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2010-06-01T07:27:27Z

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в 18:01, 31 мая 2010

2010-05-31T18:01:50Z

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== Demonstrate a simple generic method. ==



<source lang="java">
/*
Java 2, v5.0 (Tiger) New Features
by Herbert Schildt
ISBN: 0072258543
Publisher: McGraw-Hill/Osborne, 2004
*/

public class GenMethDemo {

// Determine if an object is in an array.
static <T, V extends T> boolean isIn(T x, V[] y) {

for(int i=0; i < y.length; i++)
if(x.equals(y[i])) return true;

return false;
}

public static void main(String args[]) {

// Use isIn() on Integers.
Integer nums[] = { 1, 2, 3, 4, 5 };

if(isIn(2, nums))
System.out.println("2 is in nums");

if(!isIn(7, nums))
System.out.println("7 is not in nums");

System.out.println();

// Use isIn() on Strings.
String strs[] = { "one", "two", "three",
"four", "five" };

if(isIn("two", strs))
System.out.println("two is in strs");

if(!isIn("seven", strs))
System.out.println("seven is not in strs");

// Opps! Won"t compile! Types must be compatible.
// if(isIn("two", nums))
// System.out.println("two is in strs");
}
}

</source>



== Java generic: Ambiguity caused by erasure on overloaded methods. ==



<source lang="java">
/*
Java 2, v5.0 (Tiger) New Features
by Herbert Schildt
ISBN: 0072258543
Publisher: McGraw-Hill/Osborne, 2004
*/
class MyGenClass<T, V> {
T ob1;
V ob2;

// ...

// These two overloaded methods are ambiguous.
// and will not compile.
void set(T o) {
ob1 = o;
}

void set(V o) {
ob2 = o;
}
}
// Can"t create an instance of T.
class Gen<T> {
T ob;
Gen() {
ob = new T(); // Illegal!!!
}
}
public class Wrong<T> {
// Wrong, no static variables of type T.
static T ob;

// Wrong, no static method can use T.
static T getob() {
return ob;
}

// Wrong, no static method can access object
// of type T.
static void showob() {
System.out.println(ob);
}
}

</source>



== Java generic: A situation that creates a bridge method. ==



<source lang="java">
/*
Java 2, v5.0 (Tiger) New Features
by Herbert Schildt
ISBN: 0072258543
Publisher: McGraw-Hill/Osborne, 2004
*/
//
class Gen<T> {
T ob; // declare an object of type T

// Pass the constructor a reference to
// an object of type T.
Gen(T o) {
ob = o;
}

// Return ob.
T getob() {
return ob;
}
}

// A subclass of Gen.
class Gen2 extends Gen<String> {

Gen2(String o) {
super(o);
}

// A String-specific override of getob().
String getob() {
System.out.print("You called String getob(): ");
return ob;
}
}

// Demonstrate a situation that requires a bridge method.
public class BridgeDemo {
public static void main(String args[]) {

// Create a Gen2 object for Strings.
Gen2 strOb2 = new Gen2("Generics Test");

System.out.println(strOb2.getob());
}
}

</source>



== Overriding a generic method in a generic class. ==



<source lang="java">
/*
Java 2, v5.0 (Tiger) New Features
by Herbert Schildt
ISBN: 0072258543
Publisher: McGraw-Hill/Osborne, 2004
*/
class Gen<T> {
T ob; // declare an object of type T

// Pass the constructor a reference to
// an object of type T.
Gen(T o) {
ob = o;
}

// Return ob.
T getob() {
System.out.print("Gen"s getob(): " );
return ob;
}
}

// A subclass of Gen that overrides getob().
class Gen2<T> extends Gen<T> {

Gen2(T o) {
super(o);
}

// Override getob().
T getob() {
System.out.print("Gen2"s getob(): ");
return ob;
}
}

// Demonstrate generic method override.
public class OverrideDemo {
public static void main(String args[]) {

// Create a Gen object for Integers.
Gen<Integer> iOb = new Gen<Integer>(88);

// Create a Gen2 object for Integers.
Gen2<Integer> iOb2 = new Gen2<Integer>(99);

// Create a Gen2 object for Strings.
Gen2<String> strOb2 = new Gen2<String>("Generics Test");

System.out.println(iOb.getob());
System.out.println(iOb2.getob());
System.out.println(strOb2.getob());
}
}

</source>



== Return generic value from method ==



<source lang="java">
import java.io.Serializable;
class Base {
}
class Sub1 extends Base implements Serializable {
public void run() {
}
}
class Sub2 extends Base implements Serializable {
public void run() {
}
}
public class TypeInference {
static <T extends Base> T infer(T t1, T t2) {
return null;
}
public static void main(String[] args) {
Base base = infer(new Sub1(), new Sub2());
Serializable runnable = infer(new Sub1(), new Sub2());
}
}
</source>

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Admin: 1 версия

в 18:01, 31 мая 2010