Java中的Double Dispatch自动化
作者：互联网
我有两个接口Query和Filter(Query是一个简化示例的类,我现在有1个查询),我现在想要写函数Query.applyFilter()取决于Filter是什么实际的,即NameFilter和DateFilter的不同函数和每隔一个过滤器.
我的解决方案如下：
interface Filter {
public abstract void modifyQuery(Query query);
};
class NameFilter implements Filter{
public void modifyQuery(Query query){
query.applyFilter(this);
}
};
class DateFilter implements Filter{
public void modifyQuery(Query query){
query.applyFilter(this);
}
};
class Query {
public void applyFilter(Filter filter){
filter.modifyQuery(this);
}
void applyFilter(NameFilter* filter) {
//"applying NameFilter";
}
void applyFilter(DateFilter* filter) {
//apply dateFilter
}
}
好的,这里我需要为每个Filter类重写modifyQuery()实现.
然后,我有解决方案如何在C中避免它：我们使用模板并在modifyQuery()中强制转换：
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <vector>
using namespace std;
class Query;
class IFilter
{
public:
virtual void modifyQuery(Query* query) = 0;
};
template <typename T>
class Filter : public IFilter
{
public:
virtual void modifyQuery(Query* query);
};
class DateFilter;
class NameFilter;
class Query
{
public:
void applyFilter(IFilter* filter)
{
cout << "applying Filter" << endl;
filter->modifyQuery(this);
}
void applyFilter(NameFilter* filter)
{
cout << "applying NameFilter" << endl;
}
void applyFilter(DateFilter* filter)
{
cout << "applying DateFilter" << endl;
}
};
template <typename T>
void Filter<T>::modifyQuery(Query* query)
{
query->applyFilter(dynamic_cast<T*> (this));
}
class DateFilter : public Filter<DateFilter>
{
};
class NameFilter : public Filter<NameFilter>
{
};
int main()
{
Query* query = new Query();
IFilter* nameFilter = new NameFilter();
IFilter* dateFilter = new DateFilter();
std::vector<IFilter*> filterList;
filterList.push_back(nameFilter);
filterList.push_back(dateFilter);
for (int i = 0; i < 2; ++i)
{
query->applyFilter(filterList[i]);
}
return 0;
}
但我无法在Java中使用此解决方案,因为泛型与模板的操作不同.可以在Java中使用什么来避免复制粘贴？
解决方法:
找到正确的方法时,Java不会考虑方法参数的运行时类型. Java简单地说明了变量的类型而不是值.
解：
Java 6注释可用于注释方法并实现多方法和值分派.所有这些都可以在运行时完成,无需任何特殊的编译或预处理,并且使用仍然可以合理地用户友好.
我们需要引入两个“简单”注释来注释：
方法：这种方法实现了多种方法？
参数：我们应该发布什么价值？
然后,我们可以处理注释并构建实现特定多方法的方法列表.需要对此列表进行排序,以便首先使用最具体的方法. “最具体”意味着对于每个方法参数(从左到右),参数类型/值更加专业化(例如,它是一个子类,或者它与指定值再次匹配).调用多方法意味着调用最具体的适用方法. “适用”意味着方法原型与实际运行时参数匹配,“最具体”意味着我们可以简单地搜索排序列表并找到适用的第一个.
注释处理可以包含在一个类中,然后可以在用户定义的方法中使用该方法,该方法将简单地使用实际的运行时参数调用多方法调度代码.
履行
Multi接口实现了用于标记多方法的运行时方法注释：
package jmultimethod;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Retention;
import java.lang.annotation.ElementType;
import java.lang.annotation.Target;
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Multi {
public String value();
}
接口V实现用于指定调度值的运行时参数注释：
package jmultimethod;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public @interface V {
public String value();
}
Multimethod代码如下：
package jmultimethod;
import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
public class Multimethod {
protected String name;
protected final ArrayList<Method> methods = new ArrayList<Method>();
protected final MethodComparator methodComparator = new MethodComparator();
public Multimethod(String name, Class... classes) {
this.name = name;
for(Class c: classes) {
add(c);
}
}
public void add(Class c) {
for(Method m: c.getMethods()) {
for(Annotation ma: m.getAnnotations()) {
if(ma instanceof Multi) {
Multi g = (Multi) ma;
if(this.name.equals(g.value())) {
methods.add(m);
}
}
}
}
sort();
}
protected void sort() {
Method[] a = new Method[methods.size()];
methods.toArray(a);
Arrays.sort(a, methodComparator);
methods.clear();
for(Method m: a) {
methods.add(m);
}
}
protected class MethodComparator implements Comparator<Method> {
@Override
public int compare(Method l, Method r) {
// most specific methods first
Class[] lc = l.getParameterTypes();
Class[] rc = r.getParameterTypes();
for(int i = 0; i < lc.length; i++) {
String lv = value(l, i);
String rv = value(r, i);
