Java设计模式(4) -- 模板方法

Template Method

 

英文简要描述

Intent
Define the skeleton of an algorithm in an operation, deferring some steps to subclasses.
Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.

How to
AbstractClass
defines abstract primitive operations that concrete subclasses define to implement steps of an algorithm.
implements a template method defining the skeleton of an algorithm. The template method calls primitive operations as well as operations defined in AbstractClass or those of other objects.
ConcreteClass
implements the primitive operations to carry out subclass-specific steps of the algorithm.

Known cases
A lot

 

UML

模板方法通常和工厂方法一起使用，以下是样例:

public abstract class Application { private List docs = new ArrayList(); public Document openDocument(String path) { Document doc = null; if (isDocumentExist(path)) { doc = newDocument(); } else { doc = readDocument(path); } appendDocument(doc); check(doc); return doc; } private boolean isDocumentExist(String path) { // check if the document exists // ... return true; } private void appendDocument(Document doc) { docs.add(doc); } protected abstract Document newDocument(); // factory method protected abstract Document readDocument(String path); //primitive method protected void check(Document doc) { // hook method // do thing } } public class ExcelApplication extends Application { protected Document newDocument() { return new ExcelDocument(); } protected Document readDocument(String path) { System.out.println("Read Excel Document"); return null; } protected void check(Document doc) { System.out.println("Check Excel Document"); } } public class WordApplication extends Application { protected Document newDocument() { return new WordDocument(); } protected Document readDocument(String path) { System.out.println("Read Word Document"); return null; } } public interface Document { public void show(); } public class ExcelDocument implements Document { public void show() { System.out.println("Show Excel Document"); } } public class WordDocument implements Document { public void show() { System.out.println("Show Word Document"); } } public class TestTemplateMethod { public static void main(String[] args) { Application app = null; if (args[0].equals("Word")) { app = new WordApplication(); } else if (args[0].equals("Excel")) { app = new ExcelApplication(); } else { throw new RuntimeException("Unsupport type"); } Document doc = app.openDocument("Path"); doc.show(); } }

上例的uml

primitive operation

抽象方法，子类必须实现

hook operation

钩子方法，有默认实现，子类可以重写其行为