blocks|key|1436302|text|如果你指的是像.NET匿名委托这样的东西，我想Java的匿名类也可以使用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1436303|public+class+Main+{

++++public+interface+Visitor{
++++++++int+doJob(int+a,+int+b);
++++}


++++public+static+void+main(String[]+args)+{
++++++++Visitor+adder+=+new+Visitor(){
++++++++++++public+int+doJob(int+a,+int+b)+{
++++++++++++++++return+a+%2B+b;
++++++++++++}
++++++++};

++++++++Visitor+multiplier+=+new+Visitor(){
++++++++++++public+int+doJob(int+a,+int+b)+{
++++++++++++++++return+a*b;
++++++++++++}
++++++++};

++++++++System.out.println(adder.doJob(10,+20));
++++++++System.out.println(multiplier.doJob(10,+20));

++++}
}|code-block|syntax|javascript|1436304|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you mean somthing like .NET anonymous delegate, I think Java's anonymous class can be used as well. 

<pre><code>public class Main {

 public interface Visitor{
 int doJob(int a, int b);
 }


 public static void main(String[] args) {
 Visitor adder = new Visitor(){
 public int doJob(int a, int b) {
 return a + b;
 }
 };

 Visitor multiplier = new Visitor(){
 public int doJob(int a, int b) {
 return a*b;
 }
 };

 System.out.println(adder.doJob(10, 20));
 System.out.println(multiplier.doJob(10, 20));

 }
}
</code></pre>

blocks|key|1436427|text|在Java中，方法还不是第一类对象；您不能将函数指针作为回调传递。相反，创建一个包含所需方法的对象(通常实现一个接口)并传递该方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1436428|关于Java中闭包的提议--它将提供您正在寻找的行为--已经提出，但在即将到来的Java7版本中不会包含任何内容。|1436429|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

A method is not (yet) a first-class object in Java; you can't pass a function pointer as a callback. Instead, create an object (which usually implements an interface) that contains the method you need and pass that.

Proposals for closures in Java&mdash;which would provide the behavior you are looking for&mdash;have been made, but none will be included in the upcoming Java 7 release.

blocks|key|1436489|text|当我在Java中需要这种功能时，我通常使用Observer+pattern。它确实意味着一个额外的对象，但我认为它是一种干净的方式，并且是一个被广泛理解的模式，这有助于代码的可读性。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1436490|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Observer_pattern^0|L|G|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]]]]

When I need this kind of functionality in Java, I usually use the <a href="http://en.wikipedia.org/wiki/Observer_pattern" rel="nofollow noreferrer">Observer pattern</a>. It does imply an extra object, but I think it's a clean way to go, and is a widely understood pattern, which helps with code readability.

blocks|key|1436501|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|1436502|http://code.google.com/p/lambdaj/wiki/Closures|offset|length|1436503|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|1436525|text|我尝试使用java.lang.reflect来实现“回调”，下面是一个示例：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1436526|package+StackOverflowQ443708_JavaCallBackTest;

import+java.lang.reflect.*;
import+java.util.concurrent.*;

class+MyTimer
{
++++ExecutorService+EXE+=
++++++++//Executors.newCachedThreadPool+();
++++++++Executors.newSingleThreadExecutor+();

++++public+static+void+PrintLine+()
++++{
++++++++System.out.println+("--------------------------------------------------------------------------------");
++++}

++++public+void+SetTimer+(final+int+timeout,+final+Object+obj,+final+String+methodName,+final+Object...+args)
++++{
++++++++SetTimer+(timeout,+obj,+false,+methodName,+args);
++++}
++++public+void+SetTimer+(final+int+timeout,+final+Object+obj,+final+boolean+isStatic,+final+String+methodName,+final+Object...+args)
++++{
++++++++Class<?>[]+argTypes+=+null;
++++++++if+(args+!=+null)
++++++++{
++++++++++++argTypes+=+new+Class<?>+[args.length];
++++++++++++for+(int+i=0;+i<args.length;+i%2B%2B)
++++++++++++{
++++++++++++++++argTypes[i]+=+args[i].getClass+();
++++++++++++}
++++++++}

