?在已發布的Java1.4中在核心代碼庫中增加了許多新的API(如Loging,正則表達式，NIO)等，在最新發布的JDK1.5和即將發布的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可以稱之為參數類型(parameterized types),由編譯器來驗證從客戶端將一種類型傳送給某一對象的機制。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進行Cast。我們看第二個方法，就出現了一個問題,Collection在內部用的是Object，而我們要取出Collection中的對象時,需要進行Cast，那么開發者必需用實際的類型進行Cast,像這種向下造型，編譯器無法進行檢查,如此一來我們就要冒在代碼在運行拋出ClassCastException的危險。我們看inspecCollection方法，編譯時沒有問題，但在運行時就會拋出ClassCastException異常。所以我們一定要遠離這個重大的運行時錯誤


二.使用Generics
從上一章節中的CassCastException這種異常，我們期望在代碼編譯時就能夠捕捉到,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行我們在創建ArrayList時使用了新語法，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，并沒有對它進行具體類型的定義,它只是在定義ArrayList時的類型占位符,在Example 2中的我們在定義ArrayList的實例時用String綁定在E上,當我們用add(E element)方法向ArrayList中增加對象時, 那么就像下面的寫法一樣： public void add(String element)；因為在ArrayList所有方法都會用String來替代E,無論是方法的參數還是返回值。這時我們在看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，Collection是類型安全的。
2.內在的類型轉換優于在外部的人工造型。
3.使Java 接口更加強壯,因為它增加了類型。
4.類型的匹配錯誤在編譯階段就可以捕捉到，而不是在代碼運行時。

受約束類型變量
雖然許多Class被設計成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和實現Comparable

三.Generics 方法

像Generics類一樣，方法和構造函數也可以有類型參數。方法的參數和返回值都可以有類型參數，進行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方法的參數類型為單一的T類型，而T類型繼承了Comparable，max的參數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行參數都為Integer，所以返回值也是Integer，注意返回值沒有進行造型。
在Example 5第2行參數都為String，所以返回值也是String，注意返回值沒有進行造型。以上都調用了同一個方法。
在Example 5第3行產生以下編譯錯誤：
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");

這個錯誤發生是因為編譯器無法確定返回值類型，因為String和Integer都有相同的超類Object,注意就算我們修正了第三行，這行代碼在運行仍然會報錯，因為比較了不同的對象。

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中完全兼容運行,我們要人工進行一個: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，當在第3行進行處理時， stringList也就無法保證是String類型的ArrayList,此時編譯器不允許這樣的事出現，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，編譯會報錯，此時就可以用通配符“？”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.創建自己的范型
以下代碼來自http://www.java2s.com/ExampleCode/Language-Basics
1.一個參數的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，所以我們需要進行造型，而Example 10我們不需要任何的造型

2.二個參數的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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