Decorator pattern in java# Java - 爪哇娇娃
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Anyone is good at Design Pattern in Java here? Thanks.
As for the decorator pattern, what's the real benefit of using it? In this
example, why not simply let the specific classes, like "Milk/Whip/Sprinkle",
directly extend from the base class or interface "Coffee"?
Thanks.
// The Coffee Interface defines the functionality of Coffee implemented by
decorator
public interface Coffee {
public double getCost(); // returns the cost of the coffee
public String getIngredients(); // returns the ingredients of the coffee
}
// implementation of a simple coffee without any extra ingredients
public class SimpleCoffee implements Coffee {
public double getCost() {
return 1;
}
public String getIngredients() {
return "Coffee";
}
}
// abstract decorator class - note that it implements Coffee interface
abstract public class CoffeeDecorator implements Coffee {
protected final Coffee decoratedCoffee;
protected String ingredientSeparator = ", ";
public CoffeeDecorator(Coffee decoratedCoffee) {
this.decoratedCoffee = decoratedCoffee;
}
public double getCost() { // implementing methods of the interface
return decoratedCoffee.getCost();
}
public String getIngredients() {
return decoratedCoffee.getIngredients();
}
}
// Decorator Milk that mixes milk with coffee
// note it extends CoffeeDecorator
public class Milk extends CoffeeDecorator {
public Milk(Coffee decoratedCoffee) {
super(decoratedCoffee);
}
public double getCost() { // overriding methods defined in the abstract
superclass
return super.getCost() + 0.5;
}
public String getIngredients() {
return super.getIngredients() + ingredientSeparator + "Milk";
}
}
// Decorator Whip that mixes whip with coffee
// note it extends CoffeeDecorator
public class Whip extends CoffeeDecorator {
public Whip(Coffee decoratedCoffee) {
super(decoratedCoffee);
}
public double getCost() {
return super.getCost() + 0.7;
}
public String getIngredients() {
return super.getIngredients() + ingredientSeparator + "Whip";
}
}
// Decorator Sprinkles that mixes sprinkles with coffee
// note it extends CoffeeDecorator
public class Sprinkles extends CoffeeDecorator {
public Sprinkles(Coffee decoratedCoffee) {
super(decoratedCoffee);
}
public double getCost() {
return super.getCost() + 0.2;
}
public String getIngredients() {
return super.getIngredients() + ingredientSeparator + "Sprinkles";
}
}
public class Main
{
public static void main(String[] args)
{
Coffee c = new SimpleCoffee();
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
c = new Milk(c);
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
c = new Sprinkles(c);
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
c = new Whip(c);
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
// Note that you can also stack more than one decorator of the same
type
c = new Sprinkles(c);
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
}
}
As for the decorator pattern, what's the real benefit of using it? In this
example, why not simply let the specific classes, like "Milk/Whip/Sprinkle",
directly extend from the base class or interface "Coffee"?
Thanks.
// The Coffee Interface defines the functionality of Coffee implemented by
decorator
public interface Coffee {
public double getCost(); // returns the cost of the coffee
public String getIngredients(); // returns the ingredients of the coffee
}
// implementation of a simple coffee without any extra ingredients
public class SimpleCoffee implements Coffee {
public double getCost() {
return 1;
}
public String getIngredients() {
return "Coffee";
}
}
// abstract decorator class - note that it implements Coffee interface
abstract public class CoffeeDecorator implements Coffee {
protected final Coffee decoratedCoffee;
protected String ingredientSeparator = ", ";
public CoffeeDecorator(Coffee decoratedCoffee) {
this.decoratedCoffee = decoratedCoffee;
}
public double getCost() { // implementing methods of the interface
return decoratedCoffee.getCost();
}
public String getIngredients() {
return decoratedCoffee.getIngredients();
}
}
// Decorator Milk that mixes milk with coffee
// note it extends CoffeeDecorator
public class Milk extends CoffeeDecorator {
public Milk(Coffee decoratedCoffee) {
super(decoratedCoffee);
}
public double getCost() { // overriding methods defined in the abstract
superclass
return super.getCost() + 0.5;
}
public String getIngredients() {
return super.getIngredients() + ingredientSeparator + "Milk";
}
}
// Decorator Whip that mixes whip with coffee
// note it extends CoffeeDecorator
public class Whip extends CoffeeDecorator {
public Whip(Coffee decoratedCoffee) {
super(decoratedCoffee);
}
public double getCost() {
return super.getCost() + 0.7;
}
public String getIngredients() {
return super.getIngredients() + ingredientSeparator + "Whip";
}
}
// Decorator Sprinkles that mixes sprinkles with coffee
// note it extends CoffeeDecorator
public class Sprinkles extends CoffeeDecorator {
public Sprinkles(Coffee decoratedCoffee) {
super(decoratedCoffee);
}
public double getCost() {
return super.getCost() + 0.2;
}
public String getIngredients() {
return super.getIngredients() + ingredientSeparator + "Sprinkles";
}
}
public class Main
{
public static void main(String[] args)
{
Coffee c = new SimpleCoffee();
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
c = new Milk(c);
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
c = new Sprinkles(c);
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
c = new Whip(c);
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
// Note that you can also stack more than one decorator of the same
type
c = new Sprinkles(c);
System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.
getIngredients());
}
}
