在已發(fā)布的Java1.4中在核心代碼庫中增加了許多新的API(如Loging,正則表達(dá)式，NIO)等，在最新發(fā)布的JDK1.5和即將發(fā)布的JDK1.6中也新增了許多API，其中比較有重大意義的就是Generics(范型）。
一.什么是Generics?
Generics可以稱之為參數(shù)類型(parameterized?types),由編譯器來驗(yàn)證從客戶端將一種類型傳送給某一對象的機(jī)制。如Java.util.ArrayList,
編譯器可以用Generics來保證類型安全。
在我們深入了解Generics之前,我們先來看一看當(dāng)前的java?集合框架（Collection)。在j2SE1.4中所有集合的Root?Interface是Collection
Collections?example?without?genericity:?Example?1
1?protected?void?collectionsExample()?{
2??ArrayList?list?=?new?ArrayList();
3??list.add(new?String("test?string"));
4??list.add(new?Integer(9));?//?purposely?placed?here?to?create?a?runtime?ClassCastException
5??inspectCollection(list);
6?}
7
8
9?protected?void?inspectCollection(Collection?aCollection)?{
10??Iterator?i?=?aCollection.iterator();
11??while?(i.hasNext())?{
12???String?element?=?(String)?i.next();
13??}
14?}
以上的樣例程序包含的兩個方法，collectionExample方法建立了一個簡單的集合類型ArrayList，并在ArrayList中增加了一個String和一個Integer對象.而在inspecCollection方法中，我們迭代這個ArrayList用String進(jìn)行Cast。我們看第二個方法，就出現(xiàn)了一個問題,Collection在內(nèi)部用的是Object，而我們要取出Collection中的對象時(shí),需要進(jìn)行Cast，那么開發(fā)者必需用實(shí)際的類型進(jìn)行Cast,像這種向下造型，編譯器無
法進(jìn)行檢查,如此一來我們就要冒在代碼在運(yùn)行拋出ClassCastException的危險(xiǎn)。我們看inspecCollection方法，編譯時(shí)沒有問題，但在運(yùn)行時(shí)就會拋出ClassCastException異常。所以我們一定要遠(yuǎn)離這個重大的運(yùn)行時(shí)錯誤
二.使用Generics
從上一章節(jié)中的CassCastException這種異常，我們期望在代碼編譯時(shí)就能夠捕捉到,下面我們使用范型修改上一章的樣例程序。
//Example?2
1?protected?void?collectionsExample()?{
2??ArrayList<String>?list?=?new?ArrayList<String>();
3??list.add(new?String("test?string"));
4??//?list.add(new?Integer(9));?this?no?longer?compiles
5??inspectCollection(list);
6?}
7?
8?
9?protected?void?inspectCollection(Collection<String>?aCollection)?{
10??Iterator<String>?i?=?aCollection.iterator();
11??while(i.hasNext())?{
12???String?element?=?i.next();
13??}
14?}
從上面第2行我們在創(chuàng)建ArrayList時(shí)使用了新語法，在JDK1.5中所有的Collection都加入了Generics的聲明。例:
//Example?3
1?public?class?ArrayList<E>?extends?AbstractList<E>?{
2??//?details?omitted...
3??public?void?add(E?element)?{
4???//?details?omitted
5??}
6??public?Iterator<E>?iterator()?{
7???//?details?omitted
8??}
9?}
這個E是一個類型變量，并沒有對它進(jìn)行具體類型的定義,它只是在定義ArrayList時(shí)的類型占位符,在Example?2中的我們在定義ArrayList的實(shí)
例時(shí)用String綁定在E上,當(dāng)我們用add(E?element)方法向ArrayList中增加對象時(shí),?那么就像下面的寫法一樣：?public?void?add(String?element)；因?yàn)樵贏rrayList所有方法都會用String來替代E,無論是方法的參數(shù)還是返回值。這時(shí)我們在看Example?2中的第四行，編譯就會反映出編譯錯誤。
所以在java中增加Generics主要的目的是為了增加類型安全。
通過上面的簡單的例子我們看到使用Generics的好處有：
1.在類型沒有變化時(shí)，Collection是類型安全的。
2.內(nèi)在的類型轉(zhuǎn)換優(yōu)于在外部的人工造型。
3.使Java?接口更加強(qiáng)壯,因?yàn)樗黾恿祟愋汀?br />4.類型的匹配錯誤在編譯階段就可以捕捉到，而不是在代碼運(yùn)行時(shí)。
受約束類型變量
雖然許多Class被設(shè)計(jì)成Generics，但類型變量可以是受限的
public?class?C1<T?extends?Number>?{?}
public?class?C2<T?extends?Person?&?Comparable>?{?}?
