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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