Equivalent of C# anonymous methods in Java?

Question

In C# you can define delegates anonymously (even though they are nothing more than syntactic sugar). For example, I can do this:

public string DoSomething(Func<string, string> someDelegate)
{
     // Do something involving someDelegate(string s)
} 

DoSomething(delegate(string s){ return s += "asd"; });
DoSomething(delegate(string s){ return s.Reverse(); });

Is it possible to pass code like this in Java? I'm using the processing framework, which has a quite old version of Java (it doesn't have generics).

Grilled

A q on delegates in general in Java – Broucek

Answer 1

Pre Java 8:

The closest Java has to delegates are single method interfaces. You could use an anonymous inner class.

interface StringFunc {
   String func(String s);
}

void doSomething(StringFunc funk) {
   System.out.println(funk.func("whatever"));
}

doSomething(new StringFunc() {
      public String func(String s) {
           return s + "asd";
      }
   });


doSomething(new StringFunc() {
      public String func(String s) {
           return new StringBuffer(s).reverse().toString();
      }
   });

---

Java 8 and above:

Java 8 adds lambda expressions to the language.

    doSomething((t) -> t + "asd");
    doSomething((t) -> new StringBuilder(t).reverse().toString());

Bertrando

[Google Collections][1] and Groovy[2] might be of interest to you. [1] code.google.com/p/google-collections [2] groovy.codehaus.org – Bertrando

Answer 2

Not exactly like this but Java has something similar.

It's called anonymous inner classes.

Let me give you an example:

DoSomething(new Runnable() {
   public void run() {
       // "delegate" body
   }
});

It's a little more verbose and requires an interface to implement, but other than that it's pretty much the same thing

Maclaine

Answer 3

Your example would look like this in Java, using anomymous inner classes:

interface Func {
    String execute(String s);
}

public String doSomething(Func someDelegate) {
    // Do something involving someDelegate.execute(String s)
}

doSomething(new Func() { public String execute(String s) { return s + "asd"; } });
doSomething(new Func() { public String execute(String s) { return new StringBuilder(s).reverse().toString(); } } });

Hadwin

Answer 4

Is it possible to pass code like this in Java? I'm using the processing framework, which has a quite old version of Java (it doesn't have generics).

Since the question asked about the Processing-specific answer, there is no direct equivalent. But Processing uses the Java 1.4 language level, and Java 1.1 introduced anonymous inner classes, which are a rough approximation.

Belittle

Answer 5

For example :

public class Delegate
{
    interface Func
    {
        void execute(String s);
    }

    public static void doSomething(Func someDelegate) {
        someDelegate.execute("123");
    }

    public static void main(String [] args)
    {

        Func someFuncImplementation = new Func() 
        {
            @Override
            public void execute(String s) {
                System.out.println("Bla Bla :"  + s);
            }
        };

        Func someOtherFuncImplementation = new Func() 
        {
            @Override
            public void execute(String s) {
                System.out.println("Foo Bar:"  + s);
            }
        };


        doSomething(someFuncImplementation);
        doSomething(someOtherFuncImplementation);
    }
}

Output :

Bla Bla :123

Foo Bar:123

Hug

Answer 6

You have all forgotten here that a C# delegate first of all - is thread safe. These examples are just for a single thread App..

Most of the contemporary Apps are written on multithreaded concept.. So no one answer is the answer.

There is not an equivalent in Java

Edlun

It isn't that what you are saying is wrong. It's correct. The problem is, this does not actually answer the question being asked---it just says all the other answers are wrong. Are you saying that there is not an equivalent in Java? – Aviv

The difference here would for most users be small. I'd even argue that the lack of multicast is a bigger difference between the two. As far as the question is phrased I believe the answers above are correct. All are clear that they are talking about the closest Java has. – Bertrando

As references are atomic, the Java version will not crash, but you could have stale data. To make it thread safe all you need to do is add the keyword "volatile". – Bertrando