原文位址:http://www.matrix.org.cn/resource/article/44/44344_Java+Generics
一.什麼是Generics?
Generics可以稱之為參數類型(parameterized types),由編譯器來驗證從用戶端将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,我們先來看一看目前的java 集合架構(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 } |
以上的樣例程式包含的兩個方法,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,我們疊代這個ArrayList用String進行Cast。我們看第二個方法,就出現了一個問題,Collection在内部用的是Object,而我們要取出Collection中的對象時,需要進行Cast,那麼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作抛出ClassCastException的危險。我們看inspecCollection方法,編譯時沒有問題,但在運作時就會抛出ClassCastException異常。是以我們一定要遠離這個重大的運作時錯誤
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,下面我們使用範型修改上一章的樣例程式。
//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行我們在建立ArrayList時使用了新文法,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,它隻是在定義ArrayList時的類型占位符,在Example 2中的我們在定義ArrayList的執行個體時用String綁定在E上,當我們用add(E element)方法向ArrayList中增加對象時, 那麼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傳回值。這時我們在看Example 2中的第四行,編譯就會反映出編譯錯誤。
是以在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.内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.類型的比對錯誤在編譯階段就可以捕捉到,而不是在代碼運作時。
受限制類型變量
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和實作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。方法的參數的傳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 } |
這裡,max方法的參數類型為單一的T類型,而T類型繼承了Comparable,max的參數和傳回值都有相同的超類。下面的Example 5顯示了max方法的幾個限制。
//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,是以傳回值也是Integer,注意傳回值沒有進行造型。
在Example 5第2行參數都為String,是以傳回值也是String,注意傳回值沒有進行造型。以上都調用了同一個方法。
在Example 5第3行産生以下編譯錯誤:
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确定傳回值類型,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,這行代碼在運作仍然會報錯,因為比較了不同的對象。
四.向下相容
任何一個新的特色在新的JDK版本中出來後,我們首先關心的是如何于以前編寫的代碼相容。也就是說我們編寫的Example 1程式不需要任何的改變就可以運作,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中完全相容運作,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是正确的。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,是以第3行将無法編譯。
//Example 7
void printCollection(Collection<Object> c) { for (Object e : c) { System.out.println(e); }} |
Example 7的本意是列印所有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類型就可以友善的列印了
有界通配符 <T extends Number>(上界) <T super Number>(下界)
六.建立自己的範型
以下代碼來自http://www.java2s.com/ExampleCode/Language-Basics
1.一個參數的Generics
//Example 9(沒有使用範型) 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(使用範型) 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 { 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沒有使用範型,是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 { 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 { 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 { 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(); } } } |