++++++++SetTimer+(timeout,+obj,+isStatic,+methodName,+argTypes,+args);
++++}
++++public+void+SetTimer+(final+int+timeout,+final+Object+obj,+final+String+methodName,+final+Class<?>[]+argTypes,+final+Object...+args)
++++{
++++++++SetTimer+(timeout,+obj,+false,+methodName,+argTypes,+args);
++++}
++++public+void+SetTimer+(final+int+timeout,+final+Object+obj,+final+boolean+isStatic,+final+String+methodName,+final+Class<?>[]+argTypes,+final+Object...+args)
++++{
++++++++EXE.execute+(
++++++++++++new+Runnable()
++++++++++++{
++++++++++++++++public+void+run+()
++++++++++++++++{
++++++++++++++++++++Class<?>+c;
++++++++++++++++++++Method+method;
++++++++++++++++++++try
++++++++++++++++++++{
++++++++++++++++++++++++if+(isStatic)+c+=+(Class<?>)obj;
++++++++++++++++++++++++else+c+=+obj.getClass+();

++++++++++++++++++++++++System.out.println+("Wait+for+"+%2B+timeout+%2B+"+seconds+to+invoke+"+%2B+c.getSimpleName+()+%2B+"::["+%2B+methodName+%2B+"]");
++++++++++++++++++++++++TimeUnit.SECONDS.sleep+(timeout);
++++++++++++++++++++++++System.out.println+();
++++++++++++++++++++++++System.out.println+("invoking+"+%2B+c.getSimpleName+()+%2B+"::["+%2B+methodName+%2B+"]...");
++++++++++++++++++++++++PrintLine+();
++++++++++++++++++++++++method+=+c.getDeclaredMethod+(methodName,+argTypes);
++++++++++++++++++++++++method.invoke+(obj,+args);
++++++++++++++++++++}
++++++++++++++++++++catch+(Exception+e)
++++++++++++++++++++{
++++++++++++++++++++++++e.printStackTrace();
++++++++++++++++++++}
++++++++++++++++++++finally
++++++++++++++++++++{
++++++++++++++++++++++++PrintLine+();
++++++++++++++++++++}
++++++++++++++++}
++++++++++++}
++++++++);
++++}
++++public+void+ShutdownTimer+()
++++{
++++++++EXE.shutdown+();
++++}
}

public+class+CallBackTest
{
++++public+void+onUserTimeout+()
++++{
++++++++System.out.println+("onUserTimeout");
++++}
++++public+void+onTestEnd+()
++++{
++++++++System.out.println+("onTestEnd");
++++}
++++public+void+NullParameterTest+(String+sParam,+int+iParam)
++++{
++++++++System.out.println+("NullParameterTest:+String+parameter="+%2B+sParam+%2B+",+int+parameter="+%2B+iParam);
++++}
++++public+static+void+main+(String[]+args)
++++{
++++++++CallBackTest+test+=+new+CallBackTest+();
++++++++MyTimer+timer+=+new+MyTimer+();

++++++++timer.SetTimer+((int)(Math.random+()*10),+test,+"onUserTimeout");
++++++++timer.SetTimer+((int)(Math.random+()*10),+test,+"onTestEnd");
++++++++timer.SetTimer+((int)(Math.random+()*10),+test,+"A-Method-Which-Is-Not-Exists");++++//+java.lang.NoSuchMethodException

++++++++timer.SetTimer+((int)(Math.random+()*10),+System.out,+"println",+"this+is+an+argument+of+System.out.println()+which+is+called+by+timer");
++++++++timer.SetTimer+((int)(Math.random+()*10),+System.class,+true,+"currentTimeMillis");
++++++++timer.SetTimer+((int)(Math.random+()*10),+System.class,+true,+"currentTimeMillis",+"Should-Not-Pass-Arguments");++++//+java.lang.NoSuchMethodException

++++++++timer.SetTimer+((int)(Math.random+()*10),+String.class,+true,+"format",+"%25d+%25X",+100,+200);+//+java.lang.NoSuchMethodException
++++++++timer.SetTimer+((int)(Math.random+()*10),+String.class,+true,+"format",+"%25d+%25X",+new+Object[]{100,+200});

++++++++timer.SetTimer+((int)(Math.random+()*10),+test,+"NullParameterTest",+new+Class<?>[]{String.class,+int.class},+null,+888);

