Java Delegates?

Question

Does the Java language have delegate features, similar to how C# has support for delegates?

Euphemia

@Suma How can this be a duplicate if the question you mentioned was posted a year after this one? – Insomnia

Java 8 has a feature quite like delegates. It's called lambdas. – Ionosphere

@Ionosphere to be more correct, Java 8 has a feature quite like delegates. It's called functional interfaces. Lambdas are one way of creating such delegate instances (anonymously). – Phlegethon

The answers below are from Pre-Java 8. Post Java 8 see the answers in this thread: #20312279 – Frontage

@nawfal, +1, but to be even more correct, Java 7 and below already has a feature quite like delegates. It's called plain interfaces. – Osman

@Pacerier. Only in a limited/cumbersome way. A C# delegate allows a class to pass a reference to one of its methods. Consider a class with two methods f1 and f2, having identical arguments. A c# delegate makes it trivial to dynamically pass either f1 or f2 depending on the function desired. In Java 8, a lambda can wrap a call to the desired method. How do this in Java 7? IMHO requires defining an interface and two inner classes, one to call f1, the other two call f2, then create one object of each class. Doable, but a lot of work! – Coprology

@newfal: I have mixed feelings about whether to consider a functional (one method) interface to be quite like a (C#) delegate. It does represent a single method call, but does so via inheritance. Usage requires an object that implements that interface. Whereas a C# delegate can be satisfied by any method that matches delegate's signature (regardless of method name). OK, as a practical matter, given lambdas, it is used to satisfy similar requirements. Without the cumbersome coding that was often required prior to Java 8. – Coprology

@Pacerier: Upon further reflection, I realize what you are saying. I've provided an answer here showing how to do so in Java 7. – Coprology

Answer 1

Not really, no.

You may be able to achieve the same effect by using reflection to get Method objects you can then invoke, and the other way is to create an interface with a single 'invoke' or 'execute' method, and then instantiate them to call the method your interested in (i.e. using an anonymous inner class).

You might also find this article interesting / useful : A Java Programmer Looks at C# Delegates (@blueskyprojects.com)

Conah

The solution with invoke() as the only member function of an interface is really nice. – Sprouse

But see my example here of what one would do, even in Java 7, to accomplish the equivalent to a C# delegate. – Coprology

the link is outdated – Nuristan

Thanks for the heads up - I've replaced it with a wayback machine one which should work into the future. It's worth noting, though, that quite a lot has changed in the intervening years in terms of what Java supports. – Conah

Answer 2

Depending precisely what you mean, you can achieve a similar effect (passing around a method) using the Strategy Pattern.

Instead of a line like this declaring a named method signature:

// C#
public delegate void SomeFunction();

declare an interface:

// Java
public interface ISomeBehaviour {
   void SomeFunction();
}

For concrete implementations of the method, define a class that implements the behaviour:

// Java
public class TypeABehaviour implements ISomeBehaviour {
   public void SomeFunction() {
      // TypeA behaviour
   }
}

public class TypeBBehaviour implements ISomeBehaviour {
   public void SomeFunction() {
      // TypeB behaviour
   }
}

Then wherever you would have had a SomeFunction delegate in C#, use an ISomeBehaviour reference instead:

// C#
SomeFunction doSomething = SomeMethod;
doSomething();
doSomething = SomeOtherMethod;
doSomething();

// Java
ISomeBehaviour someBehaviour = new TypeABehaviour();
someBehaviour.SomeFunction();
someBehaviour = new TypeBBehaviour();
someBehaviour.SomeFunction();

With anonymous inner classes, you can even avoid declaring separate named classes and almost treat them like real delegate functions.

// Java
public void SomeMethod(ISomeBehaviour pSomeBehaviour) {
   ...
}

...

SomeMethod(new ISomeBehaviour() { 
   @Override
   public void SomeFunction() {
      // your implementation
   }
});

This should probably only be used when the implementation is very specific to the current context and wouldn't benefit from being reused.

And then of course in Java 8, these do become basically lambda expressions:

// Java 8
SomeMethod(() -> { /* your implementation */ });

Stroy

+1. this is a decent workaround, and the verbosity may be compensated by maintainability in future. – Phlegethon

this is great... Currently working on a project where i am unable to use reflection due to project constraints and this workaround gets the job done beautifully :) – Arcature

Does Java have an I-prefix naming convention for interfaces? I haven't seen that before – Stephen

Answer 3

Short story: ­­­­­­­­­­­­­­­­­­­no.

