How to extend Java generic classes
Description
A generic class can act as a superclass or be a subclass. In a generic hierarchy, any type arguments needed by a generic superclass must be passed up the hierarchy by all subclasses.
Example
Using a Generic Superclass
class MyClass<T> {
T ob;/* w w w. jav a2s.c o m*/
MyClass(T o) {
ob = o;
}
T getob() {
return ob;
}
}
class MySubclass<T, V> extends MyClass<T> {
V ob2;
MySubclass(T o, V o2) {
super(o);
ob2 = o2;
}
V getob2() {
return ob2;
}
}
public class Main {
public static void main(String args[]) {
MySubclass<String, Integer> x = new MySubclass<String, Integer>("Value is: ", 99);
System.out.print(x.getob());
System.out.println(x.getob2());
}
}
The code above generates the following result.
Example 2
It is perfectly acceptable for a non-generic class to be the superclass of a generic subclass.
class MyClass {//w w w . j a v a 2s .c om
int num;
MyClass(int i) {
num = i;
}
int getnum() {
return num;
}
}
class MySubclass<T> extends MyClass {
T ob;
MySubclass(T o, int i) {
super(i);
ob = o;
}
T getob() {
return ob;
}
}
public class Main {
public static void main(String args[]) {
MySubclass<String> w = new MySubclass<String>("Hello", 4);
System.out.print(w.getob() + " ");
System.out.println(w.getnum());
}
}
The code above generates the following result.
Example 3
The instanceof operator can be applied to objects of generic classes.
class Gen<T> {
T ob;/* www . j ava 2 s .c o m*/
Gen(T o) {
ob = o;
}
T getob() {
return ob;
}
}
class Gen2<T> extends Gen<T> {
Gen2(T o) {
super(o);
}
}
public class Main {
public static void main(String args[]) {
Gen<Integer> iOb = new Gen<Integer>(88);
Gen2<Integer> iOb2 = new Gen2<Integer>(99);
Gen2<String> strOb2 = new Gen2<String>("Generics Test");
System.out.println("iOb2 is instance of Gen2"+(iOb2 instanceof Gen2<?>));
System.out.println("iOb2 is instance of Gen"+(iOb2 instanceof Gen<?>));
System.out.println("strOb2 is instance of Gen2"+(strOb2 instanceof Gen2<?>));
System.out.println("strOb2 is instance of Gen"+(strOb2 instanceof Gen<?>));
System.out.println("iOb is instance of Gen2"+(iOb instanceof Gen2<?>));
System.out.println("iOb is instance of Gen"+(iOb instanceof Gen<?>));
}
}
The output:
Example 4
A method in a generic class can be overridden like any other method.
class Gen<T> {
T obj;/*from w ww .j a va 2 s . c o m*/
Gen(T o) {
obj = o;
}
T getob() {
System.out.print("Gen's getob(): ");
return obj;
}
}
class Gen2<T> extends Gen<T> {
Gen2(T o) {
super(o);
}
T getob() {
System.out.print("Gen2's getob(): ");
return obj;
}
}
public class Main {
public static void main(String args[]) {
Gen<Integer> iOb = new Gen<Integer>(88);
Gen2<String> strOb2 = new Gen2<String>("Generics Test");
System.out.println(iOb.getob());
System.out.println(strOb2.getob());
}
}
The code above generates the following result.
