What's an example of duck typing in Java?

Question

I just recently heard of duck typing and I read the Wikipedia article about it, but I'm having a hard time translating the examples into Java, which would really help my understanding.

Would anyone be able to give a clear example of duck typing in Java and how I might possibly use it?

Answer 1

check this library:

interface MyInterface {
    void foo();
    int bar(int x, int y);
    int baz(int x);
}

public class Delegate {
    public int bar() {
        return 42;
    }
}

DuckPrxy duckProxy = new DuckPrxyImpl();
MyInterface prxy = duckProxy.makeProxy(MyInterface.class, new Delegate());
prxy.bar(2, 3); // Will return 42.

With an interface duck typing is simple using a Dynamic Proxy, you should match the method name and return type.

Answer 2

Java is by design not fit for duck typing. The way you might choose to do it is reflection:

public void doSomething(Object obj) throws Exception {

    obj.getClass().getMethod("getName", new Class<?>[] {}).invoke(obj);
}

But I would advocate doing it in a dynamic language, such as Groovy, where it makes more sense:

class Duck {
    quack() { println "I am a Duck" }
}

class Frog {
    quack() { println "I am a Frog" }
}

quackers = [ new Duck(), new Frog() ]
for (q in quackers) {
    q.quack()
}

Reference

Answer 3

Java doesn't implement duck typing.

Answer 4

Typing in Java is nominal - compatibility is based on names. If you need an examples on how duck-typing (or structural typing) may look like in Java please look at this page: http://whiteoak.sourceforge.net/#Examples which provides examples for program written in Whiteoak: A Java-compatible language that also supports structural typing.

Answer 5

Typically, duck typing is used with dynamically typed languages. You would check at runtime for the existence of methods or properties that are required to fulfill your needs, regardless of inheritance hierarchies.

Other than using reflection, which would get ugly, the closest you can get is by using minimal interfaces that match the criteria of what you would need for duck typing. This blog post does a good job describing the concept. It loses much of the simplicity of duck typing in python or ruby or javascript, but its actually pretty good practice in Java if you're looking for a high level of reusability.

Answer 6

See this blog post. It gives a very detailed account of how to use dynamic proxies to implement duck typing in Java.