《大话设计模式》java实现：第二章-策略模式
第二章是使用策略模式实现商场收银系统，使收银系统的促销策略可以灵活更改。
1. 原始代码实现：
package gof;
/*
* 《大话设计模式》第二章策略模式
* 实现商场收银系统，可以选择不同促销策略
*/
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class StrategyPattern {
public static void main(String[] args) {
new Gui();
}
}
// 收银系统UI
class Gui {
private JFrame frame;
private JTextField numberField;
private JTextField priceField;
private JComboBox<String> discountBox;
private JLabel resultLabel;
public Gui() {
frame = new JFrame("商场收银系统");
frame.setSize(400, 300);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new GridLayout(5, 2));
// 输入商品数量
JLabel numberLabel = new JLabel("商品数量:");
numberField = new JTextField();
frame.add(numberLabel);
frame.add(numberField);
// 输入商品单价
JLabel priceLabel = new JLabel("商品单价:");
priceField = new JTextField();
frame.add(priceLabel);
frame.add(priceField);
// 选择折扣方式
JLabel discountLabel = new JLabel("折扣方式:");
discountBox = new JComboBox<>(new String[]{"无折扣", "打八折", "满二十减五"});
frame.add(discountLabel);
frame.add(discountBox);
// 计算按钮
JButton calcButton = new JButton("计算");
calcButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
calculate();
}
});
frame.add(calcButton);
// 显示结果
resultLabel = new JLabel("总计: 0.0");
frame.add(resultLabel);
frame.setVisible(true);
}
//计算函数
private void calculate() {
try {
double number = Double.parseDouble(numberField.getText());
double price = Double.parseDouble(priceField.getText());
String discount = (String) discountBox.getSelectedItem();
//调用计算类的计算函数
double result=SimpleCount.getCount(number, price, discount);
resultLabel.setText("总计: " + result);
} catch (NumberFormatException e) {
resultLabel.setText("输入错误！");
}
}
}
// 商品价格结算类，最原始的实现。
class SimpleCount {
static double getCount(double number, double price, String discount) {
double sum = number * price;
// 打折
switch (discount) {
case "无折扣": {
return sum;
}
case "打八折": {
return sum * 0.8;
}
case "满二十减五": {
return sum >=20 ? sum - 5 : sum;
}
default:
throw new IllegalArgumentException("Unexpected value: " + discount);
}
}
}
在这个原始类中，每次更改促销策略都需要修改计算类和GUI，十分不方便。
简单工厂实现
为了节省篇幅，仅贴出需要修改的calculate()函数和其它类
private void calculate() {
try {
double number = Double.parseDouble(numberField.getText());
double price = Double.parseDouble(priceField.getText());
String discountString = (String) discountBox.getSelectedItem();
//调用折扣工厂初始化折扣类，调用折扣类获取打折后的价格
double sum=number*price;
Discount discount=DiscountFactory.getDiscount(sum, discountString);
double result=discount.getDiscount();
resultLabel.setText("总计: " + result);
} catch (NumberFormatException e) {
resultLabel.setText("输入错误！");
}
}
// 抽象打折类
abstract class Discount {
double sum;
abstract double getDiscount();
}
class NoDiscount extends Discount {
@Override
double getDiscount() {
return sum;
}
}
class P8Discount extends Discount {
@Override
double getDiscount() {
return sum * 0.8;
}
}
class Return5Discount extends Discount {
@Override
double getDiscount() {
return sum >= 20 ? sum - 5 : sum;
}
}
// 打折类工厂
class DiscountFactory {
static Discount getDiscount(double sum, String discountString) {
Discount discount;
switch (discountString) {
case "无折扣": {
discount = new NoDiscount();
break;
}
case "打八折": {
discount = new P8Discount();
break;
}
case "满二十减五": {
discount = new Return5Discount();
break;
}
default:
throw new IllegalArgumentException("Unexpected value: " + discountString);
}
// 在最后给成员变量赋值，避免写出多个赋值语句
discount.sum = sum;
return discount;
}
}
每次更改促销策略仍然需要修改计算类和GUI，十分不方便。
原始策略模式实现
删去了工厂，在计算函数中进行计算策略的实例化：
private void calculate() {
try {
double number = Double.parseDouble(numberField.getText());
double price = Double.parseDouble(priceField.getText());
String discountString = (String) discountBox.getSelectedItem();
Discount discount;
//实例化discount
switch (discountString){
case "无折扣": {
discount = new NoDiscount();
break;
}
case "打八折": {
discount = new P8Discount();
break;
}
case "满二十减五": {
discount = new Return5Discount();
break;
}
default:
throw new IllegalArgumentException("Unexpected value: " + discountString);
}
discount.sum=number*price;
//调用上下文获取折后价
DiscountContext discountContext=new DiscountContext(discount);
double result=discountContext.getDiscount();
resultLabel.setText("总计: " + result);
} catch (NumberFormatException e) {
resultLabel.setText("输入错误！");
}
}
新增了一个策略上下文类：
// 抽象打折类
abstract class Discount {
double sum;
abstract double getDiscount();
}
class NoDiscount extends Discount {
@Override
