java - 使用继承构建通用树

我正在构建一个通用的 Tree<T>类，支持子树的继承。但是我遇到了一些问题。你能帮我吗？

描述

让我们定义 Tree类和 BlueTree类，其中 BlueTree extends Tree .

让我们定义 Leaf类和 RedLeaf类，其中 RedLeaf extends Leaf .它们用作树包含的“数据”。

一个 Tree<Leaf>表示 Tree 类型的树，其“数据”的类型为 Leaf .

继承 (这不是正确的 Java 继承):

Tree<Leaf>可以有类型的 child

Tree<Leaf> , Tree<RedLeaf> , BlueTree<Leaf> , 和 BlueTree<RedLeaf> .

Tree<RedLeaf>可以有类型的 child

Tree<RedLeaf> , 和 BlueTree<RedLeaf> ,

但不是 Tree<Leaf> , 或 BlueTree<Leaf> .

BlueTree<Leaf>可以有类型的 child

BlueTree<Leaf> , 和 BlueTree<RedLeaf> ,

但不是 Tree<Leaf> , 或 Tree<RedLeaf> .

BlueTree<RedLeaf>可以有类型的 child

BlueTree<RedLeaf> ,

但不是 Tree<Leaf> , Tree<RedLeaf> , 或 BlueTree<Leaf> .

*这里，“ child ”是指树的 Twig /叶子。

(有点复杂，这就是我分开行的原因。)

编码

(如果你有一个解决方案，你可能不需要阅读下面我尝试的详细说明。如果你想一起找出解决方案，我的代码可能会给你一些想法 - 或者，它可能会混淆它们。)

初审 :(最简单的一个)

// This is the focus of this question, the class signature
public class Tree<T> {
    // some fields, but they are not important in this question
    private Tree<? super T> mParent;
    private T mData;
    private ArrayList<Tree<? extends T>> mChildren;

    // This is the focus of this question, the addChild() method signature
    public void addChild(final Tree<? extends T> subTree) {
        // add the subTree to mChildren
    }
}

这个类结构满足描述中的大部分要求。除了，它允许

class BlueTree<T> extends Tree<T> { }
class Leaf { }
class RedLeaf extends Leaf { }

Tree<Leaf> tree_leaf = new Tree<Leaf>();
BlueTree<Leaf> blueTree_leaf = new BlueTree<Leaf>();

blueTree_leaf.addChild(tree_leaf);    // should be forbidden

这违反了

BlueTree<Leaf> 不能有 Tree<Leaf> 类型的 child .

问题是因为，在 BlueTree<Leaf> ，其 addChild()方法签名还在

public void addChild(final Tree<? extends Leaf> subTree) {
     // add the subTree to mChildren
}

理想的情况是，BlueTree<Leaf>.addChild()方法签名更改(自动，继承时)为

public void addChild(final BlueTree<? extends Leaf> subTree) {
     // add the subTree to mChildren
}

(注意这个方法不能通过继承覆盖上面的方法，因为参数类型不同。)

有一个解决方法。我们可能会添加一个类继承检查，并抛出 RuntimeException对于这种情况:

public void addChild(final Tree<? extends Leaf> subTree) {
    if (this.getClass().isAssignableFrom(subTree.getClass()))
        throw new RuntimeException("The parameter is of invalid class.");
    // add the subTree to mChildren
}

但是使它成为编译时错误比运行时错误要好得多。我想在编译时强制执行此行为。

二审

第一个试验结构的问题是，参数类型Tree在方法中 addChild()不是泛型类型参数。因此它不会在继承时更新。这一次，让我们也试着让它成为一个泛型类型参数。

首先定义通用Tree类(class)。

public class Tree<T> {
    private Tree<? super T> mParent;
    private T mData;
    private ArrayList<Tree<? extends T>> mChildren;

    /*package*/ void addChild(final Tree<? extends T> subTree) {
        // add the subTree to mChildren
    }
}