if(lv == null) {
if(rv != null) {
return 1;
}
}
if(lc[i].isAssignableFrom(rc[i])) {
return 1;
}
}
return -1;
}
}
protected String value(Method method, int arg) {
Annotation[] a = method.getParameterAnnotations()[arg];
for(Annotation p: a) {
if(p instanceof V) {
V v = (V) p;
return v.value();
}
}
return null;
}
protected boolean isApplicable(Method method, Object... args) {
Class[] c = method.getParameterTypes();
for(int i = 0; i < c.length; i++) {
// must be instanceof and equal to annotated value if present
if(c[i].isInstance(args[i])) {
String v = value(method, i);
if(v != null && !v.equals(args[i])) {
return false;
}
} else {
if(args[i] != null || !Object.class.equals(c[i])) {
return false;
}
}
}
return true;
}
public Object invoke(Object self, Object... args) {
Method m = null; // first applicable method (most specific)
for(Method method: methods) {
if(isApplicable(method, args)) {
m = method;
break;
}
}
if(m == null) {
throw new RuntimeException("No applicable method '" + name + "'.");
}
try {
return m.invoke(self, args);
} catch (Exception e) {
throw new RuntimeException("Method invocation failed '" + name + "'.");
}
}
}
要使用多方法,用户代码必须：
>使用多方法名称注释方法,例如
@Multi("myMultimethod")
>带注释的方法的名称可以是Java满意的任何内容.它很可能与多方法名称不同,因为某些方法可能具有类似的原型,足以导致Java编译器的名称冲突(可能因为编译器可能存在null值问题).此外,该方法应该对Multimethod类可见(例如公共).
>通过创建Multimethod对象来处理注释,例如：
protected Multimethod mm = new Multimethod("myMultimethod", getClass());
>根据需要使用参数定义多方法“入口点”方法.该方法使用上面创建的Multimethod对象调度,例如,
public void myMultimethod(Object X, Object Y) {
mm.invoke(this, X, Y);
}
>然后,多方法可以被称为任何普通的Java方法,例如,
myMultimethod(1, null);
限制：
值分派仅适用于Java注释支持的值,例如String类型的值.
以下代码基于Multiple Dispatch example
package jmultimethod;
public class AsteroidTest {
class Asteroid {}
class Spaceship {}
@Multi("collide")
public void collideOO(Object X, Object Y) {
log("?? Bang, what happened? ", X, Y);
}
@Multi("collide")
public void collideAA(Asteroid X, Asteroid Y) {
log("AA Look at the beautiful fireworks! ", X, Y);
}
@Multi("collide")
public void collideAS(Asteroid X, Spaceship Y) {
log("AS Is it fatal? ", X, Y);
}
@Multi("collide")
public void collideSA(Spaceship X, Asteroid Y) {
log("SA Is it fatal? ", X, Y);
}
@Multi("collide")
public void collideSS(Spaceship X, Spaceship Y) {
log("SS Who's fault was it? ", X, Y);
}
@Multi("collide")
public void collide1S(String X, Spaceship Y) {
log("1S any string? ", X, Y);
}
@Multi("collide")
public void collide2S(@V("hi") String X, Spaceship Y) {
log("2S 'hi' value? ", X, Y);
}
protected Multimethod mm = new Multimethod("collide", getClass());
public void collide(Object X, Object Y) {
mm.invoke(this, X, Y);
}
public void run() {
Object A = new Asteroid();
Object S = new Spaceship();
collide(A, A);
collide(A, S);
collide(S, A);
collide(S, S);
collide(A, 1);
collide(2, A);
collide(S, 3);
collide(4, S);
collide(5, null);
collide(null, null);
collide("hi", S);
collide("hello", S);
}
public void log(Object... args) {
for(Object o: args) {
if(o instanceof String) {
System.out.print(" " + (String) o);
} else {
System.out.print(" " + o);
}
}
System.out.println();
}
public static void main(String[] args) throws Exception {
AsteroidTest t = new AsteroidTest();
t.run();
}
}
程序输出(部分编辑以适应屏幕)是：
AA Look at the beautiful fireworks! Asteroid@1f24bbbf Asteroid@1f24bbbf
AS Is it fatal? Asteroid@1f24bbbf Spaceship@24a20892
SA Is it fatal? Spaceship@24a20892 Asteroid@1f24bbbf
SS Who's fault was it? Spaceship@24a20892 Spaceship@24a20892
?? Bang, what happened? Asteroid@1f24bbbf 1
?? Bang, what happened? 2 Asteroid@1f24bbbf
?? Bang, what happened? Spaceship@24a20892 3
?? Bang, what happened? 4 Spaceship@24a20892
?? Bang, what happened? 5 null
?? Bang, what happened? null null
2S 'hi' value? hi Spaceship@24a20892
1S any string? hello Spaceship@24a20892
标签：java,double-dispatch 来源： https://codeday.me/bug/20190629/1329805.html