java中泛型理解入门例子

public class GenericTest {

	public static void main(String[] args) throws Exception {
		//jdk 1.5 之前
		ArrayList collection1 = new ArrayList();
		collection1.add(1);
		collection1.add(1L);
		collection1.add("abc");
		//int i = (int) collection1.get(1);
		
		//jdk 1.5以后
		
		ArrayList<String> collection2 = new ArrayList<String>();
		//collection2.add(1);
		//collection2.add(1L);
		collection2.add("abc");
		String  str = collection2.get(0);
		
		Constructor<String> constructor = String.class.getConstructor(String.class);
		String retVal = constructor.newInstance("abc");
		System.out.println(retVal);
		
		
		
		//ArrayList type of Integer
		ArrayList<Integer> collection3 = new ArrayList<Integer>();
		
		/**
		 * 泛型只是给编译器看的，内存中并没有泛型的概念
		 */
		System.out.println(collection2.getClass() == collection3.getClass());
		
		//collection3.add("abc");  //编译器报错
		
		//这行代码可以用作泛型的理解 
		collection3.getClass().getMethod("add", Object.class).invoke(collection3, "abc");
		
		System.out.println(collection3.get(0));
		
		
	}
}