然后是TreeManager管理一个 Tree目的。

public final class TreeManager<NodeType extends Tree<? super DataType>, DataType> {
    private NodeType mTree;

    public TreeManager(Class<NodeType> ClassNodeType) {
        try {
            mTree = ClassNodeType.newInstance();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void managerAddChild(final NodeType subTree) {
        mTree.addChild(subTree);
        // compile error: The method addChild(Tree<? extends capture#1-of ? super DataType>)
        //                in the type Tree<capture#1-of ? super DataType>
        //                is not applicable for the arguments (NodeType)
    }

    // for testing
    public static void main(String[] args) {
        @SuppressWarnings("unchecked")
        TreeManager<Tree    <Leaf>   , Leaf>    tm_TreeLeaf_Leaf           = new TreeManager<Tree    <Leaf>,    Leaf>   ((Class<Tree    <Leaf>>)    new Tree    <Leaf>   ().getClass());
        TreeManager<Tree    <RedLeaf>, RedLeaf> tm_TreeRedLeaf_RedLeaf     = new TreeManager<Tree    <RedLeaf>, RedLeaf>((Class<Tree    <RedLeaf>>) new Tree    <RedLeaf>().getClass());
        TreeManager<BlueTree<Leaf>   , Leaf>    tm_BlueTreeLeaf_Leaf       = new TreeManager<BlueTree<Leaf>,    Leaf>   ((Class<BlueTree<Leaf>>)    new BlueTree<Leaf>   ().getClass());
        TreeManager<BlueTree<RedLeaf>, RedLeaf> tm_BlueTreeRedLeaf_RedLeaf = new TreeManager<BlueTree<RedLeaf>, RedLeaf>((Class<BlueTree<RedLeaf>>) new BlueTree<RedLeaf>().getClass());

        System.out.println(tm_TreeLeaf_Leaf          .mTree.getClass());    // class Tree
        System.out.println(tm_TreeRedLeaf_RedLeaf    .mTree.getClass());    // class Tree
        System.out.println(tm_BlueTreeLeaf_Leaf      .mTree.getClass());    // class BlueTree
        System.out.println(tm_BlueTreeRedLeaf_RedLeaf.mTree.getClass());    // class BlueTree

        @SuppressWarnings("unchecked")
        TreeManager<Tree    <Leaf>   , RedLeaf> tm_TreeLeaf_RedLeaf     = new TreeManager<Tree    <Leaf>,    RedLeaf>((Class<Tree    <Leaf>>)    new Tree    <Leaf>   ().getClass());
        TreeManager<BlueTree<Leaf>   , RedLeaf> tm_BlueTreeLeaf_RedLeaf = new TreeManager<BlueTree<Leaf>,    RedLeaf>((Class<BlueTree<Leaf>>)    new BlueTree<Leaf>   ().getClass());

        System.out.println(tm_TreeLeaf_RedLeaf       .mTree.getClass());    // class Tree
        System.out.println(tm_BlueTreeLeaf_RedLeaf   .mTree.getClass());    // class BlueTree

        // the following two have compile errors, which is good and expected.
        TreeManager<Tree    <RedLeaf>, Leaf>    tm_TreeRedLeaf_Leaf     = new TreeManager<Tree    <RedLeaf>, Leaf>   ((Class<Tree    <RedLeaf>>) new Tree    <RedLeaf>().getClass());
        TreeManager<BlueTree<RedLeaf>, Leaf>    tm_BlueTreeRedLeaf_Leaf = new TreeManager<BlueTree<RedLeaf>, Leaf>   ((Class<BlueTree<RedLeaf>>) new BlueTree<RedLeaf>().getClass());
    }
}

