This class implements a mechanism to encapsulate a selector. A
selector is method signature descriptor. It is an easy way to create what may be called function pointers like in C language. I took my inspiration from Objective-C.
Java Selector
// File name: Message.java
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package util;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
/**
* <p>This class implements a mechanism to encapsulate a selector. A
* selector is like a method signature but the return type is
* optional. This enables to dynamically call the method on an
* instance or a class that match the selector.</p>
*
* <p>The idea is borrowed from the Objective-C language. In this language,
* we can encapsulate a message inside an object called a selector and
* later on use the selector to dispatch the message on some object.</p>
*
* <p>Here is an example:</p>
*
* <code>
* Class A { <br/>
* public void foo(String param) { <br/>
* // some implementation... <br/>
* } <br/>
* } <br/>
* <br/>
* // Let's create a Selector:<br/>
* Selector foo = Selector.newSelector("foo", String.class);<br/>
* <br/>
* // Let's create an A:<br/>
* A a = new A();<br/>
* <br/>
* // and call foo from 'a':<br/>
* foo.invoke(a, "Some string...");<br/>
* </code>
*
* <br/><hr/>
*
* <p>This is an Java alternative to the missing C function pointers.</p>
*
* <p>Note that the methods newSelector and invoke are overloaded as a
* conveniance.</p>
*
* <p>Instances of the class are immutable.</p>
*
* @author Pierre Thibault
* @version 2005-04-09
* @since 2004-04-08
*/
public final class Selector {
/**
* Set that countains all the selectors created to avoid duplicates.
*/
private static final Map SELECTOR_MAP = new HashMap();
/**
* Name of the method.
*/
private String name;
/**
* Parameter types of the method.
*/
private Class[] parameterTypes;
/**
* Return type of the method. May be null.
*/
private Class returnType;
private int hashCode;
/**
* Local version of NoSuchMethodException so we don't have to declare
* throws clauses.
*
*/
public static class NoSuchMethodException extends RuntimeException {
/**
* Default constructor.
*
*/
public NoSuchMethodException() {
}
/**
* Constructor with humain readable string.
* @param string The humain readable error.
*/
public NoSuchMethodException(String string) {
super(string);
}
}
/**
* Creates a selector for a method with no argument.
*
* @param name Method name.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelector(String name) {
return Selector.newSelectorRT(null, name, (Class[]) null);
}
/**
* Creates a selector for a method with one argument.
*
* @param name Method name.
* @param param1 Type of the parameter.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelector(String name, Class param1) {
return Selector.newSelectorRT(null, name, new Class[] { param1 });
}
/**
* Creates a selector for a method with two argument.
*
* @param name Method name.
* @param param1 Type of the first parameter.
* @param param2 Type of the second parameter.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelector(String name, Class param1,
Class param2) {
return Selector.newSelectorRT(null, name,
new Class[] { param1, param2 });
}
/**
* Creates a selector for a method with three argument.
*
* @param name Method name.
* @param param1 Type of the first parameter.
* @param param2 Type of the second parameter.
* @param param3 Type of the last parameter.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelector(String name, Class param1,
Class param2, Class param3) {
return Selector.newSelectorRT(null, name,
new Class[] { param1, param2, param3 });
}
/**
* Creates a selector for a method with four argument.
*
* @param name Method name.
* @param param1 Type of the first parameter.
* @param param2 Type of the second parameter.
* @param param3 Type of the third parameter.
* @param param4 Type of the last parameter.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelector(String name, Class param1,
Class param2, Class param3, Class param4) {
return Selector.newSelectorRT(null, name,
new Class[] { param1, param2, param3, param4 });
}
/**
* Creates a selector for a method with five argument.
*
* @param param1 Type of the first parameter.
* @param param2 Type of the second parameter.
* @param param3 Type of the third parameter.
* @param param4 Type of the fourth parameter.
* @param param5 Type of the last parameter.
* @param name Method name.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelector(String name, Class param1,
Class param2, Class param3, Class param4, Class param5) {
return Selector.newSelectorRT(null, name,
new Class[] { param1, param2, param3, param4, param5 });
}
/**
* Creates a selector for a method with a return type but no argument.
*
* @param returnType return type of the method.
* @param name Method name.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelectorRT(Class returnType, String name) {
return Selector.newSelectorRT(returnType, name, (Class[]) null);
}
/**
* Creates a selector for a method with a return type and a argument.
