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A java library to do duck typing

What is it

Duck typing is a style of dynamic typing where the type of an object is determined by its set of methods rather than by a defined class it belongs to.

Java does not allow duck typing out of the box. This library helps to overcome this limitation. Given an arbitrary object and an interface, this library lets you get an object that implements the interface by delegating to the object.

Project Site

More information can be found in the duckprxy project’s site (generated with Maven). Inter alia duckprxy’s api doc.


Let us assume we have to implement an interface (MyInterface), but only want to implement the method bar(), because the other methods will not be called. This can be done like this:

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

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

Now we can create an object that implements MyInterface:

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

Note: the DuckPrxy implementations are thread safe, so one instance can be safely shared / injected. The created proxies are thread safe iff the delegate is thread safe.

There is one limitation though: You can use anonymous classes for the delegate, but the methods called via duck typing must be made visible by implementing an interface or by extending a class that has these methods. Otherwise Java’s reflection API will not be able to see the methods.