第一個T變量必須繼承Number，第二個T必須繼承Person和實(shí)現(xiàn)Comparable
三.Generics?方法
像Generics類一樣，方法和構(gòu)造函數(shù)也可以有類型參數(shù)。方法的參數(shù)的返回值都可以有類型參數(shù)，進(jìn)行Generics。
//Example?4
1?public?<T?extends?Comparable>?T?max(T?t1,?T?t2)?{
2??if?(t1.compareTo(t2)?>?0)
3???return?t1;
4??else?return?t2;
5?}
這里，max方法的參數(shù)類型為單一的T類型，而T類型繼承了Comparable，max的參數(shù)和返回值都有相同的超類。下面的Example?5顯示了max方法的幾個約束。
//Example?5
1?Integer?iresult?=?max(new?Integer(100),?new?Integer(200));
2?String?sresult?=?max("AA",?"BB");
3?Number?nresult?=?max(new?Integer(100),?"AAA");?//?does?not?compile
在Example?5第1行參數(shù)都為Integer，所以返回值也是Integer，注意返回值沒有進(jìn)行造型。
在Example?5第2行參數(shù)都為String，所以返回值也是String，注意返回值沒有進(jìn)行造型。以上都調(diào)用了同一個方法。
在Example?5第3行產(chǎn)生以下編譯錯誤：
Example.java:10:?incompatible?types
found??:?java.lang.Object&java.io.Serializable&java.lang.Comparable<?>
required:?java.lang.Number
????Number?nresult?=?max(new?Integer(100),?"AAA");
這個錯誤發(fā)生是因?yàn)榫幾g器無法確定返回值類型，因?yàn)镾tring和Integer都有相同的超類Object,注意就算我們修正了第三行，這行代碼在運(yùn)行仍然會報(bào)錯，因?yàn)楸容^了不同的對象。
四.向下兼容
任何一個新的特色在新的JDK版本中出來后，我們首先關(guān)心的是如何于以前編寫的代碼兼容。也就是說我們編寫的Example?1程序不需要任何的改變就可以運(yùn)行,但是編譯器會給出一個"ROW?TYPE"的警告。在JDK1.4中編寫的代碼如何在JVM1.5中完全兼容運(yùn)行,我們要人工進(jìn)行一個:Type?erasure處理過程
五.通配符
//Example?6
List<String>?stringList?=?new?ArrayList<String>();?//1
List<Object>?objectList?=?stringList?;//2
objectList?.add(new?Object());?//?3
String?s?=?stringList?.get(0);//4
乍一看，Example?
6是正確的。但stringList本意是存放String類型的ArrayList,而objectList中可以存入任何對象，當(dāng)在第3行進(jìn)行處理時(shí)，stringList也就無法保證是String類型的ArrayList,此時(shí)編譯器不允許這樣的事出現(xiàn)，所以第3行將無法編譯。
//Example?7
void?printCollection(Collection<Object>?c)?
{?for?(Object?e?:?c)?{?
System.out.println(e);
}}
Example?7的本意是打印所有Collection的對象,但是正如Example?6所說的，編譯會報(bào)錯，此時(shí)就可以用通配符“？”來修改Example?7
//Example?8
void?printCollection(Collection<?>?c)?
{?for?(Object?e?:?c)?{?
System.out.println(e);
}}
Example?8中所有Collection類型就可以方便的打印了
有界通配符?<T?extends?Number>(上界)?<T?super?Number>(下界)?
六.創(chuàng)建自己的范型
以下代碼來自http://www.java2s.com/ExampleCode/Language-Basics
1.一個參數(shù)的Generics
//Example?9(沒有使用范型)
class?NonGen?{??
??Object?ob;?//?ob?is?now?of?type?Object?
??//?Pass?the?constructor?a?reference?to???
??//?an?object?of?type?Object?
??NonGen(Object?o)?{??
????ob?=?o;??
??}??
??//?Return?type?Object.?
??Object?getob()?{??
????return?ob;??
??}??
??//?Show?type?of?ob.??
??void?showType()?{??
????System.out.println("Type?of?ob?is?"?+??
???????????????????????ob.getClass().getName());??
??}??
}??
//?Demonstrate?the?non-generic?class.??
public?class?NonGenDemo?{??
??public?static?void?main(String?args[])?{??
????NonGen?iOb;???
????//?Create?NonGen?Object?and?store?
????//?an?Integer?in?it.?Autoboxing?still?occurs.?
????iOb?=?new?NonGen(88);??
????//?Show?the?type?of?data?used?by?iOb.?
