Dynamic Casting To Known Type
Object obj; // may be an integer
if (Integer.class.isInstance(obj)) {
Integer objAsInt = Integer.class.cast(obj);
// do something with 'objAsInt'
}Note that while in this example the class to cast to is also known at compile time, this is not necessarily so:
Dynamic Casting
Object obj; // may be an integer
Class<T> type = // may be Integer.class
if (type.isInstance(obj)) {
T objAsType = type.cast(obj);
// do something with 'objAsType'
}Because the type is unknown at compile type, we’ll call this dynamic casting.
The outcomes of tests and casts for instances of the wrong type and null references are exactly as for static casting.
Casting In Streams And Optionals
The Present
Casting the value of an Optional or the elements of a Stream is a two-step-process: First we have to filter out instances of the wrong type, then we can cast to the desired one.
With the methods on Class, we do this with method references. Using the example of Optional:
Casting In Optional
Optional<?> obj; // may contain an Integer Optional<Integer> objAsInt = obj .filter(Integer.class::isInstance) .map(Integer.class::cast);
That we need two steps to do this is no big deal but I feel like it is somewhat awkward and more verbose than necessary.
The Future (Maybe)
I propose to implement casting methods on Class which return an Optional or a Stream. If the passed instance is of the correct type, an Optional or a singleton Stream containing the cast instance would be returned. Otherwise both would be empty.
Implementing these methods is trivial:
New Methods On Class
public Optional<T> castIntoOptional(Object obj) {
if (isInstance(obj))
return Optional.of((T) obj);
else
Optional.empty();
}
public Stream<T> castIntoStream(Object obj) {
if (isInstance(obj))
return Stream.of((T) obj);
else
Stream.empty();
}This lets us use flatMap to filter and cast in one step:
FlatMap That Shit
Stream<?> stream; // may contain integers Stream<Integer> streamOfInts = stream. flatMap(Integer.class::castIntoStream);
Instances of the wrong type or null references would fail the instance test and would lead to an empty Optional or Stream. There would never be a ClassCastException.
Costs And Benefits
What is left to be determined is whether these methods would pull their own weight:
- How much code could actually use them?
- Will they improve readability for the average developer?
- Is saving one line worth it?
- What are the costs to implement and maintain them?
I’d answer these questions with not much, a little, yes, low. So it’s close to a zero-sum game but I am convinced that there is a small but non-negligible benefit.
What do you think? Do you see yourself using these methods?
| Reference: | Casting In Java 8 (And Beyond?) from our JCG partner Nicolai Parlog at the CodeFx blog. |
Static cast means converting to a more common type, like Integer to Object. Dynamic cast means the opposite direction, when you need runtime check.
Your use of the terms seems to have its root in C++ – the equivalent terms in Java would be widening and narrowing reference conversion. So “dynamic and static casting” are still free to get another meaning. I wrote above why I think they might be appropriate for what I am talking about.
Still, I know that those terms are not very common. I chose them based on the fact that the documentation of Object.isInstance calls itself the “the dynamic equivalent of the Java language instanceof operator”.
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