Java/Generics/Generic Method

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>