享元模式指通过共享的技术来支持大量细粒度对象的复用，它通过共享已经存在的又橡来大幅度减少需要创建的对象数量、避免大量相似类的开销，从而提高系统资源的利用率。例如围棋和五子棋中的黑白棋子，图像中的坐标点或颜色，局域网中的路由器、交换机和集线器，教室里的桌子和凳子等。这些对象有很多相似的地方，如果能把它们相同的部分提取出来共享，则能节省大量的系统资源，这就是享元模式的产生背景。
如何解决：用唯一标识码判断，如果内存中有，则返回这个唯一标识码所标识的对象。
关键代码：将内部状态作为标识，进行共享。
以下是享元模式的UML：
图中的 UnsharedConcreteFlyweight 是与享元角色，里面包含了非共享的外部状态信息 info；而 Flyweight 是抽象享元角色，里面包含了享元方法 operation(UnsharedConcreteFlyweight state)，非享元的外部状态以参数的形式通过该方法传入；ConcreteFlyweight 是具体享元角色，包含了关键字 key，它实现了抽象享元接口；FlyweightFactory 是享元工厂角色，它逝关键字 key 来管理具体享元；客户角色通过享元工厂获取具体享元，并访问具体享元的相关方法。
以在银行中存钱为例，支票和硬币都是货币，具有相同价值的面值，可以作为Money这一抽象享元角色的实现，而设置MoneyCoin和MoneyNote作为具体享元角色。银行可以作为享元工厂类供用户存取金钱。
C++实现：
1 #include <iostream>
2 #include <vector>
3
4 using namespace std;
5
6 //货币类型，作为享元对象标识Key
7 enum MoneyCategory{
8 Coin,
9 BankNote
10 };
11
12 //面值，外部标识，需要储存的对象
13 enum FaceValue{
14 ValueOne = 1,
15 ValueTen = 10
16 };
17
18 //抽象享元角色
19 class Money{
20 public:
21 Money(MoneyCategory _category){category = _category;}
22 virtual ~Money(){}
23 virtual void save() = 0;
24 private:
25 MoneyCategory category;
26 };
27
28 //具体享元角色
29 class MoneyCoin:public Money{
30 public:
31 MoneyCoin(MoneyCategory _moneyCategory):Money(_moneyCategory){cout << "Save With Coin" << endl;}
32 ~MoneyCoin(){}
33 void save()
34 {
35 cout << "save coin" << endl;
36 }
37 };
38
39 class MoneyNote:public Money{
40 public:
41 MoneyNote(MoneyCategory _moneyCategory):Money(_moneyCategory){cout << "Save With Note" << endl;}
42 ~MoneyNote(){}
43 void save()
44 {
45 cout << "save note" << endl;
46 }
47 };
48
49 //享元工厂类，提供用户存取享元角色的方法
50 class Bank{
51 public:
52 Bank():coin(nullptr),note(nullptr),count(0){}
53 ~Bank(){
54 if (coin != nullptr){
55 delete coin;
56 coin = nullptr;
57 }
58 if (note != nullptr){
59 delete note;
60 note = nullptr;
61 }
62 }
63 void saveMoney(MoneyCategory category, FaceValue value){
64 switch (category) {
65 case Coin:{
66 if (coin == nullptr){
67 //避免重复创建对象
68 coin = new MoneyCoin(Coin);
69 }
70 coin->save();
71 faceValueVector.push_back(value);
72 break;
73 }
74 case BankNote:{
75 if (note == nullptr){
76 note = new MoneyNote(BankNote);
77 }
78 note->save();
79 faceValueVector.push_back(value);
80 break;
81 }
82 default:{
83 break;
84 }
85 }
86 }
87 int sumSave(){
88 auto iter = faceValueVector.begin();
89 for (;iter != faceValueVector.end(); iter++){
90 count += *iter;
91 }
92 return count;
93 }
94 private:
95 vector<FaceValue> faceValueVector;
96 Money* coin;
97 Money* note;
98 int count;
99 };
100
101 int main()
102 {
103 Bank bank;
104 bank.saveMoney(Coin, ValueOne);
105 bank.saveMoney(BankNote, ValueTen);
106 bank.saveMoney(Coin, ValueOne);
107 bank.saveMoney(BankNote, ValueTen);
108 cout << "Total Money:" << bank.sumSave() << endl;
109 return 0;
110 }
Java 实现：
1 public enum MoneyCategory {
2 Coin,
3 BankNote
4 }
5
6 public enum FaceValue{
7 ValueOne,
8 ValueTen
9 }
10
11 public abstract class Money {
12
13 protected MoneyCategory category;
14
15 Money(MoneyCategory category){
16 this.category = category;
17 }
18
19 public void save(){}
20 }
21
22 public class MoneyCoin extends Money {
23
24 public MoneyCoin(MoneyCategory moneyCategory){
25 super(moneyCategory);
26 System.out.println("Save With Coin");
27 }
28
29 public void save(){
30 System.out.println("Save Coin");
31 }
32 }
33
34 public class MoneyNote extends Money {
35
36 public MoneyNote(MoneyCategory moneyCategory){
37 super(moneyCategory);
38 System.out.println("Save With Note");
39 }
40
41 public void save(){
42 System.out.println("Save Note");
43 }
44 }
45
46 public class Bank {
47
48 private static final Map<FaceValue, Integer> valueMap = new HashMap<FaceValue, Integer>(){{put(FaceValue.ValueOne, 1); put(FaceValue.ValueTen, 10);}};
49
50 private Money moneyCoin;
51
52 private Money moneyNote;
53
54 private List<FaceValue> faceValueList;
55
56 public Bank(){
57 faceValueList = new ArrayList<FaceValue>();
58 }
59
60 public void saveMoney(MoneyCategory moneyCategory, FaceValue faceValue){
61 switch (moneyCategory){
62 case Coin:{
63 if (moneyCoin == null){
64 moneyCoin = new MoneyCoin(moneyCategory);
65 }
66 moneyCoin.save();
67 faceValueList.add(faceValue);
68 break;
69 }
70 case BankNote: {
71 if (moneyNote == null) {
72 moneyNote = new MoneyNote(moneyCategory);
73 }
74 moneyNote.save();
75 faceValueList.add(faceValue);
76 }
77 default:
78 break;
79 }
80 }
81
82 public int getMoney(){
83 int total = 0;
84 for (FaceValue faceValue:
85 faceValueList) {
86 total += valueMap.get(faceValue);
87 }
88 return total;
89 }
90 }
91
92 public class Main {
93
94 public static void main(String[] args) {
95 Bank bank = new Bank();
96 bank.saveMoney(MoneyCategory.Coin, FaceValue.ValueOne);
97 bank.saveMoney(MoneyCategory.BankNote, FaceValue.ValueTen);
98 bank.saveMoney(MoneyCategory.Coin, FaceValue.ValueOne);
99 bank.saveMoney(MoneyCategory.BankNote, FaceValue.ValueTen);
100 System.out.println("Totally save money:" + bank.getMoney());
101 }
102 }