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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.util.*;
  4. import java.lang.*;
  5. import java.lang.reflect.*;
  6. import java.io.*;
  7.  
  8. class Ideone {
  9. 	public static void main (String[] args) throws java.lang.Exception {
  10. 		Builder[] registry = {   new CircleBuilder()
  11. 		                       , new SquareBuilder()
  12. 		                       , new RectangleBuilder()
  13. 		                       };
  14.  
  15. 	    for (Builder<?> b : registry) b.shape().draw();
  16. 	}
  17. }
  18.  
  19. interface Shape {
  20. 	void draw();
  21. }
  22.  
  23. class Rectangle implements Shape {
  24. 	public void draw() { System.out.println("Rect"); }
  25. }
  26.  
  27. class Square extends Rectangle {
  28. 	public void draw() { System.out.println("Square"); }
  29. }
  30.  
  31. class Circle implements Shape {
  32. 	public void draw() { System.out.println("Circle"); }
  33. }
  34.  
  35. class Builder<SHAPE extends Shape> {
  36.  
  37. 	public final SHAPE shape = build();
  38.  
  39. 	@SuppressWarnings("unchecked")
  40. 	private SHAPE build() {
  41. 		try {
  42. 		    ParameterizedType parent =
  43.               (ParameterizedType) getClass().getGenericSuperclass();
  44.             Class<?> arg = (Class<?>) parent.getActualTypeArguments()[0];
  45.             return (SHAPE) arg.newInstance();
  46. 		} catch (ReflectiveOperationException e) {
  47. 			throw new RuntimeException(e);
  48. 		}
  49. 	}
  50.  
  51. 	public SHAPE shape() { return shape; }
  52. }
  53.  
  54. class CircleBuilder extends Builder<Circle> {}
  55.  
  56. class RectangleBuilder extends Builder<Rectangle> {}
  57.  
  58. class SquareBuilder extends Builder<Square> {}
Success #stdin #stdout 0.08s 381248KB
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Standard input is empty
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Circle
Square
Rect
https://ideone.com/MRreDz
language:
Java (HotSpot 12)
created:
10 years ago
visibility:
public

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