Java 范型攻略篇3

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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6月3日

5.使用边界通配符

//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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