*
* @param returnType return type of the method.
* @param name Method name.
* @param param1 Type of the first parameter.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelectorRT(Class returnType, String name,
Class param1) {
return Selector.newSelectorRT(returnType, name,
new Class[] { param1 });
}
/**
* Creates a selector for a method with a return type but no argument.
*
* @param returnType return type of the method.
* @param name Method name.
* @param param1 Type of the first parameter.
* @param param2 Type of the second parameter.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelectorRT(Class returnType, String name,
Class param1, Class param2) {
return Selector.newSelectorRT(returnType, name, new Class[] { param1,
param2 });
}
/**
* Creates a selector for a method with a return type but no argument.
*
* @param returnType return type of the method.
* @param name Method name.
* @param param1 Type of the first parameter.
* @param param2 Type of the second parameter.
* @param param3 Type of the third parameter.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelectorRT(Class returnType, String name,
Class param1, Class param2, Class param3) {
return Selector.newSelectorRT(returnType, name, new Class[] { param1,
param2, param3 });
}
/**
* Creates a selector for a method with a return type but no argument.
*
* @param returnType return type of the method.
* @param name Method name.
* @param param1 Type of the first parameter.
* @param param2 Type of the second parameter.
* @param param3 Type of the third parameter.
* @param param4 Type of the fourth parameter.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelectorRT(Class returnType, String name,
Class param1, Class param2, Class param3, Class param4) {
return Selector.newSelectorRT(returnType, name,
new Class[] { param1, param2, param3, param4 });
}
/**
* Creates a selector for a method with a return type but no argument.
*
* @param returnType return type of the method.
* @param name Method name.
* @param param1 Type of the first parameter.
* @param param2 Type of the second parameter.
* @param param3 Type of the third parameter.
* @param param4 Type of the fourth parameter.
* @param param5 Type of the fiveth parameter.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelectorRT(Class returnType, String name,
Class param1, Class param2, Class param3, Class param4,
Class param5) {
return Selector.newSelectorRT(returnType, name,
new Class[] { param1, param2, param3, param4, param5 });
}
/**
* Creates a selector for a method with no return type from the
* argument types defined in a specified array.
*
* @param name Method name.
* @param parameterTypes Array containing the parameter types of the
* method.
* @return The selector.
* @see #newSelectorRT(Class, String, Class[])
*/
public static Selector newSelector(String name, Class[] parameterTypes) {
return newSelectorRT(null, name, parameterTypes);
}
/**
* Creates a selector for a method with a return type from the
* argument types defined in a specified array.
*
* This is the method that will be called from the other convenient
* creation method.
*
* @param returnType Return type of the method (may be null).
* @param name Method name.
* @param parameterTypes Array containing the parameter types of the
* method.
* @return The selector.
* @throws IllegalArgumentException If the name of the method was null.
*/
public static Selector newSelectorRT(Class returnType, String name,
Class[] parameterTypes) {
if (name != null) {
final Selector tempSelector = new Selector(returnType, name, parameterTypes);
final Selector selector = (Selector) SELECTOR_MAP.get(tempSelector);
if (selector == null) {
SELECTOR_MAP.put(tempSelector, tempSelector);
return tempSelector;
}
return selector;
} else {
throw new IllegalArgumentException("The name of the message was null.");
}
}
/**
* Invocate the method with no argument that match the selector on the
* object obj.
*
* @param obj The object on wich we invoke the method.
* @return The result of the invocation.
*/
public Object invoke(Object obj) {
return invoke(obj, null);
}
/**
* Invocate the method with one argument that match the selector on the
* object obj.
*
* @param obj The object on wich we invoke the method.
* @param arg1 The argument.
* @return The result of the invocation.
*/
public Object invoke(Object obj, Object arg1) {
return invoke(obj, new Object[] { arg1 } );
}
/**
* Invocate the method with two arguments that match the selector on the
* object obj.
*
* @param obj The object on wich we invoke the method.
* @param arg1 The first argument.
* @param arg2 The second argument.
* @return The result of the invocation.
*/
public Object invoke(Object obj, Object arg1, Object arg2) {
return invoke(obj, new Object[] { arg1, arg2 } );
}
/**
* Invocate the method with three arguments that match the selector on the
* object obj.
*
* @param obj The object on wich we invoke the method.
* @param arg1 The argument.
* @param arg2 The argument.
* @param arg3 The argument.
