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* to core-java-lang: * https://www.baeldung.com/java-inner-interfaces * https://www.baeldung.com/java-polymorphism * https://www.baeldung.com/java-recursion * https://www.baeldung.com/java-finalize * https://www.baeldung.com/java-method-overload-override * https://www.baeldung.com/java-deep-copy * https://www.baeldung.com/java-inheritance * https://www.baeldung.com/java-type-casting * https://www.baeldung.com/java-final * https://www.baeldung.com/a-guide-to-java-enums