blocks|key|1514399|text|你可以用一个接口代替一个函数指针。假设您想遍历一个集合，并对每个元素执行一些操作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1514400|public+interface+IFunction+{
++public+void+execute(Object+o);
}|code-block|syntax|javascript|1514401|这是我们可以传递给CollectionUtils2.doFunc(集合c，IFunction+f)的接口。|1514402|public+static+void+doFunc(Collection+c,+IFunction+f)+{
+++for+(Object+o+:+c)+{
++++++f.execute(o);
+++}
}|1514403|举个例子，假设我们有一个数字集合，你想给每个元素加1。|1514404|CollectionUtils2.doFunc(List+numbers,+new+IFunction()+{
++++public+void+execute(Object+o)+{
+++++++Integer+anInt+=+(Integer)+o;
+++++++anInt%2B%2B;
++++}
});|1514405|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

You can substitue a function pointer with an interface. Lets say you want to run through a collection and do something with each element.

<pre><code>public interface IFunction {
 public void execute(Object o);
}
</code></pre>

This is the interface we could pass to some say CollectionUtils2.doFunc(Collection c, IFunction f).

<pre><code>public static void doFunc(Collection c, IFunction f) {
 for (Object o : c) {
 f.execute(o);
 }
}
</code></pre>

As an example say we have a collection of numbers and you would like to add 1 to every element.

<pre><code>CollectionUtils2.doFunc(List numbers, new IFunction() {
 public void execute(Object o) {
 Integer anInt = (Integer) o;
 anInt++;
 }
});
</code></pre>

blocks|key|1299871|text|您可以使用反射来完成此操作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1299872|将对象和方法名(作为字符串)作为参数传递，然后调用该方法。例如：|1299873|Object+methodCaller(Object+theObject,+String+methodName)+{
+++return+theObject.getClass().getMethod(methodName).invoke(theObject);
+++//+Catch+the+exceptions
}|code-block|syntax|javascript|1299874|然后使用它，如下所示：|1299875|String+theDescription+=+methodCaller(object1,+"toString");
Class+theClass+=+methodCaller(object2,+"getClass");|1299876|当然，检查所有异常并添加所需的强制转换。|1299877|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

You can use reflection to do it.

Pass as parameter the object and the method name (as a string) and then invoke the method. For example:

<pre><code>Object methodCaller(Object theObject, String methodName) {
 return theObject.getClass().getMethod(methodName).invoke(theObject);
 // Catch the exceptions
}
</code></pre>

And then use it as in:

<pre><code>String theDescription = methodCaller(object1, "toString");
Class theClass = methodCaller(object2, "getClass");
</code></pre>

Of course, check all exceptions and add the needed casts.

blocks|key|1514330|text|不，函数在java中不是第一类对象。您可以通过实现一个处理程序类来做同样的事情--这就是在Swing中实现回调的方式。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1514331|然而，在java的未来版本中有关于闭包(您正在讨论的正式名称)的建议-+Javaworld有一篇有趣的文章。|offset|length|1514332|entityMap|0|LINK|mutability|MUTABLE|url|http://www.javaworld.com/javaworld/jw-06-2008/jw-06-closures.html^0|0|10|9|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

No, functions are not first class objects in java. You can do the same thing by implementing a handler class - this is how callbacks are implemented in the Swing etc.

There are however proposals for closures (the official name for what you're talking about) in future versions of java - <a href="http://www.javaworld.com/javaworld/jw-06-2008/jw-06-closures.html" rel="noreferrer">Javaworld</a> has an interesting article.

blocks|key|1299757|text|这让人想起Steve+Yegge的Execution+in+the+Kingdom+of+Nouns。它基本上说明Java的每个操作都需要一个对象，因此没有像函数指针这样的“纯动词”实体。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1299758|entityMap|0|LINK|mutability|MUTABLE|url|http://steve-yegge.blogspot.com/2006/03/execution-in-kingdom-of-nouns.html^0|H|X|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

This brings to mind Steve Yegge's <a href="http://steve-yegge.blogspot.com/2006/03/execution-in-kingdom-of-nouns.html" rel="noreferrer">Execution in the Kingdom of Nouns</a>. It basically states that Java needs an object for every action, and therefore does not have "verb-only" entities like function pointers.

