Decorator Pattern example

RebateDecorator.java

public abstract class RebateDecorator implements RebatePolicy {

	protected RebatePolicy decorated; 

	protected RebateDecorator(RebatePolicy decorated){
		this.decorated = decorated;
	}
}

RebatePolicy.java

public interface RebatePolicy {
	Money calculateRebate(Product product, int quantity, Money regularCost);
}

StandardRebate.java

public class StandardRebate implements RebatePolicy {

	private BigDecimal rebateRatio;
	private int mininalQuantity;
	public StandardRebate(double rebate, int mininalQuantity) {
		rebateRatio = new BigDecimal(rebate / 100);
		this.mininalQuantity = mininalQuantity;
	}

	@Override
	public Money calculateRebate(Product product, int quantity, 
	Money regularCost) {
		if (quantity >= mininalQuantity)
			return regularCost.multiplyBy(rebateRatio);
		return Money.ZERO;
	}
}

VipRebate.java

public class VipRebate extends RebateDecorator{

	private Money minimalThreshold;
	private Money rebateValue;

	public VipRebate(Money minimalThreshold, Money rebateValue) {
		this(null, minimalThreshold, rebateValue);
	}

	public VipRebate(RebatePolicy decorated, Money minimalThreshold, 
	Money rebateValue) {
		super(decorated);	
		if (rebateValue.greaterThan(minimalThreshold))
			throw new IllegalArgumentException(
			"Rabate can't be graterThan minimal threshold");
		this.minimalThreshold = minimalThreshold;
		this.rebateValue = rebateValue;
	}

	@Override
	public Money calculateRebate(Product product, int quantity,
			Money regularCost) {
		Money baseValue = (decorated == null) 
		? regularCost
		: decorated.calculateRebate(product, quantity, regularCost); 

		if (baseValue.greaterThan(minimalThreshold))
			return baseValue.subtract(rebateValue);
		return baseValue;
	}
}