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Java Duck Typing without Reflection
2022-03-06
Can we implement duck typing in Java without using reflection?
Consider there are 2 classes defined by an external library that is out of our control.
class Duck {
void swim() {...}
}
class Whale {
void swim() {...}
}
We cannot do below.
Stream.of(new Duck(), new Whale()).forEach(obj -> {
obj.swim(); // Compilation error since obj is of type `Object` which has no swim() method
});
Ideally, we want to define a common interface with a swim() method, and let Duck and Whale both implement it. But since this is an external library, we cannot modify the definitions. Instead, we can use a pattern like below to achieve duck typing.
interface CanSwim {
static CanSwim coerce(Duck duck) {
return new CanSwim() {
@Override
public void swim() {
duck.swim();
}
};
}
static CanSwim coerce(Whale whale) {
return new CanSwim() {
@Override
public void swim() {
whale.swim();
}
};
}
void swim();
}
Usage of CanSwim
Stream.of(CanSwim.coerce(new Duck()), CanSwim.coerce(new Whale())).forEach(obj -> {
obj.swim(); // obj is a CanSwim now, which has the swim() method
});