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  1. import java.lang.*;
  2. import java.io.*;
  3.  
  4. class JavaTypeClasses {
  5. 	// class Num t where ...
  6. 	public interface Num<T> {
  7. 		T add(T a, T b);
  8. 		T mul(T a, T b);
  9. 	}
  10.  
  11. 	// instance Num Int where...
  12. 	public static final class NumInt implements Num<Integer>{
  13. 		static final NumInt typeClass = new NumInt();
  14.  
  15. 		public Integer add(Integer a, Integer b) {
  16. 			return a + b;
  17. 		}
  18.  
  19. 		public Integer mul(Integer a, Integer b) {
  20. 			return a * b;
  21. 		}
  22. 	}
  23.  
  24. 	// instance Num Double where...
  25. 	public static final class NumDouble implements Num<Double> {
  26. 		static final NumDouble typeClass = new NumDouble();
  27.  
  28. 		public Double add(Double a, Double b) {
  29. 			return a + b;
  30. 		}
  31.  
  32. 		public Double mul(Double a, Double b) {
  33. 			return a * b;
  34. 		}
  35. 	}
  36.  
  37. 	// Num a => a -> a
  38. 	public static <T> T times2(Num<T> num, T x) {
  39. 		return num.add(x, x);
  40. 	}
  41.  
  42. 	// Num a => a -> a
  43. 	public static <T> T square(Num<T> num, T x) {
  44. 		return num.mul(x, x);
  45. 	}
  46.  
  47. 	public static void main (String[] args) throws java.lang.Exception
  48. 	{
  49. 		Integer intSix = times2(NumInt.typeClass, Integer.valueOf(3));
  50. 		Integer intNine = square(NumInt.typeClass, Integer.valueOf(3));
  51. 		Double doubleSix = times2(NumDouble.typeClass, Double.valueOf(3));
  52. 		Double doubleNine = square(NumDouble.typeClass, Double.valueOf(3));
  53. 		System.out.println(intSix);
  54. 		System.out.println(intNine);
  55. 		System.out.println(doubleSix);
  56. 		System.out.println(doubleNine);
  57. 	}
  58. }
Success #stdin #stdout 0.1s 320320KB
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6
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6.0
9.0
https://ideone.com/6NuhDD
language:
Java (HotSpot 12)
created:
10 years ago
visibility:
secret

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