++++++++timer.ShutdownTimer+();
++++}
}|code-block|syntax|javascript|1436527|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I tried using java.lang.reflect to implement 'callback', here's a sample:

<pre><code>package StackOverflowQ443708_JavaCallBackTest;

import java.lang.reflect.*;
import java.util.concurrent.*;

class MyTimer
{
 ExecutorService EXE =
 //Executors.newCachedThreadPool ();
 Executors.newSingleThreadExecutor ();

 public static void PrintLine ()
 {
 System.out.println ("--------------------------------------------------------------------------------");
 }

 public void SetTimer (final int timeout, final Object obj, final String methodName, final Object... args)
 {
 SetTimer (timeout, obj, false, methodName, args);
 }
 public void SetTimer (final int timeout, final Object obj, final boolean isStatic, final String methodName, final Object... args)
 {
 Class&lt;?&gt;[] argTypes = null;
 if (args != null)
 {
 argTypes = new Class&lt;?&gt; [args.length];
 for (int i=0; i&lt;args.length; i++)
 {
 argTypes[i] = args[i].getClass ();
 }
 }

 SetTimer (timeout, obj, isStatic, methodName, argTypes, args);
 }
 public void SetTimer (final int timeout, final Object obj, final String methodName, final Class&lt;?&gt;[] argTypes, final Object... args)
 {
 SetTimer (timeout, obj, false, methodName, argTypes, args);
 }
 public void SetTimer (final int timeout, final Object obj, final boolean isStatic, final String methodName, final Class&lt;?&gt;[] argTypes, final Object... args)
 {
 EXE.execute (
 new Runnable()
 {
 public void run ()
 {
 Class&lt;?&gt; c;
 Method method;
 try
 {
 if (isStatic) c = (Class&lt;?&gt;)obj;
 else c = obj.getClass ();

 System.out.println ("Wait for " + timeout + " seconds to invoke " + c.getSimpleName () + "::[" + methodName + "]");
 TimeUnit.SECONDS.sleep (timeout);
 System.out.println ();
 System.out.println ("invoking " + c.getSimpleName () + "::[" + methodName + "]...");
 PrintLine ();
 method = c.getDeclaredMethod (methodName, argTypes);
 method.invoke (obj, args);
 }
 catch (Exception e)
 {
 e.printStackTrace();
 }
 finally
 {
 PrintLine ();
 }
 }
 }
 );
 }
 public void ShutdownTimer ()
 {
 EXE.shutdown ();
 }
}

public class CallBackTest
{
 public void onUserTimeout ()
 {
 System.out.println ("onUserTimeout");
 }
 public void onTestEnd ()
 {
 System.out.println ("onTestEnd");
 }
 public void NullParameterTest (String sParam, int iParam)
 {
 System.out.println ("NullParameterTest: String parameter=" + sParam + ", int parameter=" + iParam);
 }
 public static void main (String[] args)
 {
 CallBackTest test = new CallBackTest ();
 MyTimer timer = new MyTimer ();

 timer.SetTimer ((int)(Math.random ()*10), test, "onUserTimeout");
 timer.SetTimer ((int)(Math.random ()*10), test, "onTestEnd");
 timer.SetTimer ((int)(Math.random ()*10), test, "A-Method-Which-Is-Not-Exists"); // java.lang.NoSuchMethodException

 timer.SetTimer ((int)(Math.random ()*10), System.out, "println", "this is an argument of System.out.println() which is called by timer");
 timer.SetTimer ((int)(Math.random ()*10), System.class, true, "currentTimeMillis");
 timer.SetTimer ((int)(Math.random ()*10), System.class, true, "currentTimeMillis", "Should-Not-Pass-Arguments"); // java.lang.NoSuchMethodException

 timer.SetTimer ((int)(Math.random ()*10), String.class, true, "format", "%d %X", 100, 200); // java.lang.NoSuchMethodException
 timer.SetTimer ((int)(Math.random ()*10), String.class, true, "format", "%d %X", new Object[]{100, 200});

 timer.SetTimer ((int)(Math.random ()*10), test, "NullParameterTest", new Class&lt;?&gt;[]{String.class, int.class}, null, 888);

 timer.ShutdownTimer ();
 }
}
</code></pre>

blocks|key|1651108|text|我发现使用reflect库实现的想法很有趣，并提出了这个想法，我认为它工作得很好。唯一的缺点是无法在编译时检查您是否传递了有效的参数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1651109|public+class+CallBack+{
++++private+String+methodName;
++++private+Object+scope;

