some questions from generics

[umesh rawat]

[CODE]<code>
class Box<T>
{
private T theObject;
public Box(T arg)
{ theObject = arg; }
public Box() { }
}
class Test
{
public static void main(String[] args)
{
Box<String> b1 = new Box<String>("Java"); //line 1
Object<String> b2 = new Box<String>("Struts"); //line 2
// line 10 -why it  wont compile 
Box<String> b3 = new Box("EJB"); //line 3
Box b4 = new Box<String>("Ajax"); //line 4
Object b5 = new Box<Object>(); //line 5
//Object b5 = new Box<Object>("hello object "); 
// not compiling  compiling 
}
}
 
// what is the problem in line 10 pls explain totally 
///////////////////////////////////////////////// en 
//</code>
program 2
<code>
import java.util.*;
public class HashTest{
int hashCode;
public HashTest(int hashCode){
this.hashCode = hashCode;
}
public boolean equals(Object other){
if(other instanceof HashTest){
return ((HashTest)other).hashCode == this.hashCode;
}
return false;
}
public static void main(String [] args) {
HashTest ob1 = new HashTest(3);
HashTest ob2 = new HashTest(2);
HashTest ob3 = new HashTest(1);
Map mp = new HashMap();
mp.put(ob1,"one");
mp.put(ob2,"two");
mp.put(ob3,"three");
 
ob1 = new HashTest(6);
System.out.println(mp.get(ob1));
}
}
 
// output - why the outpur is null
 

 
//program 3
 
import java.util.*;
class  HashGenerics{
public static void main(String ar[]){
 
HashSet<String> hs = new HashSet<String>();
hs.add("scjp");
hs.add("exam");
HashSet hs1 = new HashSet();
 
HashSet<Object> s = new HashSet<Object>();
 //s=hs;// wont compile 
 hs1=hs;// 
s=hs1;//possible 
//s=hs1;//possible 
//hs1=s;// it is also right 
//hs1=hs;// it is also right 
System.out.println(hs1);
}
}
 
// i have checked but not getting the logic 
 
program 4
 
class Dummy{}
class Dum extends Dummy{}
class Bum extends Dummy{}
class Gum<E> extends Dummy {
E value;
void setValue(E v)
{value =v;}
 
E getValue(){return value;}
 
public static void main(String [] args) {
 
Gum b = new Gum();
 
//b.setValue(new Object());
Gum<Dum> d = b;
Gum<Bum> e = b;
// why these two line are compiling 
Dum h = (Dum) d.getValue();
}
}
 program 5
 

 
import java.util.*;
class Gendemo3 {
public static void main(String ar[]){
List<Integer> ls1 = new LinkedList(); //line 1
List<String> ls2 = new LinkedList(); //line 2
List ls3 = new LinkedList(); //line 3 no problem
List ls4 =ls1; //line 4  no problem
List ls5 =ls2; //line 5   no problem 
//  ls5 =ls4; this is right 
//  ls4 =ls5; this is right 
//List ls6=ls5 =ls4; //line 6
 

// but directly 
//ls1=ls2; not  possible 
// why 
 
}
}
 

 
Dear friend 
please give me the suggestions for these questions 
 

 

 

 

</code>

[/CODE]

[Jesper de Jong]

https://coderanch.com/t/270622/java-programmer-SCJP/certification/questions-generics