java - 如何创建通用子类的实例？出现错误 : "Bound mismatch: The type ... is not a valid substitute for the bounded parameter ..."

Question

public class ChampionsLeague<Team extends Comparable<Team>> extends League<Team>{
...

如何创建此类的实例？

ChampionsLeague<Team> league = new ChampionsLeague<>();

这不起作用:

"Bound mismatch: The type Team is not a valid substitute for the bounded parameter <Team extends Comparable<Team>> of the type ChampionsLeague<Team>"

Answer 1

在您的代码中，Team 只是一个占位符(在本上下文中，称为类型变量)，并且恰好隐藏类型Team，而不是引用它。换句话说，该声明相当于:

public class ChampionsLeague<T extends Comparable<T>> extends League<T> {

所以它实际上只是要求一个实现(扩展)自身的Comparable的类(或接口(interface))。所以这个例子:

public class Ball implements Comparable<Ball> {
    @Override
    public int compareTo(Ball o) { return 0; }
}
// or: public interface Ball extends Comparable<Ball> { }

会起作用:

ChampionsLeague<Ball> test = new ChampionsLeague<Ball>();

编辑:

您可能想要实现的目标的一些可能性:

// ChampionsLeague needs a type Comparable to Team
public class ChampionsLeague<T extends Comparable<Team>> extends League<T> {

或

// ChampionsLeague needs a subtype of Team
// In this case, you can make Team implement Comparable<Team> in its own decl.
public class ChampionsLeague<T extends Team> extends League<T> {