Introduction

The newest version of the Microsoft Visual J++ development environment supports a language construct called delegates or bound method references. This construct, and the new keywords delegate and multicast introduced to support it, are not a part of the Java^TM programming language, which is specified by the Java Language Specification and amended by the Inner Classes Specification included in the documentation for the JDKTM 1.1 software.

It is unlikely that the Java programming language will ever include this construct. Sun already carefully considered adopting it in 1996, to the extent of building and discarding working prototypes. Our conclusion was that bound method references are unnecessary and detrimental to the language. This decision was made in consultation with Borland International, who had previous experience with bound method references in Delphi Object Pascal.

We believe bound method references are unnecessary because another design alternative, inner classes, provides equal or superior functionality. In particular, inner classes fully support the requirements of user-interface event handling, and have been used to implement a user-interface API at least as comprehensive as the Windows Foundation Classes.

We believe bound method references are harmful because they detract from the simplicity of the Java programming language and the pervasively object-oriented character of the APIs. Bound method references also introduce irregularity into the language syntax and scoping rules. Finally, they dilute the investment in VM technologies because VMs are required to handle additional and disparate types of references and method linkage efficiently.

Abnaki

As it says in what Patrick linked you want to use inner classes instead. – Materi

Great article. I LOVE their definition of "simple": Java is a designed to be a simple language as in "Java has has to be simple to write compiler/VM for" while ignoring "Java has to be simple to write/read by a person". Explains a lot. – Incunabulum

I just think SUN made a great mistake. They just have not been convinced by the functional paradigm, and that's all. – Sprouse

@Juozas: Python is explicitely made to be simple to write/read by a person and it does implements lambda functions/delegates. – Sprouse

Replaced with an archive.org link. Also, that's really stupid, Oracle. – Abnaki

That article consisted pretty much entirely of the destruction of a straw-man argument (implementation) of functions as first-class citizens. Particularly the artificially verbose button.addActionListener(new ActionDelegate(this.button_clicked)); which should have been button.addActionListener(buttonClicked); and the unjustified claim that the use of an instance method to sort retained a reference to the enclosing class, which method should have been static (and the anonymous inner-class solution, which they tout, has this problem in spades). – Ascription

Answer 4

Have you read this :

Delegates are a useful construct in event-based systems. Essentially Delegates are objects that encode a method dispatch on a specified object. This document shows how java inner classes provide a more generic solution to such problems.

What is a Delegate? Really it is very similar to a pointer to member function as used in C++. But a delegate contains the target object alongwith the method to be invoked. Ideally it would be nice to be able to say:

obj.registerHandler(ano.methodOne);

..and that the method methodOne would be called on ano when some specific event was received.

This is what the Delegate structure achieves.

Java Inner Classes

It has been argued that Java provides this functionality via anonymous inner classes and thus does not need the additional Delegate construct.

obj.registerHandler(new Handler() {
        public void handleIt(Event ev) {
            methodOne(ev);
        }
      } );

At first glance this seems correct but at the same time a nuisance. Because for many event processing examples the simplicity of the Delegates syntax is very attractive.

General Handler

However, if event-based programming is used in a more pervasive manner, say, for example, as a part of a general asynchronous programming environment, there is more at stake.

In such a general situation, it is not sufficient to include only the target method and target object instance. In general there may be other parameters required, that are determined within the context when the event handler is registered.

In this more general situation, the java approach can provide a very elegant solution, particularly when combined with use of final variables:

void processState(final T1 p1, final T2 dispatch) { 
  final int a1 = someCalculation();

  m_obj.registerHandler(new Handler() {
    public void handleIt(Event ev) {
     dispatch.methodOne(a1, ev, p1);
    }
  } );
}

final * final * final

Got your attention?

Note that the final variables are accessible from within the anonymous class method definitions. Be sure to study this code carefully to understand the ramifications. This is potentially a very powerful technique. For example, it can be used to good effect when registering handlers in MiniDOM and in more general situations.

By contrast, the Delegate construct does not provide a solution for this more general requirement, and as such should be rejected as an idiom on which designs can be based.

