Java 8 - usage of Consumer's andThen

Question

I have below POC to use Java 8 feature.

I want to update DB after accept method. Is it good to go with andThen()? When is this method called? Who calls it?

What is the basic use of andThen() method? Looking at the docs was confusing.

public class StockTest {

    public static void main(String[] args) {

    List<Trader> traders = new ArrayList<>();

    Random random = new Random();

    // Initializing trading a/c's.
    for (int i = 0; i < 10; i++) {
        Trader trader = new Trader((random.nextInt(100) + 1) * 3);
        traders.add(trader);
    }
    // Display Trade accounts.
    System.out.println("Before Bonus, Units are:");
    for (Trader trader : traders) {
        System.out.print(trader.getUnits() + "\t");
    }

    // Add bonus to each trader.
    traders.forEach(new Consumer<Trader>() {

        @Override
        public void accept(Trader trader) {
            trader.updateBonus(2);
        }

        @Override
        public Consumer<Trader> andThen(Consumer<? super Trader> after) 
       {
            System.out.println("In andThen");
            return Consumer.super.andThen(after);
        }
        });

    // Display Trade accounts after bonus applied..
    System.out.println("\nAfter bonus:");
    for (Trader trader : traders) {
        System.out.print(trader.getUnits() + "\t");
     }

   }

 }

 class Trader {
    private int units;

    public Trader(int initialUnits) {
    this.units = initialUnits;
 }

   public int getUnits() {
        return units;
    }

public void setUnits(int units) {
    this.units = units;
}

 public void updateBonus(int bonusUnits) {
    this.units = this.units * bonusUnits;
  }
 }

Please help with some example or use cases to utilize this method

Answer 1

In short andThen is used to chain consumers, so the input will go to first and second consumer, lke below:

Consumer<Trader> consumer1 = new Consumer<Trader>() {

    @Override
    public void accept(Trader trader) {
       trader.updateBonus(2);
    }
};

Consumer<Trader> consumer2 = new Consumer<Trader>() {

    @Override
    public void accept(Trader trader) {
       // do something
    }
};
// Add bonus to each trader.
traders.forEach(consumer1.andThen(consumer2));

So here the Trader will be passed to consumer1, then to consumer2 and so on.

You don't have to implement this method, or override it. When it comes to Consumers, implement only the accept.

andThen method is a helper tool to join consumers. Instead of passing the input to all of them in a loop.