++++public+CallBack(Object+scope,+String+methodName)+{
++++++++this.methodName+=+methodName;
++++++++this.scope+=+scope;
++++}

++++public+Object+invoke(Object...+parameters)+throws+InvocationTargetException,+IllegalAccessException,+NoSuchMethodException+{
++++++++Method+method+=+scope.getClass().getMethod(methodName,+getParameterClasses(parameters));
++++++++return+method.invoke(scope,+parameters);
++++}

++++private+Class[]+getParameterClasses(Object...+parameters)+{
++++++++Class[]+classes+=+new+Class[parameters.length];
++++++++for+(int+i=0;+i+<+classes.length;+i%2B%2B)+{
++++++++++++classes[i]+=+parameters[i].getClass();
++++++++}
++++++++return+classes;
++++}
}|code-block|syntax|javascript|1651110|您可以像这样使用它|1651111|public+class+CallBackTest+{
++++@Test
++++public+void+testCallBack()+throws+NoSuchMethodException,+InvocationTargetException,+IllegalAccessException+{
++++++++TestClass+testClass+=+new+TestClass();
++++++++CallBack+callBack+=+new+CallBack(testClass,+"hello");
++++++++callBack.invoke();
++++++++callBack.invoke("Fred");
++++}

++++public+class+TestClass+{
++++++++public+void+hello()+{
++++++++++++System.out.println("Hello+World");
++++++++}

++++++++public+void+hello(String+name)+{
++++++++++++System.out.println("Hello+"+%2B+name);
++++++++}
++++}
}|1651112|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I found the idea of implementing using the reflect library interesting and came up with this which I think works quite well. The only down side is losing the compile time check that you are passing valid parameters.

<pre><code>public class CallBack {
 private String methodName;
 private Object scope;

 public CallBack(Object scope, String methodName) {
 this.methodName = methodName;
 this.scope = scope;
 }

 public Object invoke(Object... parameters) throws InvocationTargetException, IllegalAccessException, NoSuchMethodException {
 Method method = scope.getClass().getMethod(methodName, getParameterClasses(parameters));
 return method.invoke(scope, parameters);
 }

 private Class[] getParameterClasses(Object... parameters) {
 Class[] classes = new Class[parameters.length];
 for (int i=0; i &lt; classes.length; i++) {
 classes[i] = parameters[i].getClass();
 }
 return classes;
 }
}
</code></pre>

You use it like this

<pre><code>public class CallBackTest {
 @Test
 public void testCallBack() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
 TestClass testClass = new TestClass();
 CallBack callBack = new CallBack(testClass, "hello");
 callBack.invoke();
 callBack.invoke("Fred");
 }

 public class TestClass {
 public void hello() {
 System.out.println("Hello World");
 }

 public void hello(String name) {
 System.out.println("Hello " + name);
 }
 }
}
</code></pre>

blocks|key|1651185|text|它有点老了，但不管怎样...我发现Peter+Wilkinson的答案很好，只是它不适用于像int/Integer这样的原始类型。问题出在parameters[i]的.getClass()，它返回例如java.lang.Integer，另一方面getMethod(methodName,parameters[])不能正确解释它(Java的错误)……|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1651186|我将其与Daniel+Spiewak+(in+his+answer+to+this)的建议相结合；成功的步骤包括:捕获NoSuchMethodException+->+getMethods()+->通过method.getName()+->查找匹配的参数，然后显式循环参数列表并应用Daniels解决方案，例如识别类型匹配和签名匹配。|1651187|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/180097/dynamically-find-the-class-that-represents-a-primitive-java-type^0|1X|D|2B|B|2S|H|3E|Y|0|1N|L|2C|C|2T|G|K|L|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@$9|15|A|16|1|17]]|E|$]]|$1|H|3|-4|5|6|7|18|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

it's a bit old, but nevertheless... I found the answer of Peter Wilkinson nice except for the fact that it does not work for primitive types like int/Integer. 
The problem is the <code>.getClass()</code> for the <code>parameters[i]</code>, which returns for instance <code>java.lang.Integer</code>, which on the other hand will not be correctly interpreted by <code>getMethod(methodName,parameters[])</code> (Java's fault) ...

