Java Tutorial/Generics/Generic Class Hierarchies
Содержание
- 1 A Generic Subclass: a nongeneric class can be the superclass of a generic subclass
- 2 A subclass can add its own type parameters, if needed.
- 3 Casting within a generic class hierarchy
- 4 Generic Class Hierarchies: uses a generic superclass
- 5 Overriding Methods in a Generic Class
- 6 Run-Time Type Comparisons Within a Generic Hierarchy
A Generic Subclass: a nongeneric class can be the superclass of a generic subclass
A nongeneric class can be the superclass of a generic subclass.
<source lang="java">
class NonGen {
int num;
NonGen(int i) {
num = i;
}
int getNumber() {
return num;
}
} class Gen<T> extends NonGen {
T ob;
Gen(T o, int i) {
super(i);
ob = o;
}
T getObject() {
return ob;
}
}
public class MainClass {
public static void main(String args[]) {
Gen<String> w = new Gen<String>("Hello", 47);
System.out.print(w.getObject() + " ");
System.out.println(w.getNumber());
}
}</source>
Hello 47
A subclass can add its own type parameters, if needed.
<source lang="java">
class Gen<T> {
T ob;
Gen(T o) {
ob = o;
}
T getob() {
return ob;
}
}
class Gen2<T, V> extends Gen<T> {
V ob2;
Gen2(T o, V o2) {
super(o);
ob2 = o2;
}
V getob2() {
return ob2;
}
} public class MainClass {
public static void main(String args[]) {
Gen2<String, Integer> x = new Gen2<String, Integer>("Value is: ", 99);
System.out.print(x.getob());
System.out.println(x.getob2());
}
}</source>
Value is: 99
Casting within a generic class hierarchy
You can cast one instance of a generic class into another only if the two are otherwise compatible and their type arguments are the same.
<source lang="java">
class Gen<T> {
T ob;
Gen(T o) {
ob = o;
}
T getObject() {
return ob;
}
}
class Gen2<T> extends Gen<T> {
Gen2(T o) {
super(o);
}
}
public class MainClass {
public static void main(String args[]) {
Gen<Integer> intObject = new Gen<Integer>(88);
Gen2<Long> longObject = new Gen2<Long>(99L);
//longObject = (Gen2<Long>)intObject;
}
}</source>
Generic Class Hierarchies: uses a generic superclass
In a generic hierarchy, any type arguments needed by a generic superclass must be passed up the hierarchy by all subclasses.
<source lang="java">
class Gen<T> {
T ob;
Gen(T o) {
ob = o;
}
T getob() {
return ob;
}
} class Gen2<T> extends Gen<T> {
Gen2(T o) {
super(o);
}
}</source>
Overriding Methods in a Generic Class
<source lang="java">
class Gen<T> {
T ob;
Gen(T o) {
ob = o;
}
T getObject() {
System.out.println("Gen"s getObject(): " );
return ob;
}
} class Gen2<T> extends Gen<T> {
Gen2(T o) {
super(o);
}
T getObject() {
System.out.println("Gen2"s getObject(): ");
return ob;
}
} public class MainClass{
public static void main(String[] arg){
Gen<Integer> intObject = new Gen<Integer>(88);
Gen2<Long> longObject = new Gen2<Long>(99L);
intObject.getObject();
longObject.getObject();
}
}</source>
Gen"s getObject(): Gen2"s getObject():
Run-Time Type Comparisons Within a Generic Hierarchy
The run-time type information operator instanceof: determines if an object is an instance of a class. The instanceof operator can be applied to objects of generic classes.
<source lang="java">
class Gen<T> {
T ob;
Gen(T o) {
ob = o;
}
T getObject() {
return ob;
}
}
class Gen2<T> extends Gen<T> {
Gen2(T o) {
super(o);
}
}
public class MainClass {
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");
if(iOb2 instanceof Gen2<?>){
System.out.println("iOb2 is instance of Gen2");
}
if(iOb2 instanceof Gen<?>){
System.out.println("iOb2 is instance of Gen");
}
if(strOb2 instanceof Gen2<?>){
System.out.println("strOb is instance of Gen2");
}
if(strOb2 instanceof Gen<?>){
System.out.println("strOb is instance of Gen");
}
if(iOb instanceof Gen2<?>){
System.out.println("iOb is instance of Gen2");
}
if(iOb instanceof Gen<?>){
System.out.println("iOb is instance of Gen");
}
// The following can"t be compiled because
// generic type info does not exist at runtime.
// if(iOb2 instanceof Gen2<Integer>) // System.out.println("iOb2 is instance of Gen2<Integer>");
}
}</source>
iOb2 is instance of Gen2 iOb2 is instance of Gen strOb is instance of Gen2 strOb is instance of Gen iOb is instance of Gen
