Question

I am implementing an interface.

I have two functions that I have to implement A and B. A checks the file is not corrupt and return then B processes the file and returns. after B return function C runs, I have no control over C as I am just implementing A and B.

The problem is that B takes a long time to run and C needs it return value to run. Ideally I would split B into B and D and put all the stuff that C dose not need to run into D. As this is impossible I want to know if it is possible to return a value from B so C can run but afterwards then have B run more code.

C cannot be called from B.

Answer 1

A must call B and C You want B to do some processing, C to be called and B to continue processing.

The simplest approach is to have

A called a method on B which opens the file and does some processing, and leaves anything open which is need later. A calls C and then calls another method on B which finishes the processing. This means B must know when to stop and return so C can be called.

The way around this is to use a listener interface below.

class A {
   void method(B b, C c) {
      b.function(new Listener() {
          public void onValue(long num) { // or any event type
             if (num > 100) C.something();
          }
      });
   }
}

This way B has no idea when C will be called.

---

You can implement a listener interface which you pass to the function and it can “return” multiple values. However once a function actually returns it is no longer running.

This function can run in another thread or the same thread.

interface Listener {
     public void onValue(long num);
     public void onError(Exception e);
     public void onEndOfData();
}

public void function(Listener l) {
    try {
       for(int i=0;i<10;i++) l.onValue(i);

       // read a file 
    } catch (Exception e) {
       l.onError(e);
    } finally {
       l.onEndOfData();
    }
}

put another way instead of doing something like

// only one value.
long value = function();
System.out.println("value returned was "+value);

you can do

// any number of values can be returned 
// and the function can keep processing.
function(new Listener() {
    public void onValue(long value) {
        System.out.println("value returned was "+value);
    }
});