We can always see many callback functions in Java,but how does that actually work?
We know that there is five step to simulate a callback process:
- Class A implements Callback interface. /The interface has abstract function operate()/
- Class A has a member Class B's instance /b/
- Class B has a method using callback /Callback instance as parameter/doSth(Callback callback)/
- A's instance a invoked B's instance b's method /b.doSth(this),use this because a is a concrete callback/
- b invoke a's method,for example, /a.operate() in b.doSth(Callback callback)/
After the above process,it just seems the b finish the job and return result for a!This is what we call "Callback" :
Pass the job to server,when the job done,the server call the client and return feedback!
The following will be some codes example:
/**
Callback.java
**/
//Whoever implement this can drink the beverage
public interface Callback(){
public void drinkBeverage(String beverage);
}/**
Me.java
**/
public class Me implements Callback{
Roommate roommate;
public Me(Roommate roommate){
this.roommate = roommate;
}
public void getBeverageAndDrink(){
//I'm too lazy to buy a beverage so I'm gonna ask my roommate.
roommate.buyBeverage(this);
}
@Override
public void drinkBeverage(String beverage){
//when my roommate call me,I can drink the beverage...
sout("I drink "+ beverage+" !");
}
}/**
Roommate.java
**/
public class Roommate{
public void buyBeverage(Callback callback){
//roommate got the beverage somewhere.we don't care.
String beverage = "Vita-lemonTea";
//call me to drink beverage,the beverage is bought.
callback.drinkBeverage(beverage);
}
}
/**
Test.java
**/
public class Test{
public static void main(String[] args){
Roommate huangyep = new Roommate();
Me wendy = new Me(huangyep);
wendy.getBeverageAndDrink();
}
}
This example is the simplest to explain this concept.Of course there is more complex examples,If I think of any I'll add it in.