Java中關於泛型擦除的小結

java由於擦除,在使用泛型的時候一些操作成了程序存在異常的可能 ,這個類中展示了一些容易出現的問題,下面的小例子做了一個簡單的總結

1. class Erased<T> {
3. public static void f(Object arg) {
4. // if (arg instanceof T) {
5. // } // Error
6. // T var = new T(); // Error
7. // T[] array = new T[SIZE]; // Error
8. // T[] array = (T) new Object[SIZE]; // Unchecked warning
9. }
10. }

下面的程序算是對擦除特性的一個彌補

1. /*casionally you can program around these issues, but sometimes you must compensate for erasure by introducing a type
2. * tag . This means you explicitly pass in the Class object for your type so that you can use it in type expressions.
3. * For example, the attempt to use instanceof in the previous program fails because the type information has been
4. * erased. If you introduce a type tag, a dynamic islnstance( ) can be used instead:
5. */
7. public class CompensatErasure<T> {
8. public static void main(String[] args) {
9. CompensatErasure<Building> ctt1 = new CompensatErasure<Building>(Building.class);
10. ctt1.f(new Building());
11. ctt1.f(new House());
12. CompensatErasure<House> ctt2 = new CompensatErasure<House>(House.class);
13. ctt2.f(new Building());
14. ctt2.f(new House());
15. }
17. private Class<T> type;
19. public CompensatErasure(Class<T> type) {
20. this.type = type;
21. }
23. /**
24. * @return the type
25. */
26. public Class<T> getType() {
27. return type;
28. }
30. /**
31. * @param type
32. * the type to set
33. */
34. public void setType(Class<T> type) {
35. this.type = type;
36. }
38. @SuppressWarnings("unchecked")
39. public void f(Object obj) {
40. // Compensate instance of
41. if (type.isInstance(obj)) {
42. System.out.println(obj + " is instance of " + type.getSimpleName());
43. } else {
44. System.out.println(obj + " doesn't instance of " + type.getSimpleName());
45. }
46. try {
47. // Compensate new T();
48. System.out.println(type.newInstance());
49. // compensate new T[]{};
50. Class<T>[] types = new Class[1];
51. } catch (InstantiationException e) {
52. e.printStackTrace();
53. } catch (IllegalAccessException e) {
54. e.printStackTrace();
55. }
56. }
58. }
60. class Building {
61. @Override
62. public String toString(){
63. return "Building";
64. }
65. }
67. class House extends Building {
68. @Override
69. public String toString(){
70. return "House";
71. }
72. }