The line "static class Impl2 extends Impl1 implements Int1" is highlighted as incorrect "Class Impl2 must either be declared abstract or implement abstract method getSupportedHints() in test.GoodCodeRed.Int1" package test; import java.awt.RenderingHints; import java.util.Set; public class GoodCodeRed { static interface Int1 { public Set<RenderingHints.Key> getSupportedHints(); } static class Impl1 implements Int1 { public Set getSupportedHints() { return null; } } static class Impl2 extends Impl1 implements Int1 { } }