java - 抽象方法中的通用列表

Question

我有一个像这样的基抽象类:

public abstract class BaseModel {
//some code
}

我用另一个 Base 抽象类来扩展它，如下所示:

public abstract class BaseChildParentModel extends BaseModel {

    public abstract <E extends BaseModel> List<E> getChildren();

    public abstract <E extends BaseModel> List<E> createChildList();

    public void addChild(BaseModel child) {
        List<BaseModel> children = getChildren();
        if (children == null)
            children = createChildList();
        children.add(child);
    }
}

我的最后一个类扩展BaseChildParentModel，如下所示:

public class Subject extends BaseChildParentModel {

 //some fields

    public ArrayList<Subject> ChildSubjects;

    @Override
    public <E extends BaseModel> List<E> getChildren() {
        return ChildSubjects;
    }

    @Override
    public <E extends BaseModel> List<E> createChildList() {
        ChildSubjects = new ArrayList<>();
        return ChildSubjects;
    }
}

编译时出现以下错误:

Error:(52, 16) error: incompatible types
required: List<E>
found:    ArrayList<Subject>
where E is a type-variable:
E extends BaseModel declared in method <E>getChildren()

和

Error:(58, 16) error: incompatible types
required: List<E>
found:    ArrayList<Subject>
where E is a type-variable:
E extends BaseModel declared in method <E>createChildList()

我的错是什么？

Answer 1

试试这个:

public abstract class Subject extends BaseChildParentModel {
    public List<Subject> ChildSubjects;

    @Override
    public <E extends BaseModel> List<E> getChildren() {
        return (List<E>) ChildSubjects;
    }

    @Override
    public <E extends BaseModel> List<E> createChildList() {
        ChildSubjects = new ArrayList<>();
        return (List<E>) ChildSubjects;
    }
}