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    copied!<p>This is the best possible code of second approach. It compiles without any warnings.</p> <pre><code>import static org.junit.Assert.fail; import org.junit.Test; import org.junit.internal.runners.JUnit4ClassRunner; import org.junit.runner.RunWith; @RunWith(JUnit4ClassRunner.class) public class TestGenerics { public static interface Copyable&lt;T&gt; { public void copy(T t); } public static class A&lt;T extends A&lt;T&gt;&gt; implements Copyable&lt;T&gt; { private String a = "--A--"; public void copy(T t) { System.out.println(t.a); } @SuppressWarnings("unchecked") public static Copyable&lt;Object&gt; getInstance() { return new A(); } } public static class B&lt;T extends B&lt;T&gt;&gt; extends A&lt;T&gt; { private String b = "--B--"; public void copy(T t) { super.copy(t); System.out.println(t.b); } @SuppressWarnings("unchecked") public static Copyable&lt;Object&gt; getInstance() { return new B(); } } @Test public void testCopy() { Copyable&lt;Object&gt; b1 = B.getInstance(); Copyable&lt;Object&gt; b2 = B.getInstance(); Copyable&lt;Object&gt; a = A.getInstance(); b1.copy(b2); // this works as intended try { b1.copy(a); // this throws ClassCastException fail(); } catch (ClassCastException cce) { } } } </code></pre> <p>And also i figured out all that happens in this program with help of reflection:</p> <pre><code> for (Method method : A.class.getMethods()) { if (method.getName().equals("copy")) { System.out.println(method.toString()); } } for (Method method : B.class.getMethods()) { if (method.getName().equals("copy")) { System.out.println(method.toString()); } } </code></pre> <p>Here is the output:</p> <pre><code>public void com.sbp.core.TestGenerics$A.copy(com.sbp.core.TestGenerics$A) public void com.sbp.core.TestGenerics$A.copy(java.lang.Object) public void com.sbp.core.TestGenerics$B.copy(com.sbp.core.TestGenerics$B) public void com.sbp.core.TestGenerics$B.copy(com.sbp.core.TestGenerics$A) public void com.sbp.core.TestGenerics$A.copy(java.lang.Object) </code></pre> <p>It means that:</p> <ol> <li><p>The copy(...) methods in A and B make compiler generate "bridges" - 2 different methods for each, one with reifed argument type from ancestor (reified T from Copyable becomes Object, reified "T extends A" from A becomes A) and that is why it's override and not overload, and the other one with reified argument type for defining class. First method (with autogenerated body) <em>downcasts</em> its argument to call the second (they call it a bridge). Because of this downcasting we get ClassCastException in runtime if we call b1.copy(a).</p></li> <li><p>It looks like direct type casting is cleaner and better tool for my problem and generics are better used in their direct purpose - to enforce compile time type checking.</p></li> </ol>
 

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