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/* Adapted from: http://thinking-in-code.blogspot.com/2008/11/duck-typing-in-java-using-dynamic.html */
package de.danoeh.antennapod.util;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
/**
* Allows "duck typing" or dynamic invocation based on method signature rather
* than type hierarchy. In other words, rather than checking whether something
* IS-a duck, check whether it WALKS-like-a duck or QUACKS-like a duck.
*
* To use first use the coerce static method to indicate the object you want to
* do Duck Typing for, then specify an interface to the to method which you want
* to coerce the type to, e.g:
*
* public interface Foo { void aMethod(); } class Bar { ... public void
* aMethod() { ... } ... } Bar bar = ...; Foo foo =
* DuckType.coerce(bar).to(Foo.class); foo.aMethod();
*
*
*/
public class DuckType {
private final Object objectToCoerce;
private DuckType(Object objectToCoerce) {
this.objectToCoerce = objectToCoerce;
}
private class CoercedProxy implements InvocationHandler {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
Method delegateMethod = findMethodBySignature(method);
assert delegateMethod != null;
return delegateMethod.invoke(DuckType.this.objectToCoerce, args);
}
}
/**
* Specify the duck typed object to coerce.
*
* @param object
* the object to coerce
* @return
*/
public static DuckType coerce(Object object) {
return new DuckType(object);
}
/**
* Coerce the Duck Typed object to the given interface providing it
* implements all the necessary methods.
*
* @param
* @param iface
* @return an instance of the given interface that wraps the duck typed
* class
* @throws ClassCastException
* if the object being coerced does not implement all the
* methods in the given interface.
*/
@SuppressWarnings({ "rawtypes", "unchecked" })
public <T> T to(Class iface) {
assert iface.isInterface() : "cannot coerce object to a class, must be an interface";
if (isA(iface)) {
return (T) iface.cast(objectToCoerce);
}
if (quacksLikeA(iface)) {
return generateProxy(iface);
}
throw new ClassCastException("Could not coerce object of type " + objectToCoerce.getClass() + " to " + iface);
}
@SuppressWarnings("rawtypes")
private boolean isA(Class iface) {
return objectToCoerce.getClass().isInstance(iface);
}
/**
* Determine whether the duck typed object can be used with the given
* interface.
*
* @param Type
* of the interface to check.
* @param iface
* Interface class to check
* @return true if the object will support all the methods in the interface,
* false otherwise.
*/
@SuppressWarnings("rawtypes")
public boolean quacksLikeA(Class iface) {
for (Method method : iface.getMethods()) {
if (findMethodBySignature(method) == null) {
return false;
}
}
return true;
}
@SuppressWarnings({ "unchecked", "rawtypes" })
private <T> T generateProxy(Class iface) {
return (T) Proxy.newProxyInstance(iface.getClassLoader(), new Class[] { iface }, new CoercedProxy());
}
private Method findMethodBySignature(Method method) {
try {
return objectToCoerce.getClass().getMethod(method.getName(), method.getParameterTypes());
} catch (NoSuchMethodException e) {
return null;
}
}
}
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