????iOb.showType();?
????//?Get?the?value?of?iOb.?
????//?This?time,?a?cast?is?necessary.?
????int?v?=?(Integer)?iOb.getob();??
????System.out.println("value:?"?+?v);??
????System.out.println();??
????//?Create?another?NonGen?object?and??
????//?store?a?String?in?it.?
????NonGen?strOb?=?new?NonGen("Non-Generics?Test");??
????//?Show?the?type?of?data?used?by?strOb.?
????strOb.showType();?
????//?Get?the?value?of?strOb.?
????//?Again,?notice?that?a?cast?is?necessary.??
????String?str?=?(String)?strOb.getob();??
????System.out.println("value:?"?+?str);??
????//?This?compiles,?but?is?conceptually?wrong!?
????iOb?=?strOb;?
????v?=?(Integer)?iOb.getob();?//?runtime?error!?
??}??
}
??
//Example?10(使用范型)
class?Example1<T>{
?private?T?t;
?Example1(T?o){
??this.t=o;
??}
?T?getOb(){
??return?t;
?}
?void?ShowObject(){
??System.out.println("對象的類型是："+t.getClass().getName());
?}
}
public?class?GenericsExample1?{
?/**
??*?@param?args
??*/
?public?static?void?main(String[]?args)?{
??//?TODO?Auto-generated?method?stub
??Example1<Integer>?examplei=new?Example1<Integer>(100);
??examplei.ShowObject();
??System.out.println("對象是："+examplei.getOb());
??Example1<String>?examples=new?Example1<String>("Bill");
??examples.ShowObject();
??System.out.println("對象是："+examples.getOb());
?}
}
我們來看Example?9沒有使用范型，所以我們需要進(jìn)行造型，而Example?10我們不需要任何的造型
2.二個參數(shù)的Generics
//Example?11
class?TwoGen<T,?V>?{?
???T?ob1;?
???V?ob2;?
???//?Pass?the?constructor?a?reference?to??
???//?an?object?of?type?T.?
???TwoGen(T?o1,?V?o2)?{?
?????ob1?=?o1;?
?????ob2?=?o2;?
???}?
???//?Show?types?of?T?and?V.?
???void?showTypes()?{?
?????System.out.println("Type?of?T?is?"?+?
????????????????????????ob1.getClass().getName());?
?????System.out.println("Type?of?V?is?"?+?
????????????????????????ob2.getClass().getName());?
???}?
???T?getob1()?{?
?????return?ob1;?
???}?
???V?getob2()?{?
?????return?ob2;?
???}?
?}?
public?class?GenericsExampleByTwoParam?{
?/**
??*?@param?args
??*/
?public?static?void?main(String[]?args)?{
??//?TODO?Auto-generated?method?stub
??TwoGen<Integer,?String>?tgObj?=?
???????new?TwoGen<Integer,?String>(88,?"Generics");?
?????//?Show?the?types.?
?????tgObj.showTypes();?
?????//?Obtain?and?show?values.?
?????int?v?=?tgObj.getob1();?
?????System.out.println("value:?"?+?v);?
?????String?str?=?tgObj.getob2();?
?????System.out.println("value:?"?+?str);?
???}?
?}
3.Generics的Hierarchy
//Example?12
class?Stats<T?extends?Number>?{??
???T[]?nums;?//?array?of?Number?or?subclass?
???//?Pass?the?constructor?a?reference?to???
???//?an?array?of?type?Number?or?subclass.?
???Stats(T[]?o)?{??
?????nums?=?o;??
???}??
???//?Return?type?double?in?all?cases.?
???double?average()?{??
?????double?sum?=?0.0;?
?????for(int?i=0;?i?<?nums.length;?i++)??
???????sum?+=?nums[i].doubleValue();?
?????return?sum?/?nums.length;?
???}??
?}??
public?class?GenericsExampleByHierarchy?{
?
?/**
??*?@param?args
??*/
?public?static?void?main(String[]?args)?{
??//?TODO?Auto-generated?method?stub
???Integer?inums[]?=?{?1,?2,?3,?4,?5?};?
?????Stats<Integer>?iob?=?new?Stats<Integer>(inums);???
?????double?v?=?iob.average();?
?????System.out.println("iob?average?is?"?+?v);?
?????Double?dnums[]?=?{?1.1,?2.2,?3.3,?4.4,?5.5?};?
?????Stats<Double>?dob?=?new?Stats<Double>(dnums);???
?????double?w?=?dob.average();?
?????System.out.println("dob?average?is?"?+?w);?
?????//?This?won't?compile?because?String?is?not?a?
?????//?subclass?of?Number.?
