java - 如何创建泛型子类的实例？出现错误：“绑定不匹配：类型......不是有界参数的有效替代品......”

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：

“绑定不匹配：该类型Team不是该类型的有界参数<Team extends Comparable<Team>>的有效替代品ChampionsLeague<Team>”

Answer 1

在您的代码中，Team它只是一个占位符（在这种情况下，称为Type Variable）并且恰好隐藏了 type Team，而不是引用它。换句话说，该声明等同于：

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）Comparable自身的类（或接口）。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> { }

将工作：

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

编辑：

您可能试图实现的一些可能性：

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

或者

// 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> {