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  1. import java.util.*;
  2. import java.util.concurrent.atomic.AtomicInteger;
  3.  
  4. class Covariance {
  5.   public static void main(String[] args) {
  6.     X x = new X();
  7.     x.getChildren().add(new Y());
  8.     x.getChildren().add(new Y());
  9.     Test test = new Test();
  10.     test.methodTest(x);
  11.   }
  12.  
  13.   public static abstract class Data {
  14.     public abstract Collection<? extends Data> getChildren();
  15.   }
  16.  
  17.   public static class Y extends Data {
  18.     private static AtomicInteger idGen = new AtomicInteger();
  19.  
  20.     public final int id = idGen.getAndIncrement(); 
  21.  
  22.     public Collection<? extends Data> getChildren() {
  23.       return null;
  24.     }
  25.  
  26.     @Override
  27.     public String toString() {
  28.       return String.format("Y [id=%s]", id);
  29.     }
  30.   }
  31.  
  32.   public static class X extends Data {
  33.     public final Collection<Y> ys = new ArrayList<>();
  34.  
  35.     public Collection<Y> getChildren() {
  36.       return ys;
  37.     }
  38.   }
  39.  
  40.   public static class Test {
  41.     public void methodTest(X someX) {
  42.       for (Y y : someX.getChildren()) { //NOPE: type mismatch
  43.         System.out.println(y);
  44.       }
  45.     }
  46.   }
  47. }
Success #stdin #stdout 0.13s 320576KB
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Y [id=0]
Y [id=1]
https://ideone.com/a10fPI
language:
Java (HotSpot 12)
created:
10 years ago
visibility:
public

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