This time I understood what a closure is...

1. CallBack.java

package com.fs;

public interface CallBack {
    void m(int e);
}

2. CallBackImplementation.java

package com.impl.callback;

import com.fs.CallBack;
import java.util.ArrayList;
import java.util.List;

public class CCallBack {
    
    List<CallBack> cbs = new ArrayList<>();
    
    public void registerCallBack(CallBack f){
        cbs.add(f);
    }
    
    public void removeCallBack(CallBack f){
        if(cbs.contains(f)){
            cbs.remove(f);
        }
    }
    
    public void onAction(int i){
        for (CallBack callBack : cbs) {
            callBack.m(i);
        }
    }
}

3. Tester.java

package basic;

import com.fs.CallBack;
import com.impl.callback.CCallBack;
import org.junit.Test;


public class CallBackTester {
    CCallBack cb = new CCallBack();
    
    @Test
    public void test_callback(){
 
        CallBack cb1 = new CallBack() {
            int x = 1;
            @Override
            public void m(int e) {
                if(e==1){
                    System.out.println("You register this callback " + x + " time/times");
                    x++;
                }
            }
        };
        
        cb.registerCallBack(cb1);
        cb.registerCallBack(cb1);
        cb.registerCallBack(cb1);
        cb.removeCallBack(cb1);
        
        cb.onAction(1);
    }
         
}

4. Output.txt

You register this callback 1 time/times
You register this callback 2 time/times