Generic Subtyping - Method Call with Subtype

[Salman Riaz]

Respected Gurus!

I am new to generics and cannot figure out the difference in the two implementations shown in the methods below:

GenericMethodSubtyping.someMethod1(l1);
System.out.println(l.get(counter));// why i cannot call getSValue() method?


GenericMethodSubtyping.someMethod(l2);
System.out.println(l.get(counter).getSValue()); // Why i can call getSValue() here

i know
l2 = l1; //compile time error
l3 = l1; // compile time error
=======================================================

package scjp.ch07;
 
import java.util.List;
import java.util.ArrayList;
 
class A{
private String x;
public A(String x) {
this.x=x;
}
public String getSValue () {
return x;
}
public String toString() {
return x;
}
}
class B extends A{
private String xxVal;
public B(String xx) { 
super("B Class");
this.xxVal = xx;
}
public String getSValue() {return xxVal;}
public String toString() {
return xxVal;
}
}
class C extends A {
private String xVal;
public C(String x) {
super("C Class");
this.xVal = x;
}
public String getSValue() {return xVal;}
public String toString() {
return xVal;
}
}
public class GenericMethodSubtyping {

public static void main(String[] args) {
List<A> l1 = new ArrayList<A>();
l1.add(new A("A1"));
l1.add(new A("A2"));
l1.add(new A("A3"));

List<B> l2 = new ArrayList<B>();
l2.add(new B("B1"));
l2.add(new B("B2"));
l2.add(new B("B3"));

List<C> l3 = new ArrayList<C>();
l3.add(new C("C1"));
l3.add(new C("C2"));
l3.add(new C("C3"));

GenericMethodSubtyping.someMethod1(l1);
GenericMethodSubtyping.someMethod(l2);

}

public static <A> void someMethod1(List<A> l){
int counter = 0;
for (A a : l){
System.out.println(l.get(counter));
counter++;
}
}

public static <T extends A> void someMethod(List<T> l){
int counter = 0;
for (T a : l){
System.out.println(l.get(counter).getSValue());
counter++;
}
}
}

=====================================================
OUTPUT:
A1
A2
A3
B1
B2
B3

*****************************************************

please explain!

thanks
salman

[BPSouther: Added code tags]
[ April 03, 2008: Message edited by: Ben Souther ]

[Ashok Kumar Babu]

Hi Salman,

1.

GenericMethodSubtyping.someMethod1(l1);
System.out.println(l.get(counter));// why i cannot call getSValue() method?

Let us check this condition first.

public static <A> void someMethod1(List<A> l){}

In the above declaration <A> does not represent Class A, so only the method getSValue is not available. <A> is just to indicate the generics - any instance, again the <A> before the void gives the explanation about it. To understand it clearly, the above statement is very similiar to the below.

public static <B> void someMethod1(List<B> l){
public static <T> void someMethod1(List<T> l){
public static <X> void someMethod1(List<X> l){

So whats the way to say only Instance A.

public static void someMethod1(List<A> l){

See the missing before the void. this above declaration clearly says only instance of class A. With this declaration you can get getSValue() method.

2.

public static <T extends A> void someMethod(List<T> l){}

This is similar to Somemethod1 but while describing the generics it clearly says any class which is a sub class of Class A. So obviously objects in the list can be cast to Class B automatically, hence the getSValue method is available.


If anything is wrong in my explanation. please mark it out.

[Salman Riaz]

Ashok,

Many Thanks DUDE!
that explains...

Salman