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  1. import java.util.ArrayList;
  2. import java.util.Collection;
  3.  
  4. public 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. 		public Collection<? extends Data> getChildren() {
  32. 			return this.children;
  33. 		}
  34. 	}
  35.  
  36. 	public static class Beta extends Data {
  37. 		private int id;
  38.  
  39. 		public Beta(int id) {
  40. 			this.id = id;
  41. 		}
  42.  
  43. 		public Collection<? extends Data> getChildren() {
  44. 			return null;
  45. 		}
  46.  
  47. 		public int getID() {
  48. 			return this.id;
  49. 		}
  50. 	}
  51. }
Compilation error #stdin compilation error #stdout 0s 0KB
comments (?)
stdin 
Standard input is empty
compilation info 
Main.java:4: error: class CovarianceTest is public, should be declared in a file named CovarianceTest.java
public class CovarianceTest {
       ^
Main.java:12: error: incompatible types: CAP#1 cannot be converted to Beta
		for (Beta beta : alpha.getChildren()) {
		                                  ^
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Data from capture of ? extends Data
2 errors
stdout 
Standard output is empty
https://ideone.com/Rvym07
language:
Java (HotSpot 12)
created:
10 years ago
visibility:
public

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