TreeManager初始化没有问题；虽然线路有点长。它也符合描述中的规则。

但是，在调用 Tree.addChild() 时会出现编译错误。内TreeManager ，如上图所示。

三审

为了修复第二次试验中的编译错误，我尝试更改类签名(甚至更长)。现在 mTree.addChild(subTree);编译没有问题。

// T is not used in the class. T is act as a reference in the signature only
public class TreeManager3<T, NodeType extends Tree<T>, DataType extends T> {
    private NodeType mTree;

    public TreeManager3(Class<NodeType> ClassNodeType) {
        try {
            mTree = ClassNodeType.newInstance();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void managerAddChild(final NodeType subTree) {
        mTree.addChild(subTree);    // compile-error is gone
    }
}

我已经用与第二次试验非常相似的代码对其进行了测试。它没有任何问题，就像第二次审判一样。 (甚至更长。)

(您可以跳过下面的代码块，因为它只是在逻辑上重复。)

public static void main(String[] args) {
    @SuppressWarnings("unchecked")
    TreeManager3<Leaf   , Tree    <Leaf>   , Leaf>    tm_TreeLeaf_Leaf           = new TreeManager3<Leaf   , Tree    <Leaf>,    Leaf>   ((Class<Tree    <Leaf>>)    new Tree    <Leaf>   ().getClass());
    TreeManager3<RedLeaf, Tree    <RedLeaf>, RedLeaf> tm_TreeRedLeaf_RedLeaf     = new TreeManager3<RedLeaf, Tree    <RedLeaf>, RedLeaf>((Class<Tree    <RedLeaf>>) new Tree    <RedLeaf>().getClass());
    TreeManager3<Leaf   , BlueTree<Leaf>   , Leaf>    tm_BlueTreeLeaf_Leaf       = new TreeManager3<Leaf   , BlueTree<Leaf>,    Leaf>   ((Class<BlueTree<Leaf>>)    new BlueTree<Leaf>   ().getClass());
    TreeManager3<RedLeaf, BlueTree<RedLeaf>, RedLeaf> tm_BlueTreeRedLeaf_RedLeaf = new TreeManager3<RedLeaf, BlueTree<RedLeaf>, RedLeaf>((Class<BlueTree<RedLeaf>>) new BlueTree<RedLeaf>().getClass());

    System.out.println(tm_TreeLeaf_Leaf          .mTree.getClass());    // class Tree
    System.out.println(tm_TreeRedLeaf_RedLeaf    .mTree.getClass());    // class Tree
    System.out.println(tm_BlueTreeLeaf_Leaf      .mTree.getClass());    // class BlueTree
    System.out.println(tm_BlueTreeRedLeaf_RedLeaf.mTree.getClass());    // class BlueTree

    @SuppressWarnings("unchecked")
    TreeManager3<Leaf   , Tree    <Leaf>   , RedLeaf> tm_TreeLeaf_RedLeaf     = new TreeManager3<Leaf   , Tree    <Leaf>,    RedLeaf>((Class<Tree    <Leaf>>)    new Tree    <Leaf>   ().getClass());
    TreeManager3<Leaf   , BlueTree<Leaf>   , RedLeaf> tm_BlueTreeLeaf_RedLeaf = new TreeManager3<Leaf   , BlueTree<Leaf>,    RedLeaf>((Class<BlueTree<Leaf>>)    new BlueTree<Leaf>   ().getClass());

    System.out.println(tm_TreeLeaf_RedLeaf       .mTree.getClass());    // class Tree
    System.out.println(tm_BlueTreeLeaf_RedLeaf   .mTree.getClass());    // class BlueTree

    // the following two have compile errors, which is good and expected.
    TreeManager3<RedLeaf, Tree    <RedLeaf>, Leaf>    tm_TreeRedLeaf_Leaf     = new TreeManager3<RedLeaf, Tree    <RedLeaf>, Leaf>   ((Class<Tree    <RedLeaf>>) new Tree    <RedLeaf>().getClass());
    TreeManager3<RedLeaf, BlueTree<RedLeaf>, Leaf>    tm_BlueTreeRedLeaf_Leaf = new TreeManager3<RedLeaf, BlueTree<RedLeaf>, Leaf>   ((Class<BlueTree<RedLeaf>>) new BlueTree<RedLeaf>().getClass());
}