//?????String?strs[]?=?{?"1",?"2",?"3",?"4",?"5"?};?
//?????Stats<String>?strob?=?new?Stats<String>(strs);???
//?????double?x?=?strob.average();?
//?????System.out.println("strob?average?is?"?+?v);?
???}??
?}
??
4.使用通配符
//Example?14
class?StatsWildCard<T?extends?Number>?{
?T[]?nums;?//?array?of?Number?or?subclass
?//?Pass?the?constructor?a?reference?to
?//?an?array?of?type?Number?or?subclass.
?StatsWildCard(T[]?o)?{
??nums?=?o;
?}
?//?Return?type?double?in?all?cases.
?double?average()?{
??double?sum?=?0.0;
??for?(int?i?=?0;?i?<?nums.length;?i++)
???sum?+=?nums[i].doubleValue();
??return?sum?/?nums.length;
?}
?//?Determine?if?two?averages?are?the?same.
?//?Notice?the?use?of?the?wildcard.
?boolean?sameAvg(StatsWildCard<?>?ob)?{
??if?(average()?==?ob.average())
???return?true;
??return?false;
?}
}
public?class?GenericsExampleByWildcard?{
?/**
??*?@param?args
??*/
?public?static?void?main(String[]?args)?{
??//?TODO?Auto-generated?method?stub
??Integer?inums[]?=?{?1,?2,?3,?4,?5?};
??StatsWildCard<Integer>?iob?=?new?StatsWildCard<Integer>(inums);
??double?v?=?iob.average();
??System.out.println("iob?average?is?"?+?v);
??Double?dnums[]?=?{?1.1,?2.2,?3.3,?4.4,?5.5?};
??StatsWildCard<Double>?dob?=?new?StatsWildCard<Double>(dnums);
??double?w?=?dob.average();
??System.out.println("dob?average?is?"?+?w);
??Float?fnums[]?=?{?1.0F,?2.0F,?3.0F,?4.0F,?5.0F?};
??StatsWildCard<Float>?fob?=?new?StatsWildCard<Float>(fnums);
??double?x?=?fob.average();
??System.out.println("fob?average?is?"?+?x);
??//?See?which?arrays?have?same?average.
??System.out.print("Averages?of?iob?and?dob?");
??if?(iob.sameAvg(dob))
???System.out.println("are?the?same.");
??else
???System.out.println("differ.");
??System.out.print("Averages?of?iob?and?fob?");
??if?(iob.sameAvg(fob))
???System.out.println("are?the?same.");
??else
???System.out.println("differ.");
?}
}
5.使用邊界通配符
//Example?15
class?TwoD?{?
??int?x,?y;?
??TwoD(int?a,?int?b)?{?
????x?=?a;?
????y?=?b;?
??}?
}?
//?Three-dimensional?coordinates.?
class?ThreeD?extends?TwoD?{?
??int?z;?
??ThreeD(int?a,?int?b,?int?c)?{?
????super(a,?b);?
????z?=?c;?
??}?
}?
//?Four-dimensional?coordinates.?
class?FourD?extends?ThreeD?{?
??int?t;?
??FourD(int?a,?int?b,?int?c,?int?d)?{?
????super(a,?b,?c);?
????t?=?d;??
??}?
}?
//?This?class?holds?an?array?of?coordinate?objects.?
class?Coords<T?extends?TwoD>?{?
??T[]?coords;?
??Coords(T[]?o)?{?coords?=?o;?}?
}?
//?Demonstrate?a?bounded?wildcard.?
public?class?BoundedWildcard?{?
??static?void?showXY(Coords<?>?c)?{?
????System.out.println("X?Y?Coordinates:");?
????for(int?i=0;?i?<?c.coords.length;?i++)?
??????System.out.println(c.coords[i].x?+?"?"?+?
?????????????????????????c.coords[i].y);?
????System.out.println();?
??}?
??static?void?showXYZ(Coords<??extends?ThreeD>?c)?{?
????System.out.println("X?Y?Z?Coordinates:");?
????for(int?i=0;?i?<?c.coords.length;?i++)?
??????System.out.println(c.coords[i].x?+?"?"?+?
?????????????????????????c.coords[i].y?+?"?"?+?
?????????????????????????c.coords[i].z);?
????System.out.println();?
??}?
??static?void?showAll(Coords<??extends?FourD>?c)?{?
????System.out.println("X?Y?Z?T?Coordinates:");?
????for(int?i=0;?i?<?c.coords.length;?i++)?
??????System.out.println(c.coords[i].x?+?"?"?+?
?????????????????????????c.coords[i].y?+?"?"?+?