Pantoja

Answer 5

I know this post is old, but Java 8 has added lambdas, and the concept of a functional interface, which is any interface with only one method. Together these offer similar functionality to C# delegates. See here for more info, or just google Java Lambdas. http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-final.html

Tole

Answer 6

No, but they're fakeable using proxies and reflection:

  public static class TestClass {
      public String knockKnock() {
          return "who's there?";
      }
  }

  private final TestClass testInstance = new TestClass();

  @Test public void
  can_delegate_a_single_method_interface_to_an_instance() throws Exception {
      Delegator<TestClass, Callable<String>> knockKnockDelegator = Delegator.ofMethod("knockKnock")
                                                                   .of(TestClass.class)
                                                                   .to(Callable.class);
      Callable<String> callable = knockKnockDelegator.delegateTo(testInstance);
      assertThat(callable.call(), is("who's there?"));
  }

The nice thing about this idiom is that you can verify that the delegated-to method exists, and has the required signature, at the point where you create the delegator (although not at compile-time, unfortunately, although a FindBugs plug-in might help here), then use it safely to delegate to various instances.

See the karg code on github for more tests and implementation.

Deed

Answer 7

I have implemented callback/delegate support in Java using reflection. Details and working source are available on my website.

How It Works

There is a principle class named Callback with a nested class named WithParms. The API which needs the callback will take a Callback object as a parameter and, if neccessary, create a Callback.WithParms as a method variable. Since a great many of the applications of this object will be recursive, this works very cleanly.

With performance still a high priority to me, I didn't want to be required to create a throwaway object array to hold the parameters for every invocation - after all in a large data structure there could be thousands of elements, and in a message processing scenario we could end up processing thousands of data structures a second.

In order to be threadsafe the parameter array needs to exist uniquely for each invocation of the API method, and for efficiency the same one should be used for every invocation of the callback; I needed a second object which would be cheap to create in order to bind the callback with a parameter array for invocation. But, in some scenarios, the invoker would already have a the parameter array for other reasons. For these two reasons, the parameter array does not belong in the Callback object. Also the choice of invocation (passing the parameters as an array or as individual objects) belongs in the hands of the API using the callback enabling it to use whichever invocation is best suited to its inner workings.

The WithParms nested class, then, is optional and serves two purposes, it contains the parameter object array needed for the callback invocations, and it provides 10 overloaded invoke() methods (with from 1 to 10 parameters) which load the parameter array and then invoke the callback target.

What follows is an example using a callback to process the files in a directory tree. This is an initial validation pass which just counts the files to process and ensure none exceed a predetermined maximum size. In this case we just create the callback inline with the API invocation. However, we reflect the target method out as a static value so that the reflection is not done every time.

static private final Method             COUNT =Callback.getMethod(Xxx.class,"callback_count",true,File.class,File.class);

...

IoUtil.processDirectory(root,new Callback(this,COUNT),selector);

...

private void callback_count(File dir, File fil) {
    if(fil!=null) {                                                                             // file is null for processing a directory
        fileTotal++;
        if(fil.length()>fileSizeLimit) {
            throw new Abort("Failed","File size exceeds maximum of "+TextUtil.formatNumber(fileSizeLimit)+" bytes: "+fil);
            }
        }
    progress("Counting",dir,fileTotal);
    }

IoUtil.processDirectory():

/**
 * Process a directory using callbacks.  To interrupt, the callback must throw an (unchecked) exception.
 * Subdirectories are processed only if the selector is null or selects the directories, and are done
 * after the files in any given directory.  When the callback is invoked for a directory, the file
 * argument is null;
 * <p>
 * The callback signature is:
 * <pre>    void callback(File dir, File ent);</pre>
 * <p>
 * @return          The number of files processed.
 */
static public int processDirectory(File dir, Callback cbk, FileSelector sel) {
    return _processDirectory(dir,new Callback.WithParms(cbk,2),sel);
    }

static private int _processDirectory(File dir, Callback.WithParms cbk, FileSelector sel) {
    int                                 cnt=0;

    if(!dir.isDirectory()) {
        if(sel==null || sel.accept(dir)) { cbk.invoke(dir.getParent(),dir); cnt++; }
        }
    else {
        cbk.invoke(dir,(Object[])null);

        File[] lst=(sel==null ? dir.listFiles() : dir.listFiles(sel));
        if(lst!=null) {
            for(int xa=0; xa<lst.length; xa++) {
                File ent=lst[xa];
                if(!ent.isDirectory()) {
                    cbk.invoke(dir,ent);
                    lst[xa]=null;
                    cnt++;
                    }
                }
            for(int xa=0; xa<lst.length; xa++) {
                File ent=lst[xa];
                if(ent!=null) { cnt+=_processDirectory(ent,cbk,sel); }
                }
            }
        }
    return cnt;
    }

This example illustrates the beauty of this approach - the application specific logic is abstracted into the callback, and the drudgery of recursively walking a directory tree is tucked nicely away in a completely reusable static utility method. And we don't have to repeatedly pay the price of defining and implementing an interface for every new use. Of course, the argument for an interface is that it is far more explicit about what to implement (it's enforced, not simply documented) - but in practice I have not found it to be a problem to get the callback definition right.