double getDiscount() {
return sum;
}
}
class P8Discount extends Discount {
@Override
double getDiscount() {
return sum * 0.8;
}
}
class Return5Discount extends Discount {
@Override
double getDiscount() {
return sum >= 20 ? sum - 5 : sum;
}
}
//使用原始策略模式
class DiscountContext{
private Discount discountClass;
//构造时传入具体折扣策略类
public DiscountContext(Discount discountSuper) {
this.discountClass=discountSuper;
}
//调用策略类的方法得到值。
public double getDiscount() {
return discountClass.getDiscount();
}
}
这样一来，在客户端实例化算法类，如果需要修改算法类，就需要修改客户端的实例化代码。还是很不方便。
策略模式+简单工厂实现
将工厂模式和策略模式上下文结合，在策略模式上下文中实例化算法类。
修改calculate函数：
private void calculate() {
try {
double number = Double.parseDouble(numberField.getText());
double price = Double.parseDouble(priceField.getText());
String discountString = (String) discountBox.getSelectedItem();
//调用上下文获取折后价
DiscountContext discountContext=new DiscountContext(number, price, discountString);
double result=discountContext.getDiscount();
resultLabel.setText("总计: " + result);
} catch (NumberFormatException e) {
resultLabel.setText("输入错误！");
}
}
在DiscountContxt类增加实例化语句：
//策略模式与简单工厂模式组合
class DiscountContext{
private Discount discount;
//构造函数与简单工厂结合
public DiscountContext(double number, double price, String discountString) {
switch (discountString) {
case "无折扣": {
discount = new NoDiscount();
break;
}
case "打八折": {
discount = new P8Discount();
break;
}
case "满二十减五": {
discount = new Return5Discount();
break;
}
default:
throw new IllegalArgumentException("Unexpected value: " + discountString);
}
// 在最后给成员变量赋值，避免写出多个赋值语句
discount.sum = number*price;
}
//调用策略类的方法得到值。
public double getDiscount() {
return discount.getDiscount();
}
}
这种设计模式比上面几种更加清楚。GUI的计算函数调用上下文，算法的实例化和计算都由上下文来调用。这样GUI只依赖于上下文类。
与简单工厂的区别就在于简单工厂中，GUI的计算函数需要调用工厂类和算法类两个类，而策略模式+简单工厂只需要调用上下文类，进一步降低了耦合。
下面附上完全体代码：
package gof;
/*
* 《大话设计模式》第二章策略模式
* 实现商场收银系统，可以选择不同促销策略
*/
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class StrategyPattern {
public static void main(String[] args) {
new Gui();
}
}
// 收银系统UI
class Gui {
private JFrame frame;
private JTextField numberField;
private JTextField priceField;
private JComboBox<String> discountBox;
private JLabel resultLabel;
public Gui() {
frame = new JFrame("商场收银系统");
frame.setSize(400, 300);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new GridLayout(5, 2));
// 输入商品数量
JLabel numberLabel = new JLabel("商品数量:");
numberField = new JTextField();
frame.add(numberLabel);
frame.add(numberField);
// 输入商品单价
JLabel priceLabel = new JLabel("商品单价:");
priceField = new JTextField();
frame.add(priceLabel);
frame.add(priceField);
// 选择折扣方式
JLabel discountLabel = new JLabel("折扣方式:");
discountBox = new JComboBox<>(new String[]{"无折扣", "打八折", "满二十减五"});
frame.add(discountLabel);
frame.add(discountBox);
// 计算按钮
JButton calcButton = new JButton("计算");
calcButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
calculate();
}
});
frame.add(calcButton);
// 显示结果
resultLabel = new JLabel("总计: 0.0");
frame.add(resultLabel);
frame.setVisible(true);
}
private void calculate() {
try {
double number = Double.parseDouble(numberField.getText());
double price = Double.parseDouble(priceField.getText());
String discountString = (String) discountBox.getSelectedItem();
//调用上下文获取折后价
DiscountContext discountContext=new DiscountContext(number, price, discountString);
double result=discountContext.getDiscount();
resultLabel.setText("总计: " + result);
} catch (NumberFormatException e) {
resultLabel.setText("输入错误！");
}
}
}
// 抽象打折类
abstract class Discount {
double sum;
abstract double getDiscount();
}
class NoDiscount extends Discount {
@Override
double getDiscount() {
return sum;
}
}
class P8Discount extends Discount {
@Override
double getDiscount() {
return sum * 0.8;
}
}
class Return5Discount extends Discount {
@Override
double getDiscount() {
return sum >= 20 ? sum - 5 : sum;
}
}
//策略模式与简单工厂模式组合
class DiscountContext{
private Discount discount;
//构造函数与简单工厂结合
public DiscountContext(double number, double price, String discountString) {
switch (discountString) {
case "无折扣": {
discount = new NoDiscount();
break;
}
case "打八折": {
discount = new P8Discount();
break;
}
case "满二十减五": {
discount = new Return5Discount();
break;
}
default:
throw new IllegalArgumentException("Unexpected value: " + discountString);
}
// 在最后给成员变量赋值，避免写出多个赋值语句
discount.sum = number*price;
}
//调用策略类的方法得到值。
public double getDiscount() {
return discount.getDiscount();
}
}