StrategyDemo.java

package com.art.designpatterns.strategy;

import com.google.common.collect.Lists;
import java.util.List;

public class StrategyDemo {

  public static void main(String[] args) {
    PromotionStrategy halfOffPromo = new HalfOffPromotionStrategy();
    PromotionStrategy clearancePromo = new ClearancePromotionStrategy();

    Customer customer1 = new Customer(halfOffPromo);
    customer1.addItem(100.0);
    customer1.addItem(50.0);
    customer1.addItem(20.0);

    System.out.println("Customer 1");
    System.out.println("Total: " + customer1.getTotalBeforePromo());
    System.out.println("Total (After Promo): " + customer1.getTotalAfterPromo());

    Customer customer2 = new Customer(clearancePromo);
    customer2.addItem(100.0);
    customer2.addItem(50.0);
    customer2.addItem(20.0);

    System.out.println("Customer 2");
    System.out.println("Total: " + customer2.getTotalBeforePromo());
    System.out.println("Total (After Promo):" + customer2.getTotalAfterPromo());
  }
}

class Customer {
  private List<Double> items;
  private PromotionStrategy promotionStrategy;

  public Customer(PromotionStrategy promotionStrategy) {
    this.promotionStrategy = promotionStrategy;
    this.items = Lists.newArrayList();
  }

  public void addItem(Double item){
    items.add(item);
  }

  public  double getTotalBeforePromo(){
    double total = items.stream().mapToDouble(i -> i).sum();
    return total;
  }

  public double getTotalAfterPromo(){
    double total = getTotalBeforePromo();
    double promotion = promotionStrategy.getPromotion(total);
    return total - promotion;
  }
}

interface PromotionStrategy{
  double getPromotion(Double totalAmount);
}

class HalfOffPromotionStrategy implements PromotionStrategy {
  @Override
  public double getPromotion(Double totalAmount) {
    return totalAmount * 0.5;
  }
}

class ClearancePromotionStrategy implements PromotionStrategy{
  @Override
  public double getPromotion(Double totalAmount) {
    return totalAmount * 0.75;
  }
}