但是，当我尝试拨打 TreeManager3.managerAddChild() 时出现问题。 .

tm_TreeLeaf_Leaf.managerAddChild(new Tree<Leaf>());
tm_TreeLeaf_Leaf.managerAddChild(new Tree<RedLeaf>());      // compile error: managerAddChild(Tree<RedLeaf>) cannot cast to managerAddChild(Tree<Leaf>)
tm_TreeLeaf_Leaf.managerAddChild(new BlueTree<Leaf>());
tm_TreeLeaf_Leaf.managerAddChild(new BlueTree<RedLeaf>());  // compile error: managerAddChild(BlueTree<RedLeaf>) cannot cast to managerAddChild(BlueTree<Leaf>)

这是可以理解的。 TreeManager3.managerAddChild(NodeType)意味着 TreeManager3.managerAddChild(Tree<T>)并且没有通配符 Tree<? extends T>在参数类型中，如 Tree.addChild(final Tree<? extends T> subTree)在第一次审判中。

恳求你的帮助...

我已经没有想法了。我是不是走错了方向来解决这个问题？我花了很多时间输入这个问题，并尽我最大的努力使它更具可读性、更易于理解和遵循。我不得不说对不起，它仍然很长很冗长。但是，如果您知道路，请您帮忙，或者请给我您的任何想法？非常感谢您的每一个意见。非常感谢!

编辑 #1(对于 comment below )

位于初审 , 只允许 mChildren由 addChild() 修改(以及其他带有 isAssignableFrom() 检查的方法)，因此甚至允许用户继承 Tree和覆盖 addChild()不会破坏 Tree 的完整性。

/developer/util/Tree.java

package developer.util;

import java.util.ArrayList;

public class Tree<T> {

    private Tree<? super T> mParent;
    private final ArrayList<Tree<? extends T>> mChildren = new ArrayList<Tree<? extends T>>();

    public int getChildCount() { return mChildren.size(); }
    public Tree<? extends T> getLastChild() { return mChildren.get(getChildCount()-1); }

    public void addChild(final Tree<? extends T> subTree) {
        if (this.getClass().isAssignableFrom(subTree.getClass()) == false)
            throw new RuntimeException("The child (subTree) must be a sub-class of this Tree.");

        subTree.mParent = this;
        mChildren.add(subTree);
    }
}

/user/pkg/BinaryTree.java

package user.pkg;

import developer.util.Tree;

public class BinaryTree<T> extends Tree<T> {
    @Override
    public void addChild(final Tree<? extends T> subTree) {
        if (getChildCount() < 2) {
            super.addChild(subTree);
        }
    }
}

/Main.java

import user.pkg.BinaryTree;
import developer.util.Tree;

public class Main {

    public static void main(String[] args) {
        Tree<Integer> treeOfInt = new Tree<Integer>();
        BinaryTree<Integer> btreeOfInt = new BinaryTree<Integer>();

        treeOfInt.addChild(btreeOfInt);
        System.out.println(treeOfInt.getLastChild().getClass());
        // class user.pkg.BinaryTree

        try {
            btreeOfInt.addChild(treeOfInt);
        } catch (Exception e) {
            System.out.println(e);
            // java.lang.RuntimeException: The child (subTree) must be a sub-class of this Tree.
        }

        System.out.println("done.");
    }
}

你怎么认为？

最佳答案

在我看来，这个问题没有完美的解决方案。这基本上是由于类型删除。 Erasure of Generic Methods文章说明您的 addChild(final Tree<? extends Leaf> subTree)函数将变成 addChild(final Tree subTree)功能。所以，即使你能以某种方式拥有一个通用参数 <TreeType extends Tree<? extends Leaf>> addChild(final TreeType subTree) (无效的语法!)它将被删除为 addChild(final Tree subTree)在编译时。添加您的运行时测试将起作用，因此您所做的编辑将完成这项工作。

关于java - 使用继承构建通用树，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/18399922/

java - 使用继承构建通用树

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