?????????????????????????c.coords[i].z?+?"?"?+?
?????????????????????????c.coords[i].t);?
????System.out.println();?
??}?
??public?static?void?main(String?args[])?{?
????TwoD?td[]?=?{?
??????new?TwoD(0,?0),?
??????new?TwoD(7,?9),?
??????new?TwoD(18,?4),?
??????new?TwoD(-1,?-23)?
????};?
????Coords<TwoD>?tdlocs?=?new?Coords<TwoD>(td);?????
????System.out.println("Contents?of?tdlocs.");?
????showXY(tdlocs);?//?OK,?is?a?TwoD?
//??showXYZ(tdlocs);?//?Error,?not?a?ThreeD?
//??showAll(tdlocs);?//?Erorr,?not?a?FourD?
????//?Now,?create?some?FourD?objects.?
????FourD?fd[]?=?{?
??????new?FourD(1,?2,?3,?4),?
??????new?FourD(6,?8,?14,?8),?
??????new?FourD(22,?9,?4,?9),?
??????new?FourD(3,?-2,?-23,?17)?
????};?
????Coords<FourD>?fdlocs?=?new?Coords<FourD>(fd);?????
????System.out.println("Contents?of?fdlocs.");?
????//?These?are?all?OK.?
????showXY(fdlocs);??
????showXYZ(fdlocs);?
????showAll(fdlocs);?
??}?
}?
6.ArrayList的Generics
//Example?16
public?class?ArrayListGenericDemo?{
??public?static?void?main(String[]?args)?{
????ArrayList<String>?data?=?new?ArrayList<String>();
????data.add("hello");
????data.add("goodbye");
????//?data.add(new?Date());?This?won't?compile!
????Iterator<String>?it?=?data.iterator();
????while?(it.hasNext())?{
??????String?s?=?it.next();
??????System.out.println(s);
????}
??}
}?
7.HashMap的Generics
//Example?17
public?class?HashDemoGeneric?{
??public?static?void?main(String[]?args)?{
????HashMap<Integer,String>?map?=?new?HashMap<Integer,String>();
????map.put(1,?"Ian");
????map.put(42,?"Scott");
????map.put(123,?"Somebody?else");
????String?name?=?map.get(42);
????System.out.println(name);
??}
}?
8.接口的Generics
//Example?18
interface?MinMax<T?extends?Comparable<T>>?{?
??T?min();?
??T?max();?
}?
//?Now,?implement?MinMax?
class?MyClass<T?extends?Comparable<T>>?implements?MinMax<T>?{?
??T[]?vals;?
??MyClass(T[]?o)?{?vals?=?o;?}?
??//?Return?the?minimum?value?in?vals.?
??public?T?min()?{?
????T?v?=?vals[0];?
????for(int?i=1;?i?<?vals.length;?i++)?
??????if(vals[i].compareTo(v)?<?0)?v?=?vals[i];?
????return?v;?
??}?
??//?Return?the?maximum?value?in?vals.?
??public?T?max()?{?
????T?v?=?vals[0];?
????for(int?i=1;?i?<?vals.length;?i++)?
??????if(vals[i].compareTo(v)?>?0)?v?=?vals[i];?
????return?v;?
??}?
}?
public?class?GenIFDemo?{?
??public?static?void?main(String?args[])?{?
????Integer?inums[]?=?{3,?6,?2,?8,?6?};?
????Character?chs[]?=?{'b',?'r',?'p',?'w'?};?
????MyClass<Integer>?iob?=?new?MyClass<Integer>(inums);?
????MyClass<Character>?cob?=?new?MyClass<Character>(chs);?
????System.out.println("Max?value?in?inums:?"?+?iob.max());?
????System.out.println("Min?value?in?inums:?"?+?iob.min());?
????System.out.println("Max?value?in?chs:?"?+?cob.max());?
????System.out.println("Min?value?in?chs:?"?+?cob.min());?
??}?
}
9.Exception的Generics
//Example?20
interface?Executor<E?extends?Exception>?{
????void?execute()?throws?E;
}
public?class?GenericExceptionTest?{
????public?static?void?main(String?args[])?{
????????try?{
????????????Executor<IOException>?e?=
????????????????new?Executor<IOException>()?{
????????????????public?void?execute()?throws?IOException
????????????????{
????????????????????//?code?here?that?may?throw?an
????????????????????//?IOException?or?a?subtype?of
????????????????????//?IOException
????????????????}
????????????};
????????????e.execute();
????????}?catch(IOException?ioe)?{
????????????System.out.println("IOException:?"?+?ioe);
????????????ioe.printStackTrace();
????????}
????}
}??
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