Vector<A> a= new Vector<A>();
Vector<A> b= a.clone(); //1.
Vector<A> b=(Vector) a.clone();//2.
Vector<A> b=(Vector<A>) a. clone(); //3

i have tried all the three ways of casting, but all of them will produce either an error or a warning. Can anyone suggest how to do casting in this case? thanks a lot in advance!

Vector[]?

[atp@gateway ~]$ cat aaa.java
import java.util.Vector;

public class aaa
{
　　public static void main (String [] s)
　　{
　　　　Vector<String> a = new Vector<String>();
　　　　a.add("hi");
　　　　Vector<String> b = (Vector<String>)a.clone();
　　　　System.out.println(b);
　　}
}
[atp@gateway ~]$ javac aaa.java
Note: aaa.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
[atp@gateway ~]$ java aaa
[hi]
[atp@gateway ~]$　

try this

Vector<String> b = new Vector<String>(a);

Vector<String> b = new Vector<String>(a); // this is shallow clone
Vector<String> b = (Vector<String>)a.clone(); // this is deep clone

还是有
Note: aaa.java uses unchecked or unsafe operations
难道就没有办法可以避免吗？

Shallow clone: if original object's value is changed, and the copy of that object also changes.

this warning message just shows that compiler cannot assure correctness of this casting operation as clone() method returns type of object.

It is really programmer's responsibility to avoid any casting failure.

in this case, a shallow clone will make an extra copy of Vector object. ( only container object is copied)

however, a deep clone will create memory for Vector object and all String objects inside this Vector. ( container and contained objects are all duplicated)