Java 通用对象重用

Question

我有一个类层次结构，并且想为层次结构中的每个类构建属性 setter 列表。我要编写的代码类似于以下内容:

import java.util.ArrayList;
import java.util.List;
import java.util.function.BiConsumer;

class Attribute<E, T> {
    public final Class<T> dataType;
    public final BiConsumer<E, T> setter;
    public Attribute(final Class<T> dataType, final BiConsumer<E, T> setter) {
        this.dataType = dataType;
        this.setter   = setter;
    }
}

class Foo1 {
    public static final List<Attribute<Foo1, ?>> foo1Attrs = new ArrayList<>();
    static {
        foo1Attrs.add(new Attribute<>(String.class, Foo1::setProp1));
    }
    public void setProp1(final String prop1) {
    }
}

class Foo2 extends Foo1 {
    // The following line results in an error
    public static final List<Attribute<Foo2, ?>> foo2Attrs = new ArrayList<>(foo1Attrs);
    static {
        foo2Attrs.add(new Attribute<>(Integer.class, Foo2::setProp2));
    }
    public void setProp2(final Integer prop2) {
    }
}

当我编译上面的代码时，出现错误:

error: incompatible types: cannot infer type arguments for ArrayList<>  public static final List<Attribute<Foo2, ?>> foo2Attrs = new ArrayList<>(foo1Attrs);                                                                          ^  reason: inference variable E has incompatible bounds    equality constraints: Attribute<Foo2,?>    lower bounds: Attribute<Foo1,?>where E is a type-variable:  E extends Object declared in class ArrayList

I understand why I am getting the above error. My question is, what is the typically pattern employed to make the above code compile and run?

The only way I can figure out how to make this work is to create a copy constructor on the Attribute class that takes an Attribute<? extends E, T>, then duplicate the data members (pointers) stored in the Attribute class into a second memory location with a different type. This seems really heavy for what should be the equivalent of a static_cast in C++. The following changes exemplify this approach:

class Attribute<E, T> {
    ...
    public final BiConsumer<? super E, T> setter;
    ...
    public Attribute(final Attribute<? super E, T> other) {
        this.dataType = other.dataType;
        this.setter   = other.setter;
    }
}

class Foo2 extends Foo1 {
    public static final List<Attribute<Foo2, ?>> foo2Attrs = new ArrayList<>();
    static {
        for (Attribute<Foo1, ?> attr : foo1Attrs)
            foo2Attrs.add(new Attribute<>(attr));
        ...
    }
    ...
}

重申一下这个问题:是否有比上述方法更好的方法来解决此编译时错误？

Answer 1

自 foo2Attrs可以包含Attribute<Foo1, ?>哪里Foo1是 Foo2 的父类(super class)型，它应该声明为类型 List<Attribute<? super Foo2, ?>> 。这是有道理的，因为 Foo2对象可以由任何针对 Foo2 父类(super class)的属性 setter 设置其属性。 .

同样，foo1Attrs应声明为类型 List<Attribute<? super Foo1, ?>> .

这消除了由第一个类型参数引起的编译错误，但是当您尝试编写像foo2Attrs.get(i).setter.accept(foo, 23)这样的代码时仍然会出现问题。 ，因为setter被推断为类型 BiConsumer<? super Foo2, ?>和整数 23无法转换为第二个? .