Java 范型攻略篇(2)

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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)。

作者：管斌来源：IT专家网 2008年5月5日

六.创建自己的范型

以下代码来自http://www.java2s.com/ExampleCode/Language-Basics

1.一个参数的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 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.");
}
}

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Java 范型攻略篇(2)

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