blocks|key|1299710|text|要实现类似的功能，您可以使用匿名内部类。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1299711|如果您要定义一个接口Foo|offset|length|style|CODE|1299712|interface+Foo+{
++++Object+myFunc(Object+arg);
}|code-block|syntax|javascript|1299713|创建一个方法bar，它将接收一个'function+pointer‘作为参数：|1299714|public+void+bar(Foo+foo)+{
++++//+.....
++++Object+object+=+foo.myFunc(argValue);
++++//+.....
}|1299715|最后，按如下方式调用该方法：|1299716|bar(new+Foo()+{
++++public+Object+myFunc(Object+arg)+{
++++++++//+Function+code.
++++}
}|1299717|entityMap^0|0|A|3|0|0|6|3|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|10|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|11|8|@$D|12|E|13|F|G]]|9|@]|A|$]]|$1|O|3|P|5|J|7|14|8|@]|9|@]|A|$K|L]]|$1|Q|3|R|5|6|7|15|8|@]|9|@]|A|$]]|$1|S|3|T|5|J|7|16|8|@]|9|@]|A|$K|L]]|$1|U|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|V|$]]

To achieve similar functionality you could use anonymous inner classes.

If you were to define a interface <code>Foo</code>:

<pre><code>interface Foo {
 Object myFunc(Object arg);
}
</code></pre>

Create a method <code>bar</code> which will receive a 'function pointer' as an argument:

<pre><code>public void bar(Foo foo) {
 // .....
 Object object = foo.myFunc(argValue);
 // .....
}
</code></pre>

Finally call the method as follows:

<pre><code>bar(new Foo() {
 public Object myFunc(Object arg) {
 // Function code.
 }
}
</code></pre>

blocks|key|1300004|text|Java8引入了lambda和method+references。因此，如果您的函数与functional+interface匹配(您可以创建自己的)，则可以在本例中使用方法引用。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1300005|Java提供了一组common+functional+interfaces。而您可以执行以下操作：|1300006|public+class+Test+{
+++public+void+test1(Integer+i)+{}
+++public+void+test2(Integer+i)+{}
+++public+void+consumer(Consumer<Integer>+a)+{
+++++a.accept(10);
+++}
+++public+void+provideConsumer()+{
+++++consumer(this::test1);+++//+method+reference
+++++consumer(x+->+test2(x));+//+lambda
+++}
}|code-block|syntax|javascript|1300007|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html|1|https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html|2|https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html^0|F|H|0|17|K|1|0|9|S|2|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]|$A|10|B|11|1|12]]|C|$]]|$1|D|3|E|5|6|7|13|8|@]|9|@$A|14|B|15|1|16]]|C|$]]|$1|F|3|G|5|H|7|17|8|@]|9|@]|C|$I|J]]|$1|K|3|-4|5|6|7|18|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]|S|$5|N|O|P|C|$Q|T]]|U|$5|N|O|P|C|$Q|V]]]]

Java8 has introduced lambdas and <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html" rel="noreferrer">method references</a>. So if your function matches a <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html" rel="noreferrer">functional interface</a> (you can create your own) you can use a method reference in this case.

Java provides a set of <a href="https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html" rel="noreferrer">common functional interfaces</a>. whereas you could do the following:

<pre><code>public class Test {
 public void test1(Integer i) {}
 public void test2(Integer i) {}
 public void consumer(Consumer&lt;Integer&gt; a) {
 a.accept(10);
 }
 public void provideConsumer() {
 consumer(this::test1); // method reference
 consumer(x -&gt; test2(x)); // lambda
 }
}
</code></pre>

blocks|key|1514449|text|Java中没有这样的东西。您需要将函数包装到某个对象中，并传递对该对象的引用，以便传递对该对象上的方法的引用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1514450|在语法上，这可以通过使用就地定义的匿名类或定义为类的成员变量的匿名类在一定程度上得到缓解。|1514451|示例：|1514452|class+MyComponent+extends+JPanel+{
++++private+JButton+button;
++++public+MyComponent()+{
++++++++button+=+new+JButton("click+me");
++++++++button.addActionListener(buttonAction);
++++++++add(button);
++++}

++++private+ActionListener+buttonAction+=+new+ActionListener()+{
++++++++public+void+actionPerformed(ActionEvent+e)+{
++++++++++++//+handle+the+event...
++++++++++++//+note+how+the+handler+instance+can+access+
++++++++++++//+members+of+the+surrounding+class
++++++++++++button.setText("you+clicked+me");
++++++++}
++++}
}|code-block|syntax|javascript|1514453|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|O|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|P|8|@]|9|@]|A|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

There is no such thing in Java. You will need to wrap your function into some object and pass the reference to that object in order to pass the reference to the method on that object.

