java - 泛型需要类型转换

Question

我有课:

class ColumnBuilder < C extends ICompositeColumn > implements IColumnBuilder < C , 
List < ColumnNameComponent > > {

    @Override
    public C getColumn(List<ColumnNameComponent> columnNameComponents) {
        Collections.sort(columnNameComponents);
        C compositeColumn = (C)new CompositeColumnImpl();

        for (ColumnNameComponent component : columnNameComponents){
            compositeColumn.add(component.getOrdinal(),component.getValue());
        }
        return  compositeColumn;
    }

类 CompositeColumnImpl 实现 ICompositeColumn 接口(interface)那么为什么编译器需要在这一行中进行转换:

C compositeColumn = (C)new CompositeColumnImpl();

谢谢。

Answer 1

因为您声明了 C extends ICompositeColumn。这意味着扩展 ICompositeColumn 的每个类型都可以绑定(bind)到 C，而您的类型 CompositeColumnImpl 只是其中之一。

想想这个子类

public class SubColumnBuilder extends ColumnBuilder<AnotherCompositeColumnImpl> {
}

public class AnotherCompositeColumnImpl implements ICompositeColumn{
}