Java Tutorial/Generics/Bounded Types

Bounded Types

The type argument for T must be either Number, or a class derived from Number.

   <source lang="java">

class Stats<T extends Number> {

 T[] nums;
  
 Stats(T[] o) { 
   nums = o; 
 } 

 double average() { 
   double sum = 0.0;
   for(int i=0; i < nums.length; i++) 
     sum += nums[i].doubleValue();
   return sum / nums.length;
 } 

}

public class MainClass {

 public static void main(String args[]) { 
   Integer inums[] = { 1, 2, 3, 4, 5 };
   Stats<Integer> iob = new Stats<Integer>(inums);
   double v = iob.average();
   System.out.println("iob average is " + v);
   Double dnums[] = { 1.1, 2.2, 3.3, 4.4, 5.5 };
   Stats<Double> dob = new Stats<Double>(dnums);
   double w = dob.average();
   System.out.println("dob average is " + w);
 } 

}</source>

iob average is 3.0
dob average is 3.3

Bounded Wildcards

   <source lang="java">

class Two {

 int x, y;
 Two(int a, int b) {
   x = a;
   y = b;
 }

} class Three extends Two {

 int z;
 Three(int a, int b, int c) {
   super(a, b);
   z = c;
 }

} class Four extends Three {

 int t;
 Four(int a, int b, int c, int d) {
   super(a, b, c);
   t = d;
 }

} class Gen<T extends Two> {

 T[] coords;
 Gen(T[] o) {
   coords = o;
 }

} // Demonstrate a bounded wildcard. public class MainClass {

 static void showTwo(Gen<?> c) {
   System.out.println("X Y Coordinates:");
   for (int i = 0; i < c.coords.length; i++)
     System.out.println(c.coords[i].x + " " + c.coords[i].y);
   System.out.println();
 }
 static void showThree(Gen<? extends Three> c) {
   System.out.println("X Y Z Coordinates:");
   for (int i = 0; i < c.coords.length; i++)
     System.out.println(c.coords[i].x + " " + c.coords[i].y + " " + c.coords[i].z);
   System.out.println();
 }
 static void showAll(Gen<? extends Four> c) {
   System.out.println("X Y Z T Coordinates:");
   for (int i = 0; i < c.coords.length; i++)
     System.out.println(c.coords[i].x + " " + c.coords[i].y + " " + c.coords[i].z + " "
         + c.coords[i].t);
   System.out.println();
 }
 public static void main(String args[]) {
   Two td[] = { new Two(0, 0), new Two(7, 9), new Two(18, 4), new Two(-1, -23) };
   Gen<Two> tdlocs = new Gen<Two>(td);
   System.out.println("Contents of tdlocs.");
   showTwo(tdlocs); // OK, is a TwoD
   Four fd[] = { new Four(1, 2, 3, 4), new Four(6, 8, 14, 8), new Four(22, 9, 4, 9),
       new Four(3, -2, -23, 17) };
   Gen<Four> fdlocs = new Gen<Four>(fd);
   System.out.println("Contents of fdlocs.");
   // These are all OK.
   showTwo(fdlocs);
   showThree(fdlocs);
   showAll(fdlocs);
 }

}</source>

Contents of tdlocs.
X Y Coordinates:
0 0
7 9
18 4
-1 -23
Contents of fdlocs.
X Y Coordinates:
1 2
6 8
22 9
3 -2
X Y Z Coordinates:
1 2 3
6 8 14
22 9 4
3 -2 -23
X Y Z T Coordinates:
1 2 3 4
6 8 14 8
22 9 4 9
3 -2 -23 17

Upper/lower bound for a wildcard

Establishing an upper bound for a wildcard

   <source lang="java">

<? extends superclass></source>

Using Wildcard Arguments

   <source lang="java">

// Use a wildcard. class GenericStats<T extends Number> {

 T[] nums;
   
 GenericStats(T[] o) {
   nums = o;
 }
 double average() {
   double sum = 0.0;
    for(int i=0; i < nums.length; i++){
     sum += nums[i].doubleValue();
    }
   return sum / nums.length;
 }
 boolean sameAvg(GenericStats<?> ob) {
   if(average() == ob.average()) 
     return true;
   return false;
 }

}

public class MainClass {

 public static void main(String args[]) {
   Integer inums[] = { 1, 2, 3, 4, 5 }; 
   GenericStats<Integer> iob = new GenericStats<Integer>(inums);
   double v = iob.average();
   System.out.println("iob average is " + v);

   Double dnums[] = { 1.1, 2.2, 3.3, 4.4, 5.5 };
   GenericStats<Double> dob = new GenericStats<Double>(dnums);
   double w = dob.average();
   System.out.println("dob average is " + w); 

   Float fnums[] = { 1.0F, 2.0F, 3.0F, 4.0F, 5.0F }; 
   GenericStats<Float> fob = new GenericStats<Float>(fnums);
   double x = fob.average(); 
   System.out.println("fob average is " + x);

   System.out.print("Averages of iob and dob ");
   if(iob.sameAvg(dob)){
     System.out.println("are the same."); 
   }else{
     System.out.println("differ."); 
   }
   
   System.out.print("Averages of iob and fob ");
   if(iob.sameAvg(fob)){
     System.out.println("are the same."); 
   }else{
     System.out.println("differ.");
   }
 }

}</source>

iob average is 3.0
dob average is 3.3
fob average is 3.0
Averages of iob and dob differ.
Averages of iob and fob are the same.