Defining and implementing an interface is not really so bad (unless you're distributing applets, as I am, where avoiding creating extra classes actually matters), but where this really shines is when you have multiple callbacks in a single class. Not only is being forced to push them each into a separate inner class added overhead in the deployed application, but it's downright tedious to program and all that boiler-plate code is really just "noise".

Ascription

Answer 8

Yes & No, but delegate pattern in Java could be thought of this way. This video tutorial is about data exchange between activity - fragments, and it has great essence of delegate sorta pattern using interfaces.

Puma

Answer 9

It doesn't have an explicit delegate keyword as C#, but you can achieve similar in Java 8 by using a functional interface (i.e. any interface with exactly one method) and lambda:

private interface SingleFunc {
    void printMe();
}

public static void main(String[] args) {
    SingleFunc sf = () -> {
        System.out.println("Hello, I am a simple single func.");
    };
    SingleFunc sfComplex = () -> {
        System.out.println("Hello, I am a COMPLEX single func.");
    };
    delegate(sf);
    delegate(sfComplex);
}

private static void delegate(SingleFunc f) {
    f.printMe();
}

Every new object of type SingleFunc must implement printMe(), so it is safe to pass it to another method (e.g. delegate(SingleFunc)) to call the printMe() method.

Dilemma

Answer 10

While it is nowhere nearly as clean, but you could implement something like C# delegates using a Java Proxy.

Tattler

While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. – Rood

@Rood Strip the link, and what do you get? A post with information. Vote to keep. – Soppy

@Soppy what would happen if the link is not valid any more ? – Rood

@Rood It still provides an answer - use a Java Proxy. – Soppy

then it could be left as a comment? – Rood

Answer 11

No, but it has similar behavior, internally.

In C# delegates are used to creates a separate entry point and they work much like a function pointer.

In java there is no thing as function pointer (on a upper look) but internally Java needs to do the same thing in order to achieve these objectives.

For example, creating threads in Java requires a class extending Thread or implementing Runnable, because a class object variable can be used a memory location pointer.

London

Answer 12

No, Java doesn't have that amazing feature. But you could create it manually using the observer pattern. Here is an example: Write C# delegate in java

Loadstone

Answer 13

The code described offers many of the advantages of C# delegates. Methods, either static or dynamic, can be treated in a uniform manner. The complexity in calling methods through reflection is reduced and the code is reusable, in the sense of requiring no additional classes in the user code. Note we are calling an alternate convenience version of invoke, where a method with one parameter can be called without creating an object array.Java code below:

  class Class1 {
        public void show(String s) { System.out.println(s); }
    }

    class Class2 {
        public void display(String s) { System.out.println(s); }
    }

    // allows static method as well
    class Class3 {
        public static void staticDisplay(String s) { System.out.println(s); }
    }

    public class TestDelegate  {
        public static final Class[] OUTPUT_ARGS = { String.class };
        public final Delegator DO_SHOW = new Delegator(OUTPUT_ARGS,Void.TYPE);

        public void main(String[] args)  {
            Delegate[] items = new Delegate[3];

            items[0] = DO_SHOW .build(new Class1(),"show,);
            items[1] = DO_SHOW.build (new Class2(),"display");
            items[2] = DO_SHOW.build(Class3.class, "staticDisplay");

            for(int i = 0; i < items.length; i++) {
                items[i].invoke("Hello World");
            }
        }
    }

Mosaic

Answer 14

Java doesn't have delegates and is proud of it :). From what I read here I found in essence 2 ways to fake delegates: 1. reflection; 2. inner class

Reflections are slooooow! Inner class does not cover the simplest use-case: sort function. Do not want to go into details, but the solution with inner class basically is to create a wrapper class for an array of integers to be sorted in ascending order and an class for an array of integers to be sorted in descending order.

Enclosure

What has inner class got to do with sorting? And what has sorting to do with the question? – Phlegethon