在MySearchTree的K基本类型为Course。因此K必须“扩展” Comparable<Keyable<Course>> & Keyable<Course>。但是，它没有扩展Comparable<Keyable<DataElement>> & Keyable<DataElement>。
我猜 应该以类似的方式对泛型 或 。
public interface Keyable <T> {public String getKey();}
public interface DataElement<THIS extends DataElement<THIS>> extends Comparable<Keyable<THIS>>, Keyable<THIS>, Serializable {...}
public class Course implements DataElement<Course> {...}
public interface SearchTree<K extends Comparable<Keyable<K>> & Keyable<K>> extends Serializable {...}
public class MySearchTree implements SearchTree<Course> {
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