How to call function with callbacks in Java like I do it in C#?

Question

I'm pretty new to Java but I need to write something like this C# code (this is hand-typed prototype just to illustrate what I need)

private void ParentFunc()
{
    var worker = new WorkerClass()

    worker.DoWork(e => console.Write("Progress" + e));


}

public class WorkerClass()
{
    public method DoWork(Action<int> callback)
    {
        for (int i=1; i<1000; i++) callback.Invoke(i);  
    }
}

Little explanation. I'm using AsyncTask in android and calling outside processor classed but would like them to signal back so I can publishProgress. I prefer not to put interface over my AsyncTask

possible duplicate of [Callback functions in Java](http://stackoverflow.com/questions/443708/callback-functions-in-java) — assylias, Jun 06 '12 at 17:21

score 6 · Accepted Answer · answered Jun 06 '12 at 17:27

Since closures are not supported yet you have to use interface and anonymous inner class.

private void ParentFunc {
    WorkerClass worker = new WorkerClass();

    worker.doWork(new Callback<Integer>() {
        public void invoke(Integer arg) {
            System.out.println("Progress" + arg);
        }
    });
}

public class WorkerClass {
    public doWork(Callback<Integer> callback) {
        for (int i=1; i<1000; i++) callback.invoke(i);  
    }
}

public interface Callback<T> {
    public void invoke(T arg);
}

Sergey Kalinichenko · Answer 2 · 2012-06-06T17:36:18.640

5

In java, you use anonymous classes implementing interfaces. This is somewhat more verbose, but it works just fine:

interface MyAction {
    void doit(String str);
}

class WorkerClass {
    void doWork(MyAction action);
}

void main(String args[]) {
    WorkerClass worker = new WorkerClass();
    worker.doWork(new MyAction {
        public void doit(String e) {
            System.out.println("Progress" + e)
        }
    });
}

edited Jun 06 '12 at 17:36

answered Jun 06 '12 at 17:23

Sergey Kalinichenko

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@Eng.Fouad Oops, I've been coding in C# for too long :) Thanks! – Sergey Kalinichenko Jun 06 '12 at 17:37

Jack · Answer 3 · 2012-06-06T17:31:37.677

With the aid of a support class:

interface CallBack<P>
{
  public void callback(P p);
}

class MyCallBack implements CallBack<String> {
  public void callBack(String p) {
    System.out.println(p);
  }
}

class WorkerClass {
  public void doWork(CallBack<String> callback)
  {
    // do work
    callback.callBack("Done!");
  }
}

WorkerClass wc = new WorkerClass();
wc.doWork(new MyCallBack());

I'm not going to suggest you to use anonymous classes because I find them quite verbose until a slicker syntax won't be available but it's just a matter of personal taste, I end up losing my self between braces when I use them..

score 0 · Answer 4 · edited May 23 '17 at 10:34

0

It is possible with java 8. See my answer to the duplicate question:

https://stackoverflow.com/a/26776520/1206998

Use a function as argument:

import java.util.function.Function;

public String applyFunction(Function<String,String> function){
        return function.apply("what ever");
}

Call

// with someObject.someMethod(String) => String
xxx.applyFunction(someObject::someMethod);

edited May 23 '17 at 10:34

Community

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answered Nov 06 '14 at 12:28

Juh_

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How to call function with callbacks in Java like I do it in C#?

4 Answers4