I combined it with the suggestion of Daniel Spiewak (<a href="https://stackoverflow.com/questions/180097/dynamically-find-the-class-that-represents-a-primitive-java-type">in his answer to this</a>); steps to success included: catching <code>NoSuchMethodException</code> -> <code>getMethods()</code> -> looking for the matching one by <code>method.getName()</code> -> and then explicitly looping through the list of parameters and applying Daniels solution, such identifying the type matches and the signature matches.

blocks|key|1651242|text|public+class+HelloWorldAnonymousClasses+{

++++//this+is+an+interface+with+only+one+method
++++interface+HelloWorld+{
++++++++public+void+printSomething(String+something);
++++}

++++//this+is+a+simple+function+called+from+main()
++++public+void+sayHello()+{

++++//this+is+an+object+with+interface+reference+followed+by+the+definition+of+the+interface+itself

++++++++new+HelloWorld()+{
++++++++++++public+void+printSomething(String+something)+{
++++++++++++++++System.out.println("Hello+"+%2B+something);
++++++++++++}
++++++++}.printSomething("Abhi");

+++++//imagine+this+as+an+object+which+is+calling+the+function'printSomething()"
++++}

++++public+static+void+main(String...+args)+{
++++++++HelloWorldAnonymousClasses+myApp+=
++++++++++++++++new+HelloWorldAnonymousClasses();
++++++++myApp.sayHello();
++++}
}
//Output+is+"Hello+Abhi"|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1651243|基本上，如果你想使一个接口的对象，这是不可能的，因为接口不能有对象。|unstyled|1651244|选项是让某个类实现接口，然后使用该类的对象调用该函数。但是这种方法非常冗长。|1651245|或者，编写新的HelloWorld()+(*oberserve这是一个接口，而不是一个类)，然后定义接口方法本身。(*这个定义实际上是匿名类)。然后，您将获得对象引用，通过该对象引用可以调用方法本身。|1651246|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|N|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|F|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|F|7|Q|8|@]|9|@]|A|$]]]|L|$]]

<pre><code>public class HelloWorldAnonymousClasses {

 //this is an interface with only one method
 interface HelloWorld {
 public void printSomething(String something);
 }

 //this is a simple function called from main()
 public void sayHello() {

 //this is an object with interface reference followed by the definition of the interface itself

 new HelloWorld() {
 public void printSomething(String something) {
 System.out.println("Hello " + something);
 }
 }.printSomething("Abhi");

 //imagine this as an object which is calling the function'printSomething()"
 }

 public static void main(String... args) {
 HelloWorldAnonymousClasses myApp =
 new HelloWorldAnonymousClasses();
 myApp.sayHello();
 }
}
//Output is "Hello Abhi"
</code></pre>

Basically if you want to make the object of an interface it is
not possible, because interface cannot have objects.

The option is to let some class implement the interface and then call that function using the object of that class.
But this approach is really verbose.

Alternatively, write new HelloWorld() (*oberserve this is an interface not a class) and then follow it up with the defination of the interface methods itself. (*This defination is in reality the anonymous class).
Then you get the object reference through which you can call the method itself.

blocks|key|1436779|text|我最近开始做这样的事情：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1436780|public+class+Main+{
++++@FunctionalInterface
++++public+interface+NotDotNetDelegate+{
++++++++int+doSomething(int+a,+int+b);
++++}

++++public+static+void+main(String[]+args)+{
++++++++//+in+java+8+(lambdas):
++++++++System.out.println(functionThatTakesDelegate((a,+b)+->+{return+a*b;}+,+10,+20));

++++}

++++public+static+int+functionThatTakesDelegate(NotDotNetDelegate+del,+int+a,+int+b)+{
++++++++//+...
++++++++return+del.doSomething(a,+b);
++++}
}|code-block|syntax|javascript|1436781|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I've recently started doing something like this:

<pre><code>public class Main {
 @FunctionalInterface
 public interface NotDotNetDelegate {
 int doSomething(int a, int b);
 }

 public static void main(String[] args) {
 // in java 8 (lambdas):
 System.out.println(functionThatTakesDelegate((a, b) -&gt; {return a*b;} , 10, 20));

