相关疑难解决方法(0)

我经常发现自己想要编写表单的泛型类定义

public class Foo<ActualType extends Foo<ActualType>>

例如,在这样的设置中:

public interface ChangeHandler<SourceType> {
    public void onChange(SourceType source);
}


public class Foo<ActualType extends Foo<ActualType>> {

    private final List<ChangeHandler<ActualType>> handlers = new ArrayList<>();

    public void addChangeHandler(ChangeHandler<ActualType> handler) {
        handlers.add(handler);
    }

    @SuppressWarnings("unchecked")
    protected void reportChange() {
        for (ChangeHandler<ActualType> handler: handlers)
            handler.onChange((ActualType) this);
    }
}


public class Bar extends Foo<Bar> {
    // things happen in here that call super.reportChange();
}


public static void main(String[] args) throws IOException {

    Bar bar = new Bar();
    bar.addChangeHandler(new ChangeHandler<Bar>() { …

我有一个构建器类,它从大多数方法返回以允许菊花链.为了使这个工作与子类,我希望父方法返回子的实例,以便子方法可用于链到最后.

public class BaseBuilder<T extends BaseBuilder<T>> {
    public T buildSomething() {
        doSomeWork();
        /* OPTION #1: */ return this;     // "Type mismatch: cannot convert from BaseBuilder<T> to T"
        /* OPTION #2: */ return T.this;   // "Type mismatch: cannot convert from BaseBuilder<T> to T"
        /* OPTION #3: */ return (T) this; // "Type safety: Unchecked cast from SqlBuilder<T> to T"
    }
}

public class ChildBuilder extends BaseBuilder<ChildBuilder> {}

选项#1和#2导致编译错误,选项#3导致警告(尽管可以抑制@SuppressWarnings("unchecked")).这里有更好的方法吗？我怎样才能安全地将Basebuilder投降给Childbuilder？