gistfile1.txt

import java.util.ArrayList;
import java.util.Collection;

public class CovarianceTest {
	public static void main(String[] args) {
		Alpha alpha = new Alpha();
		
		// Type mismatch: cannot convert from element type capture#1-of ?
		// extends CovarianceTest.Data to CovarianceTest.Beta
		// Quick fix: Change type of beta to Beta.
		// That would work, but nasty type casting.
		for (Beta beta : alpha.getChildren()) {
			System.out.println(beta.getID());
		}
	}
	
	public static abstract class Data {
		public abstract Collection<? extends Data> getChildren();
	}
	
	public static class Alpha extends Data {
		public Collection<Beta> children;
		
		public Alpha() {
			this.children = new ArrayList<Beta>();
			this.children.add(new Beta(1));
			this.children.add(new Beta(2));
			this.children.add(new Beta(3));
		}
		
		@Override
		public Collection<? extends Data> getChildren() {
			return this.children;
		}
	}
	
	public static class Beta extends Data {
		private int id;
		
		public Beta(int id) {
			this.id = id;
		}
		
		@Override
		public Collection<? extends Data> getChildren() {
			return null;
		}
		
		public int getID() {
			return this.id;
		}
	}
}