泛型总结

1 List myIntList=new LinkedList();
3 myIntList.add(newInteger(0));
5 Integer x=(Integer)myIntList.iterator().next();

1 List<Integer> myIntList=newLinkedList<Integer>;
3 myIntList.add(newInteger(0));
5 Integer x=myIntList.iterator().next();

 1 public interface List<E>{
 3    void add(E,x);
 5         Iterator<E> iterator();
 7 }
 9 public interface Iterator<E>{
11     E next();
13     boolean hasNext();
15 }

1 public interface List<Integer>{
2     void add(Integer x);
3         terator<Integer> iterator();
4     }  

1 /*会导致编译时出错*/
2 public class Erasure{
3     public void test(List<String> ls){
4         System.out.println("String");
5     }
6     public void test(List<Integer> ls){
7         System.out.println("Integer");
8     }
9 }    

1 List<Apple> apples=new ArrayList<Apple>();
2 List<Fruit> fruits=apples;

1 //不使用泛型
2 void printCollection(Collection c){
3     Iterator i=c.iterator();
4         for(k=0;k<c.size();k++){
5         System.out.println(i.next());
6     }
7 }    

1 //使用泛型
2 void printCollection(Collection c)<Object> c{
3     for(Object e:c){
4         System.out.println(e);
5     }
6 }

1 void printCollection(Collection c)<？> c{
2     for(Object e:c){
3         System.out.println(e);
4     }
5 }    

1 Collection<?> c=new ArrayList<String>();
2 c.add(newObject());//编译出错，不管加入什么对象都出错，除了null外。
3 c.add(null);//OK!

 1 public abstract class Shape{
 2     public abstract void draw(Canvas c);
 3 }
 4 public class Circle extends Shape{
 5     private int x,y;
 6     public void draw(Canvas c){...};
 7 }
 8 public class Rectangle extends Shape{
 9     private int x,y,width,height;
10     public void draw(Canvas c){...}
11 }
12 
13 //原始版本
14 public void drawAll(List<Shape> shapes){
15     for(Shapes :shapes){
16         s.draw(this);
17     }
18 }
19 
20 //使用边界通配符的版本
21 public void drawALL(List<? extends Shape> shapes){
22     for(Shapes :shapes){
23         s.draw(this);
24     }
25 }    

1 List<Shape> shapes=new ArrayList<Shape>();
2 List<? super Cicle> cicleSupers=shapes;
3 circleSupers.add(new Cicle());//OK,subclss of Cicle also OK
4 cicleSupers.add(new Shape());//ERROR

1 static void fromArrayToCollection(Object[]a,Collection<?>c){
2     for(Object o:a){
3         c.add(o);//编译错误
4     }
5 }

1 static<T> void fromArrayToCollection(T[] a,Collection<T>c){
2     for(T O:a){
3         c.add(o);//OK
4     }
5 }

 1 Object[] oa=new Object[100];
 2 Collection<Object>co=new ArrayList<Object>();
 3 fromArrayToCollection(oa,co);//T inferred to be Object
 4 String[] sa=new String[100];
 5 Collection<String>cs=new ArrayList<String>();
 6 fromArrayToCollection(sa,cs);//T inferred to be String
 7 fromArrayToCollection(sa,co);//T inferred to be Object
 8 Integer[] ia=new Integer[100];
 9 Float[] fa=new Float[100];
10 Number[] na=new Number[100];
11 Collection<Number>cn=new ArrayList<Number>();
12 fromArrayToCollection(ia,cn);//T inferred to be Number
13 fromArrayToCollection(fa,cn);//T inferred to be Number
14 fromArrayToCollection(na,cn);//T inferred to be Number
15 fromArrayToCollection(na,co);//T inferred to be Object
16 fromArrayToCollection(na,cs);//编译错误

 1 public <T> void go(T t){
 2     System.out.println("generic function");
 3 }
 4 public void go(String str){
 5     System.out.println("normal function");
 6 }
 7 public static void main(String[] args){
 8     FuncGenric fg=new FuncGenric();
 9     fg.go("haha");//打印normal function
10     fg.<String>go("haha");//打印normal function,String是多余的，并且不报错。
11     fg.go(new Object());//打印generic function
12     fg.<Object>go(new Object());//打印generic function
13 }

1 //代码一：编译出错
2 public class Erasure{
3     public void test(int i){
4         System.out.printlnl("String");
5     }
6     public void test(int i){
7         System.out.println("Integer");
8     }
9 }

1 //代码二：正确
2 public class Erasure{
3     public void test(List<String>ls){
4         System.out.println("String");
5     }    
6     public void test(List<Integer>li){
7         System.out.println("Integer");
8     }
9 }    

1 Collection cs=new CollectionList<String>();
2 if (cs instanceof Collection<String>){....}//编译出错，如果改成instanceof Collection<?>则正确。

1 List<String>[] lsa=new ArrayList<String>[10];//错误。因为如果可以这样，那么考虑如下代码，会导致运行时错误。
2 List<String>[] lsa=new ArrayList<String>[10];//实际上并不允许这样创建数组
3 Object 0=lsa;
4 Object[] oa=(Object[])o;
5 List<Integer>li=new ArrayList<Integer>();
6 li.add(new Integer(3));
7 oa[1]=li;//unsound,but passes run time store check
8 String s=lsa[1].get[0];//run-time error-classCastException

1 List<?>[] lsa=new List<?>[10];
2 Object 0=lsa;
3 Object[] oa=(Object[])o;
4 List<Integer>li=new ArrayList<Integer>();
5 li.add(new Integer(3));
6 oa[1]=li;/OK
7 String s=lsa[1].get(0);//run-time error,but cast is explicit
8 Integer it=(Integer)lsa[1].get(0);//OK