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2010-06-01T05:20:50Z

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в 17:44, 31 мая 2010

2010-05-31T17:44:27Z

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== 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>



<pre class=codeResult>Hello 47</pre>

== 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>



<pre class=codeResult>Value is: 99</pre>

== 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>



<pre class=codeResult>Gen"s getObject():
Gen2"s getObject():</pre>

== 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>



<pre class=codeResult>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</pre>

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Admin: 1 версия

в 17:44, 31 мая 2010