泛型树，自我有界泛型 [英] Generic tree, self bounded-generics

问题描述

我要为我的一个项目添加通用性。
我喜欢泛型，因为这使得我的代码更加健壮，自我记录并擦除所有那些丑陋的表演。

然而，我遇到了一个棘手的案例，一些问题试图为我的一个结构表达递归约束。

这基本上是某种通用树，具有双向链接（对于孩子和父母）。我最大限度地简化了类，以显示问题：

  public class GenericTree< 
 ParentClass扩展GenericTree<？，？>，
 ChildClass扩展GenericTree<？，？>> 
 {
 //属性
 private ArrayList< ChildClass> children = new ArrayList< ChildClass>（）; 
 private ParentClass parent = null; 
 
 //方法
 public void setParent（ParentClass parent）{
 this.parent = parent; 
} 
 
 public void addChild（ChildClass child）{
 child.setParent（this）; 
 this.children.add（child）; 
 
 
 $ / code> 

问题在于指令：小孩.setParent（this）。

Java给出以下错误：

绑定不匹配：ChildClass类型的setParent（？）方法不适用于

参数（GenericTree）。通配符参数？没有低于
的界限，实际上可能比参数
更具限制性GenericTree

我想要的是能够表达如下：

  public class GenericTree< 
 ParentClass扩展GenericTree<？，？>，
 ChildClass扩展GenericTree< [THIS_VERY_CLASS]，？>> 
  

要说孩子类的父类应该是它自己的...

我看过一些关于自定义泛型的文章，但我不知道如何在这种情况下应用它。

任何帮助将不胜感激。

解决方案

我能得到的最接近的是遵循Enum的自我引用模式。它仍然需要一个未经检查的强制转换，但是假设你的子类正确定义了T，它应该是安全的。

  public class GenericTree< T extends GenericTree< T，P，C>，P扩展GenericTree< P，？，T>，C扩展GenericTree< C，T，>> {
 
 //属性
 private ArrayList< C> children = new ArrayList< C>（）; 
 private P parent = null; 
 
 //方法
 public void setParent（P parent）{
 this.parent = parent; 
} 
 $ b $ public void addChild（C child）{
 @SuppressWarnings（unchecked）
 final T thisAsType =（T）this; 
 child.setParent（thisAsType）; 
 this.children.add（child）; 
 
 
 
 
 
 
编辑：
执行示例
  public static class SingleTypeTree< T>扩展
 GenericTree< SingleTypeTree< T>，SingleTypeTree< T> ;, SingleTypeTree< T> {
 
} 
  
 
I am to adding genericity to one of my projects.
I love generics as this makes my code more robust, self-documented and erases all those ugly casts.

However, I came across a tough case and have some issues trying to express a "recursive" constraint for one of my structures. 

This is basically some kind of "generic" tree, with double links (to children and parent). I have simplified the class at maximum to show the issue :
public class GenericTree<
    ParentClass extends GenericTree<?, ?>, 
    ChildClass extends GenericTree<?, ?>> 
{
    // Attributes
    private ArrayList<ChildClass> children = new ArrayList<ChildClass>();
    private ParentClass parent = null;

    // Methods 
    public void setParent(ParentClass parent) {
        this.parent = parent;
    }

    public void addChild(ChildClass child) {
        child.setParent(this);
        this.children.add(child);
    }
}
The problem is for the instruction : child.setParent(this).

Java gives the following error :

  
Bound mismatch: The method setParent(?) of type ChildClass is not applicable for the

  arguments (GenericTree). The wildcard parameter ? has no lower 
  bound, and may actually be more restrictive than argument 
  GenericTree
What I would like is to be able to express something like :
public class GenericTree<
    ParentClass extends GenericTree<?, ?>, 
    ChildClass extends GenericTree<[THIS_VERY_CLASS], ?>> 
To say that the parent class of the child class should be itself ...

I have looked at some articles about the self bounding generics, but I don't know how to apply it in this case.

Any help would be appreciated.
解决方案
The closest I could get is to follow Enum's pattern of self reference. It still requires an unchecked cast, but assuming your subclasses define T correctly, it should be safe.
public class GenericTree<T extends GenericTree<T, P, C>, P extends GenericTree<P, ?, T>, C extends GenericTree<C, T, ?>> {

    // Attributes
    private ArrayList<C> children = new ArrayList<C>();
    private P parent = null;

    // Methods
    public void setParent(P parent) {
        this.parent = parent;
    }

    public void addChild(C child) {
        @SuppressWarnings("unchecked")
        final T thisAsType = (T) this;
        child.setParent(thisAsType);
        this.children.add(child);
    }
}
Edit:
Implementation example
public static class SingleTypeTree<T> extends
    GenericTree<SingleTypeTree<T>, SingleTypeTree<T>, SingleTypeTree<T>> {

}


                        
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