What's an example of duck typing in Java?

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

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

In summary:

create an interface that represents the methods you want to use via duck typing
create a dynamic proxy that uses this interface and an implementation object that invokes methods of the interface on the underlying object by reflection (assuming the signatures match)

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.

What's an example of duck typing in Java?

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