//--------A.java file---------//
pubilc class A{
  private String str;
  public setStr(String str){
    this.str = str;
  }
  public A( ){}
  public A(A a){
    str = a.str; //why can i use a.str??private member should be used in the 
                     //obj itself rather than another obj!!But why.....
    a.str = "fff";//It's dangerous! No protection at all
  }
  public void setStr(String str){
    this.str = str;
  }
  public String getStr( ){
    return str;
  }
}

//--------B.java file--------//
public class B{
  A a1 = new A( );
  a1.setStr("Hello");
  A a2 = new A(a1); //it's all right?!
  System.out.println("a1 is " + a1.getStr());
  System.out.println("a2 is " + a2.getStr());
}

//---------output ---------//
a1 is fff

a2 is Hello