introduce
Intent : Define the skeleton of an algorithm in an operation, while deferring some steps to subclasses. Template methods allow subclasses to redefine certain steps of an algorithm without changing its structure.
The main solution : some methods are common, but this method is rewritten in each subclass.
When to use : There are some general methods.
How to solve it : abstract these general algorithms out.
Key code : Implemented in the abstract class, other steps are implemented in the subclass.
accomplish
Basic implementation
public abstract class CaffeineBeverage {
final void prepareRecipe() {
boilWater();
brew();
pourInCup();
addCondiments();
}
abstract void brew();
abstract void addCondiments();
void boilWater() {
System.out.println("Boiling water");
}
void pourInCup() {
System.out.println("Pouring into cup");
}
}
public class Coffee extends CaffeineBeverage {
public void brew() {
System.out.println("Dripping Coffee through filter");
}
public void addCondiments() {
System.out.println("Adding Sugar and Milk");
}
}
public class Tea extends CaffeineBeverage {
public void brew() {
System.out.println("Steeping the tea");
}
public void addCondiments() {
System.out.println("Adding Lemon");
}
}
run class
public class BeverageTestDrive {
public static void main(String[] args) {
Tea tea = new Tea();
Coffee coffee = new Coffee();
System.out.println("\nMaking tea...");
tea.prepareRecipe();
System.out.println("\nMaking coffee...");
coffee.prepareRecipe();
}
}
operation result
Making tea...
Boiling water
Steeping the tea
Pouring into cup
Adding Lemon
Making coffee...
Boiling water
Dripping Coffee through filter
Pouring into cup
Adding Sugar and Milk
Hook up template methods
A hook is a method that is declared in an abstract class, but only has an empty or default implementation. The existence of hooks allows subclasses to have the ability to hook different points of the algorithm (in order to allow template methods to be implemented). It is up to the subclass to decide whether to hook it or not.
public abstract class CaffeineBeverageWithHook {
final void prepareRecipe() {
boilWater();
brew();
pourInCup();
if (customerWantsCondiments()) {
addCondiments();
}
}
abstract void brew();
abstract void addCondiments();
void boilWater() {
System.out.println("Boiling water");
}
void pourInCup() {
System.out.println("Pouring into cup");
}
//钩子
boolean customerWantsCondiments() {
return true;
}
}
public class CoffeeWithHook extends CaffeineBeverageWithHook {
public void brew() {
System.out.println("Dripping Coffee through filter");
}
public void addCondiments() {
System.out.println("Adding Sugar and Milk");
}
//钩子
public boolean customerWantsCondiments() {
String answer = getUserInput();
if (answer.toLowerCase().startsWith("y")) {
return true;
} else {
return false;
}
}
private String getUserInput() {
String answer = null;
System.out.print("Would you like milk and sugar with your coffee (y/n)? ");
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
try {
answer = in.readLine();
} catch (IOException ioe) {
System.err.println("IO error trying to read your answer");
}
if (answer == null) {
return "no";
}
return answer;
}
}
public class CoffeeWithHook extends CaffeineBeverageWithHook {
public void brew() {
System.out.println("Dripping Coffee through filter");
}
public void addCondiments() {
System.out.println("Adding Sugar and Milk");
}
//钩子
public boolean customerWantsCondiments() {
String answer = getUserInput();
if (answer.toLowerCase().startsWith("y")) {
return true;
} else {
return false;
}
}
private String getUserInput() {
String answer = null;
System.out.print("Would you like milk and sugar with your coffee (y/n)? ");
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
try {
answer = in.readLine();
} catch (IOException ioe) {
System.err.println("IO error trying to read your answer");
}
if (answer == null) {
return "no";
}
return answer;
}
}
run class
public class BeverageTestDrive {
public static void main(String[] args) {
TeaWithHook teaHook = new TeaWithHook();
CoffeeWithHook coffeeHook = new CoffeeWithHook();
System.out.println("\nMaking tea...");
teaHook.prepareRecipe();
System.out.println("\nMaking coffee...");
coffeeHook.prepareRecipe();
}
}
operation result
Making tea...
Boiling water
Steeping the tea
Pouring into cup
Would you like lemon with your tea (y/n)? y
Adding Lemon
Making coffee...
Boiling water
Dripping Coffee through filter
Pouring into cup
Would you like milk and sugar with your coffee (y/n)? y
Adding Sugar and Milk
Sort by template method
public class Duck implements Comparable<Duck> {
String name;
int weight;
public Duck(String name, int weight) {
this.name = name;
this.weight = weight;
}
public String toString() {
return name + " weighs " + weight;
}
//模板方法
public int compareTo(Duck object) {
Duck otherDuck = object;
if (this.weight < otherDuck.weight) {
return -1;
} else if (this.weight == otherDuck.weight) {
return 0;
} else { // this.weight > otherDuck.weight
return 1;
}
}
}
run class
public class DuckSortTestDrive {
public static void main(String[] args) {
Duck[] ducks = {
new Duck("Daffy", 8),
new Duck("Dewey", 2),
new Duck("Howard", 7),
new Duck("Louie", 2),
new Duck("Donald", 10),
new Duck("Huey", 2)
};
System.out.println("Before sorting:");
display(ducks);
Arrays.sort(ducks);
System.out.println("\nAfter sorting:");
display(ducks);
}
public static void display(Duck[] ducks) {
for (Duck d : ducks) {
System.out.println(d);
}
}
}
operation result
Before sorting:
Daffy weighs 8
Dewey weighs 2
Howard weighs 7
Louie weighs 2
Donald weighs 10
Huey weighs 2
After sorting:
Dewey weighs 2
Louie weighs 2
Huey weighs 2
Howard weighs 7
Daffy weighs 8
Donald weighs 10
References and citations
1. "Head First Design Patterns" Eric Freeman et al