 }

 public static int functionThatTakesDelegate(NotDotNetDelegate del, int a, int b) {
 // ...
 return del.doSomething(a, b);
 }
}
</code></pre>

blocks|key|1436819|text|您还可以使用Delegate模式执行Callback：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1436820|Callback.java|1436821|public+interface+Callback+{
++++void+onItemSelected(int+position);
}|code-block|syntax|javascript|1436822|PagerActivity.java|1436823|public+class+PagerActivity+implements+Callback+{

++++CustomPagerAdapter+mPagerAdapter;

++++public+PagerActivity()+{
++++++++mPagerAdapter+=+new+CustomPagerAdapter(this);
++++}

++++@Override
++++public+void+onItemSelected(int+position)+{
++++++++//+Do+something
++++++++System.out.println("Item+"+%2B+postion+%2B+"+selected")
++++}
}|1436824|CustomPagerAdapter.java|1436825|public+class+CustomPagerAdapter+{
++++private+static+final+int+DEFAULT_POSITION+=+1;
++++public+CustomPagerAdapter(Callback+callback)+{
++++++++callback.onItemSelected(DEFAULT_POSITION);
++++}
}|1436826|entityMap^0|6|8|I|8|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|15|8|@]|D|@]|E|$]]|$1|S|3|T|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|U|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|V|$]]

You also can do the<code>Callback</code> using the <code>Delegate</code> pattern:

Callback.java

<pre><code>public interface Callback {
 void onItemSelected(int position);
}
</code></pre>

PagerActivity.java

<pre><code>public class PagerActivity implements Callback {

 CustomPagerAdapter mPagerAdapter;

 public PagerActivity() {
 mPagerAdapter = new CustomPagerAdapter(this);
 }

 @Override
 public void onItemSelected(int position) {
 // Do something
 System.out.println("Item " + postion + " selected")
 }
}
</code></pre>

CustomPagerAdapter.java

<pre><code>public class CustomPagerAdapter {
 private static final int DEFAULT_POSITION = 1;
 public CustomPagerAdapter(Callback callback) {
 callback.onItemSelected(DEFAULT_POSITION);
 }
}
</code></pre>

blocks|key|1651345|text|在使用lambdas的Java8中，这非常容易。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1651346|public+interface+Callback+{
++++void+callback();
}

public+class+Main+{
++++public+static+void+main(String[]+args)+{
++++++++methodThatExpectsACallback(()+->+System.out.println("I+am+the+callback."));
++++}
++++private+static+void+methodThatExpectsACallback(Callback+callback){
++++++++System.out.println("I+am+the+method.");
++++++++callback.callback();
++++}
}|code-block|syntax|javascript|1651347|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

This is very easy in Java 8 with lambdas.

<pre><code>public interface Callback {
 void callback();
}

public class Main {
 public static void main(String[] args) {
 methodThatExpectsACallback(() -&gt; System.out.println("I am the callback."));
 }
 private static void methodThatExpectsACallback(Callback callback){
 System.out.println("I am the method.");
 callback.callback();
 }
}
</code></pre>

blocks|key|1436949|text|创建一个接口，并在回调类中创建相同的接口属性。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1436950|interface+dataFetchDelegate+{
++++void+didFetchdata(String+data);
}
//callback+class
public+class+BackendManager{
+++public+dataFetchDelegate+Delegate;

+++public+void+getData()+{
+++++++//Do+something,+Http+calls/+Any+other+work
+++++++Delegate.didFetchdata("this+is+callbackdata");
+++}

}|code-block|syntax|javascript|1436951|现在，在您想要回调的类中，实现上面创建的接口。并传递要回调的类的"this“对象/引用。|1436952|public+class+Main+implements+dataFetchDelegate
{+++++++
++++public+static+void+main(+String[]+args+)
++++{
++++++++new+Main().getDatafromBackend();
++++}

++++public+void+getDatafromBackend()+{
++++++++BackendManager+inc+=+new+BackendManager();
++++++++//Pass+this+object+as+reference.in+this+Scenario+this+is+Main+Object++++++++++++
++++++++inc.Delegate+=+this;
++++++++//make+call
++++++++inc.getData();
++++}

++++//This+method+is+called+after+task/Code+Completion
++++public+void+didFetchdata(String+callbackData)+{
++++++++//+TODO+Auto-generated+method+stub
++++++++System.out.println(callbackData);
++++}
}|1436953|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Create an Interface, and Create the Same Interface Property in Callback Class.