* @return The result of the invocation.
*/
public Object invoke(Object obj, Object arg1, Object arg2, Object arg3) {
return invoke(obj, new Object[] { arg1, arg2, arg3 } );
}
/**
* Invocate the method with four arguments that match the selector on the
* object obj.
*
* @param obj The object on wich we invoke the method.
* @param arg1 The first argument.
* @param arg2 The second argument.
* @param arg3 The third argument.
* @param arg4 The fourth argument.
* @return The result of the invocation.
*/
public Object invoke(Object obj, Object arg1, Object arg2, Object arg3,
Object arg4) {
return invoke(obj, new Object[] { arg1, arg2, arg3, arg4 } );
}
/**
* Invocate the method with five arguments that match the selector on the
* object obj.
*
* @param obj The object on wich we invoke the method.
* @param arg1 The first argument.
* @param arg2 The second argument.
* @param arg3 The third argument.
* @param arg4 The fourth argument.
* @param arg5 The fiveth argument.
* @return The result of the invocation.
* @see #invoke(Object, Object[])
*/
public Object invoke(Object obj, Object arg1, Object arg2, Object arg3,
Object arg4, Object arg5) {
return invoke(obj, new Object[] { arg1, arg2, arg3, arg4, arg5 } );
}
/**
* Invocate the method with the arguments countain in args that match the
* selector on the object obj.
*
* This is the main method called for the other convenience methods of
* the same type.
*
* @param obj The object on wich we invoke the method.
* @param args The array of arguments.
* @return The result of the invocation.
* @throws NoSuchMethodException If the method can not be called. It
* could be because one of the following errors occured internaly:
* SecurityException, NoSuchMethodException, IllegalArgumentException,
* IllegalAccessException or InvocationTargetException.
* (NoSuchMethodException is defined internally in this class.)
*/
public Object invoke(Object obj, Object[] args) {
try {
final Method method = obj.getClass().getMethod(name,
parameterTypes);
if (returnType == null || method.getReturnType() == returnType) {
try {
return method.invoke(obj, args);
} catch (Exception e) {
throwException(obj);
}
} else {
throwException(obj);
}
} catch (Exception e) {
throwException(obj);
}
return null;
}
/**
* Returns true if a method is implemented for the selector on the object
* obj.
*
* @param obj The object on wich we do the verification.
* @return True if and only if obj implements the method.
*/
public boolean isImplemented(Object obj) {
if (obj != null) {
try {
Method method = obj.getClass().getMethod(name, parameterTypes);
return returnType == null || method.getReturnType() == returnType;
} catch (Exception e) {
return false;
}
}
return false;
}
/**
* Return the method name associated with the selector.
* @return The name of the method.
*/
public String getName() {
return name;
}
/**
* Return the parameter types of the method associated with the selector.
* @return The parameter types.
*/
public Class[] getParameterTypes() {
return parameterTypes;
}
/**
* Return the return type of method associated with the selector.
* It may be null. In such case the return type is not used when matching
* a method.
* @return the return type.
*/
public Class getReturnType() {
return returnType;
}
//------------------------------------------------------------------------
/**
* Equality comparator.
* @return True if equal.
*/
public final boolean equals(Object anObject) {
if (anObject == this) return true;
boolean result = false;
if (anObject instanceof Selector) {
final Selector selector = (Selector) anObject;
result = selector.name.equals(name)
&& selector.returnType == returnType
&& Arrays.equals(parameterTypes, selector.parameterTypes);
}
return result;
}
/**
* Hash code.
* @return Hash code.
*/
public final int hashCode() {
if (hashCode == 0) {
hashCode = name.hashCode();
if (parameterTypes != null) {
for (int i = 0; i < parameterTypes.length; i++) {
hashCode ^= parameterTypes[i].getName().hashCode();
}
}
}
return hashCode;
}
/**
* Private constructor of the class.
* @param returnType Return type of the selector.
* @param name Method name associeted with the selector.
* @param parameterTypes Parameter types of the arguments.
*/
private Selector(Class returnType, String name, Class[] parameterTypes) {
this.name = name;
this.parameterTypes = parameterTypes;
this.returnType = returnType;
}
/**
* Throw a NoSuchMethodException nicelly.
* @throws NoSuchMethodException
*/
private void throwException(Object obj) {
throw new NoSuchMethodException("Mehtod " + name + " was not found for "+obj);
}
}
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