“当看到一只鸟走起来像鸭子、游泳起来像鸭子、叫起来也像鸭子，那么这只鸟就可以被称为鸭子。“【引用维基】

一般而言，鸭子类型机制常见/用于动态语言，如Python、Ruby、JS。来段Python代码实例：

class Duck:

def quack(self):

print "Quaaaaaack!"

def feathers(self):

print "The duck has white and gray feathers."

class Person:

def quack(self):

print "The person imitates a duck."

def feathers(self):

print "The person takes a feather from the ground and shows it."

def in_the_forest(duck):

duck.quack()

duck.feathers()

def game():

donald = Duck()

john = Person()

in_the_forest(donald)

in_the_forest(john)

game()

从上面代码片段可以看出，鸭子类型设计风格更关注对象的使用，如正则(非成员)函数in_the_forest的形参duck，只要有方法quack和feathers即可，不用关注具体类型，这样使设计更加”范化“。当然鸭子类型机制并不仅限动态语言，在静态语言中也可见，本文主要阐述在Java中的鸭子类型机制(亦称潜在类型机制或结构化类型机制)实现。

实现方式一： 使用一个类或接口，并用有边界泛型方法实现，见代码：

// interface Performs.class

public interface Performs {

void quack();

void feathers();

}

// class Duck.class

public class Duck implements Performs {

public void quack() {

System.out.println("Quaaaaak!");

}

public void feathers() {

System.out.println("The duck has white and gry feathers!");

}

}

// class Person.class

public class Person implements Performs {

public void quack() {

System.out.println("The person imitates a duck!");

}

public void feathers() {

System.out.println("The person has yellow feather!");

}

}

// class MainClass.class

public class MainClass {

public static void inTheForest(T duck) {

duck.quack();

duck.feathers();

}

public static void main(String[] args) {

Duck d = new Duck();

Person p = new Person();

MainClass.inTheForest(d);

MainClass.inTheForest(p);

}

}方式一使用的前提是都有共同的基类或者接口，但是当两个类之间没有这个条件仅有相同的方法签名时如何实现呢？这时可以采用如下两种方法：

方式二： 利用反射，见代码

// class Duck.class

public class Duck {

public void quack() {

System.out.println("Quaaaaak!");

}

public void feathers() {

System.out.println("The duck has white and gry feathers!");

}

}

// class Person.class

public class Person {

public void quack() {

System.out.println("The person imitates a duck!");

}

public void feathers() {

System.out.println("The person has yellow feather!");

}

}

// class MainClass.class

import java.lang.reflect.*;

public class MainClass {

public static void inTheForest(Object duck) {

Class> duk = duck.getClass();

try {

try {

Method quack = duk.getMethod("quack");

quack.invoke(duck);

} catch(NoSuckMethodException e) {

System.err.println(duck + " cannot quack");

}

try {

Method feathers = duk.getMethod("feathers");

feathers.invoke(duck);

} catch(NoSuckMethodException e) {

System.err.println(duck + " cannot feathers");

}

} catch(Exception e) {

throw new RuntimeException(duck.toString(), e);

}

}

public static void main(String[] args) {

Duck d = new Duck();

Person p = new Person();

MainClass.inTheForest(d);

MainClass.inTheForest(p);

}

}

方式三： 模拟接口，使用适配器设计模式

import java.lang.reflect.InvocationTargetException;

import java.lang.reflect.Method;

// interface Performs.class

// 先前不包含这个接口，也不继承这个接口

interface Performs {

void quack();

void feathers();

}

// class Duck.class

class Duck {

public void quack() {

System.out.println("Quaaaaak!");

}

public void feathers() {

System.out.println("The duck has white and gry feathers!");

}

}

// class Person.class

class Person {

public void quack() {

System.out.println("The person imitates a duck!");

}

public void feathers() {

System.out.println("The person has yellow feather!");

}

}

// Adapter

class PerformsAdapter implements Performs {

private Object d;

public PerformsAdapter(Object d) {

this.d = d;

}

public void quack() {

Class> dk = d.getClass();

try {

Method qk = dk.getMethod("quack");

qk.invoke(d);

} catch (NoSuchMethodException e) {

System.out.println(d + " cannot quack");

} catch (InvocationTargetException e) {

e.printStackTrace();

} catch (IllegalAccessException e) {

e.printStackTrace();

}

}

public void feathers() {

Class> dk = d.getClass();

try {

Method ft = dk.getMethod("feathers");

ft.invoke(d);

} catch (NoSuchMethodException e) {

System.out.println(d + " cannot have feathers");

} catch (InvocationTargetException e) {

e.printStackTrace();

} catch (IllegalAccessException e) {

e.printStackTrace();

}

}

}

// class  MainClass.class

public class DuckTest {

public static void inTheForest(T duck) {

duck.quack();

duck.feathers();

}

public static void main(String[] args) {

Duck d = new Duck();

Person p = new Person();

DuckTest.inTheForest(new PerformsAdapter(d));

DuckTest.inTheForest(new PerformsAdapter(p));

}

}

参考文献：

1. 鸭子类型 http://zh.wikipedia.org/wiki/%E9%B8%AD%E5%AD%90%E7%B1%BB%E5%9E%8B

2. 《Thinking in Java》