Other than java.lang, every other package you use must be named in an import statement : package « Java Source And Data Type « SCJP

Or you must use the fully qualified name of the class every time it is mentioned. 

import java.util.Vector;

public class MainClass {
  public static void main(String[] argv) {
    System.out.println(new Vector());
    System.out.println(new java.util.ArrayList());
  }
}

1.3.package
	1.3.1.	The package name is a series of elements separated by periods.
	1.3.2.	Packages are identified by the package statement.
	1.3.3.	Use only alphanumeric characters in package names.
	1.3.4.	java.lang package is imported by default and does not need to be imported by an import statement.
	1.3.5.	Other than java.lang, every other package you use must be named in an import statement