Syntactically, this can be eased to a certain extent by using anonymous classes defined in-place or anonymous classes defined as member variables of the class. 

Example:

<pre><code>class MyComponent extends JPanel {
 private JButton button;
 public MyComponent() {
 button = new JButton("click me");
 button.addActionListener(buttonAction);
 add(button);
 }

 private ActionListener buttonAction = new ActionListener() {
 public void actionPerformed(ActionEvent e) {
 // handle the event...
 // note how the handler instance can access 
 // members of the surrounding class
 button.setText("you clicked me");
 }
 }
}
</code></pre>

blocks|key|1514574|text|检查闭包是如何在lambdaj库中实现的。它们实际上具有与C#代理非常相似的行为：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1514575|http://code.google.com/p/lambdaj/wiki/Closures|offset|length|1514576|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|1A|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@$D|O|E|P|1|Q]]|A|$]]|$1|F|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|C]]]]

Check the closures how they have been implemented in the lambdaj library. They actually have a behavior very similar to C# delegates:

<a href="http://code.google.com/p/lambdaj/wiki/Closures" rel="nofollow noreferrer">http://code.google.com/p/lambdaj/wiki/Closures</a>

blocks|key|1299937|text|相对于这里的大多数人来说，我是java的新手，但由于我还没有看到类似的建议，所以我有另一个建议。我不确定这是不是一个好的实践，甚至不确定之前的建议，我只是没有得到它。我只是喜欢它，因为我认为它是自我描述的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1299938|+/*Just+to+merge+functions+in+a+common+name*/
+public+class+CustomFunction{+
+public+CustomFunction(){}
+}

+/*Actual+functions*/
+public+class+Function1+extends+CustomFunction{
+public+Function1(){}
+public+void+execute(){...something+here...}
+}

+public+class+Function2+extends+CustomFunction{
+public+Function2(){}
+public+void+execute(){...something+here...}
+}

+.....
+/*in+Main+class*/
+CustomFunction+functionpointer+=+null;|code-block|syntax|javascript|1299939|然后根据应用程序，分配|1299940|+functionpointer+=+new+Function1();
+functionpointer+=+new+Function2();|1299941|等。|1299942|并通过以下方式调用|1299943|+functionpointer.execute();|1299944|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

Relative to most people here I am new to java but since I haven't seen a similar suggestion I have another alternative to suggest. Im not sure if its a good practice or not, or even suggested before and I just didn't get it. I just like it since I think its self descriptive.

<pre><code> /*Just to merge functions in a common name*/
 public class CustomFunction{ 
 public CustomFunction(){}
 }

 /*Actual functions*/
 public class Function1 extends CustomFunction{
 public Function1(){}
 public void execute(){...something here...}
 }

 public class Function2 extends CustomFunction{
 public Function2(){}
 public void execute(){...something here...}
 }

 .....
 /*in Main class*/
 CustomFunction functionpointer = null;
</code></pre>

then depending on the application, assign

<pre><code> functionpointer = new Function1();
 functionpointer = new Function2();
</code></pre>

etc. 

and call by

<pre><code> functionpointer.execute();
</code></pre>

This may be something common and trivial, but I seem to be having trouble finding a concrete answer. In C# there is a concept of delegates, which relates strongly to the idea of function pointers from C++. Is there a similar functionality in Java? Given that pointers are somewhat absent, what is the best way about this? And to be clear, we're talking first class here.

Function Pointers in Java

Java

这可能是常见和琐碎的事情，但我似乎很难找到一个具体的答案。在C#中有一个委托的概念，它与C++中的函数指针的思想密切相关。Java中有没有类似的功能？考虑到指针的缺失，最好的方法是什么？需要说明的是，我们在这里谈论的是头等舱。

问Java中的函数指针
EN

回答 9

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问Java中的函数指针EN

回答 9

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问Java中的函数指针
EN