玻璃杯中的花生壳--Java 范型攻略篇

Java 范型攻略篇

[ 2006/12/9 15:15:39 | By: 玻璃杯中的花生壳 ]

　 Java 范型攻略篇摘要:在已发布的Java1.4中在核心代码库中增加了许多新的API(如Loging,正则表达式，NIO)等，在最新发布的JDK1.5和即将发布的JDK1.6中也新增了许多API，其中比较有重大意义的就是Generics(范型）。一.什么是Generics?Generics可以称之为参数类型(parameterized types),由编译器来验证从客户端将一种类型传送给某一对象的机制。如Java.util.ArrayList,编译器可以用Generics来保证类型安全。在我们深入了解Generics之前,我们先来看一看当前的java 集合框架（Collection)。在j2SE1.4中所有集合的Root Interface是CollectionCollections 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 ClassCastException5 inspectCollection(list);6 }789 protected void inspectCollection(Collection aCollection) {10 Iterator i = aCollection.iterator();11 while (i.hasNext()) {12 String element = (String) i.next();13 }14 }以上的样例程序包含的两个方法，collectionExample方法建立了一个简单的集合类型ArrayList，并在ArrayList中增加了一个String和一个Integer对象.而在inspecCollection方法中，我们迭代这个ArrayList用String进行Cast。我们看第二个方法，就出现了一个问题,Collection在内部用的是Object，而我们要取出Collection中的对象时,需要进行Cast，那么开发者必需用实际的类型进行Cast,像这种向下造型，编译器无法进行检查,如此一来我们就要冒在代码在运行抛出ClassCastException的危险。我们看inspecCollection方法，编译时没有问题，但在运行时就会抛出ClassCastException异常。所以我们一定要远离这个重大的运行时错误二.使用Generics从上一章节中的CassCastException这种异常，我们期望在代码编译时就能够捕捉到,下面我们使用范型修改上一章的样例程序。//Example 21 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 compiles5 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 }从上面第2行我们在创建ArrayList时使用了新语法，在JDK1.5中所有的Collection都加入了Generics的声明。例://Example 31 public class ArrayList<E> extends AbstractList<E> {2 // details omitted...3 public void add(E element) {4 // details omitted5 }6 public Iterator<E> iterator() {7 // details omitted8 }9 }这个E是一个类型变量，并没有对它进行具体类型的定义,它只是在定义ArrayList时的类型占位符,在Example 2中的我们在定义ArrayList的实例时用String绑定在E上,当我们用add(E element)方法向ArrayList中增加对象时, 那么就像下面的写法一样： public void add(String element)；因为在ArrayList所有方法都会用String来替代E,无论是方法的参数还是返回值。这时我们在看Example 2中的第四行，编译就会反映出编译错误。所以在java中增加Generics主要的目的是为了增加类型安全。通过上面的简单的例子我们看到使用Generics的好处有：1.在类型没有变化时，Collection是类型安全的。2.内在的类型转换优于在外部的人工造型。3.使Java 接口更加强壮,因为它增加了类型。4.类型的匹配错误在编译阶段就可以捕捉到，而不是在代码运行时。受约束类型变量虽然许多Class被设计成Generics，但类型变量可以是受限的public class C1<T extends Number> { }public class C2<T extends Person & Comparable> { } 第一个T变量必须继承Number，第二个T必须继承Person和实现Comparable三.Generics 方法像Generics类一样，方法和构造函数也可以有类型参数。方法的参数的返回值都可以有类型参数，进行Generics。//Example 41 public <T extends Comparable> T max(T t1, T t2) {2 if (t1.compareTo(t2) > 0)3 return t1;4 else return t2;5 }这里，max方法的参数类型为单一的T类型，而T类型继承了Comparable，max的参数和返回值都有相同的超类。下面的Example 5显示了max方法的几个约束。//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行参数都为Integer，所以返回值也是Integer，注意返回值没有进行造型。在Example 5第2行参数都为String，所以返回值也是String，注意返回值没有进行造型。以上都调用了同一个方法。在Example 5第3行产生以下编译错误：Example.java:10: incompatible typesfound : java.lang.Object&java.io.Serializable&java.lang.Comparable<?>required: java.lang.Number Number nresult = max(new Integer(100), "AAA");这个错误发生是因为编译器无法确定返回值类型，因为String和Integer都有相同的超类Object,注意就算我们修正了第三行，这行代码在运行仍然会报错，因为比较了不同的对象。四.向下兼容任何一个新的特色在新的JDK版本中出来后，我们首先关心的是如何于以前编写的代码兼容。也就是说我们编写的Example 1程序不需要任何的改变就可以运行,但是编译器会给出一个"ROW TYPE"的警告。在JDK1.4中编写的代码如何在JVM1.5中完全兼容运行,我们要人工进行一个:Type erasure处理过程五.通配符//Example 6List<String> stringList = new ArrayList<String>(); //1List<Object> objectList = stringList ;//2objectList .add(new Object()); // 3String s = stringList .get(0);//4乍一看，Example 6是正确的。但stringList本意是存放String类型的ArrayList,而objectList中可以存入任何对象，当在第3行进行处理时，stringList也就无法保证是String类型的ArrayList,此时编译器不允许这样的事出现，所以第3行将无法编译。//Example 7void printCollection(Collection<Object> c) { for (Object e : c) { System.out.println(e);}}Example 7的本意是打印所有Collection的对象,但是正如Example 6所说的，编译会报错，此时就可以用通配符“？”来修改Example 7//Example 8void printCollection(Collection<?> c) { for (Object e : c) { System.out.println(e);}}Example 8中所有Collection类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.创建自己的范型以下代码来自http://www.java2s.com/ExampleCode/Language-Basics1.一个参数的Generics//Example 9(没有使用范型)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("对象的类型是："+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("对象是："+examplei.getOb()); Example1<String> examples=new Example1<String>("Bill"); examples.ShowObject(); System.out.println("对象是："+examples.getOb()); }}我们来看Example 9没有使用范型，所以我们需要进行造型，而Example 10我们不需要任何的造型2.二个参数的Generics//Example 11class 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 12class 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 14class 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.使用边界通配符//Example 15class 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 16public 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 17public 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 18interface 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 20interface 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(); } }} package test;public class genericusage { /** * 测试范型编程。 * * @param <t> * @param e * @return * @throws instantiationexception * @throws illegalaccessexception */ public <t> t getobject(class<? extends t> e) throws instantiationexception, illegalaccessexception { return e.newinstance(); }}package test;import junit.framework.testcase;public class genericusagetest extends testcase { genericusage gu = new genericusage(); public void testgetobject() throws instantiationexception, illegalaccessexception { string str = gu.getobject(string.class); assertnotnull(str); }}package test;public class genericusage { /** * 测试范型编程。 * * @param <t> * @param e * @return * @throws instantiationexception * @throws illegalaccessexception */ public <t> t getobject(class<? extends t> e) throws instantiationexception, illegalaccessexception { return e.newinstance(); }}package test;import junit.framework.testcase;public class genericusagetest extends testcase { genericusage gu = new genericusage(); public void testgetobject() throws instantiationexception, illegalaccessexception { string str = gu.getobject(string.class); assertnotnull(str); }}

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