<pre><code>interface dataFetchDelegate {
 void didFetchdata(String data);
}
//callback class
public class BackendManager{
 public dataFetchDelegate Delegate;

 public void getData() {
 //Do something, Http calls/ Any other work
 Delegate.didFetchdata("this is callbackdata");
 }

}
</code></pre>

Now in the class where you want to get called back implement the above Created Interface.
and Also Pass "this" Object/Reference of your class to be called back.

<pre><code>public class Main implements dataFetchDelegate
{ 
 public static void main( String[] args )
 {
 new Main().getDatafromBackend();
 }

 public void getDatafromBackend() {
 BackendManager inc = new BackendManager();
 //Pass this object as reference.in this Scenario this is Main Object 
 inc.Delegate = this;
 //make call
 inc.getData();
 }

 //This method is called after task/Code Completion
 public void didFetchdata(String callbackData) {
 // TODO Auto-generated method stub
 System.out.println(callbackData);
 }
}
</code></pre>

blocks|key|1437038|text|最简单和最容易的方法是创建一个可重用的模型和触发器……https://onecompiler.com/java/3wejrcby2?fbclid=IwAR0dHbGDChRUJoCZ3CIDW-JQu7Dz3iYGNGYjxYVCPCWfEqQDogFGTwuOuO8|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1437039|entityMap|0|LINK|mutability|MUTABLE|url|https://onecompiler.com/java/3wejrcby2?fbclid=IwAR0dHbGDChRUJoCZ3CIDW-JQu7Dz3iYGNGYjxYVCPCWfEqQDogFGTwuOuO8^0|R|2Z|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]]]]

Simpliest and easiest way is by creating a reusable model and trigger.... <a href="https://onecompiler.com/java/3wejrcby2?fbclid=IwAR0dHbGDChRUJoCZ3CIDW-JQu7Dz3iYGNGYjxYVCPCWfEqQDogFGTwuOuO8" rel="nofollow noreferrer">https://onecompiler.com/java/3wejrcby2?fbclid=IwAR0dHbGDChRUJoCZ3CIDW-JQu7Dz3iYGNGYjxYVCPCWfEqQDogFGTwuOuO8</a>

blocks|key|1651661|text|在java8中，这个任务很简单，如果你想在多线程场景中使用回调，你可以做类似如下的事情：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1651662|public++void+methodA+(int+n,+IntConsumer+consumer)+{
++++
++++//+create+a+thread
++++Thread+t+=+new+Thread(()+->+{
++++++++//+some+time+consuming+operation
++++++++int+result+=+IntStream.range(0,+n).sum();
++++++++//+after+the+result+is+ready+do+something+with+it.
++++++++consumer.accept(result);
++++});
++++t.start();
}|code-block|syntax|javascript|1651663|要使用此方法，请执行以下操作：|1651664|methodA(1000000,+System.out::println);|1651665|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

with java 8 this task is kinda easy, if you want to use callback in multi-thread scenario you can do something similar like the following:
<pre><code>public void methodA (int n, IntConsumer consumer) {
 
 // create a thread
 Thread t = new Thread(() -&gt; {
 // some time consuming operation
 int result = IntStream.range(0, n).sum();
 // after the result is ready do something with it.
 consumer.accept(result);
 });
 t.start();
}
</code></pre>
and to use this method do:
<pre><code>methodA(1000000, System.out::println);
</code></pre>

Is there a way to pass a call back function in a Java method?

The behavior I'm trying to mimic is a .Net Delegate being passed to a function.

I've seen people suggesting creating a separate object but that seems overkill, however I am aware that sometimes overkill is the only way to do things.

Callback functions in Java

Java

有没有办法在Java方法中传递回调函数？我试图模仿的行为是将.Net委托传递给函数。我见过有人建议创建一个单独的对象，但这似乎有点过头了，但我意识到有时过度杀伤力是做事情的唯一方法。

问Java中的回调函数
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