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  1. import java.util.ArrayList;
  2. import java.util.Collection;
  3.  
  4. class CovarianceTest {
  5. 	public static void main(String[] args) {
  6. 		Alpha alpha = new Alpha();
  7.  
  8. 		// Type mismatch: cannot convert from element type capture#1-of ?
  9. 		// extends CovarianceTest.Data to CovarianceTest.Beta
  10. 		// Quick fix: Change type of beta to Beta.
  11. 		// That would work, but nasty type casting.
  12. 		for (Beta beta : alpha.getChildren()) {
  13. 			System.out.println(beta.getID());
  14. 		}
  15. 	}
  16.  
  17. 	public static abstract class Data {
  18. 		public abstract Collection<? extends Data> getChildren();
  19. 	}
  20.  
  21. 	public static class Alpha extends Data {
  22. 		public Collection<Beta> children;
  23.  
  24. 		public Alpha() {
  25. 			this.children = new ArrayList<Beta>();
  26. 			this.children.add(new Beta(1));
  27. 			this.children.add(new Beta(2));
  28. 			this.children.add(new Beta(3));
  29. 		}
  30.  
  31. 		@Override
  32. 		public Collection<Beta> getChildren() {
  33. 			return this.children;
  34. 		}
  35. 	}
  36.  
  37. 	public static class Beta extends Data {
  38. 		private int id;
  39.  
  40. 		public Beta(int id) {
  41. 			this.id = id;
  42. 		}
  43.  
  44. 		@Override
  45. 		public Collection<Data> getChildren() {
  46. 			return null;
  47. 		}
  48.  
  49. 		public int getID() {
  50. 			return this.id;
  51. 		}
  52. 	}
  53. }
Success #stdin #stdout 0.1s 320576KB
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https://ideone.com/djh4lR
language:
Java (HotSpot 12)
created:
10 years ago
visibility:
public

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