我正在构建一个通用的Tree<T>
类,支持子树的继承。但我遇到了一些问题。请您帮帮我好吗?
描述
让我们定义Tree
类和BlueTree
班级,哪里BlueTree extends Tree
.
让我们定义Leaf
类和RedLeaf
班级,哪里RedLeaf extends Leaf
。它们被用作树包含的“数据”。
A Tree<Leaf>
表示类型为 TreeTree
,其“数据”类型为Leaf
.
为了继承(这是not正确的 Java 继承):
- a
Tree<Leaf>
can have child of type
-
Tree<Leaf>
, Tree<RedLeaf>
, BlueTree<Leaf>
, and BlueTree<RedLeaf>
.
.
- a
Tree<RedLeaf>
can have child of type
-
Tree<RedLeaf>
, and BlueTree<RedLeaf>
,
-
but not
Tree<Leaf>
, or BlueTree<Leaf>
.
.
- a
BlueTree<Leaf>
can have child of type
-
BlueTree<Leaf>
, and BlueTree<RedLeaf>
,
-
but not
Tree<Leaf>
, or Tree<RedLeaf>
.
.
- a
BlueTree<RedLeaf>
can have child of type
-
BlueTree<RedLeaf>
,
-
but not
Tree<Leaf>
, Tree<RedLeaf>
, or BlueTree<Leaf>
.
*这里,“孩子”是指树的树枝/叶子。
(有点复杂,这就是我分开线条的原因。)
The code
(如果你有解决方案,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,它可能会让他们感到困惑。)
一审:(最简单的)
// 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
}
}
该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
- a
BlueTree<Leaf>
cannot有类型的孩子Tree<Leaf>
.
问题是因为,在BlueTree<Leaf>
, its addChild()
方法签名仍然是
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(继承时自动)为
public void addChild(final BlueTree<? extends Leaf> subTree) {
// add the subTree to mChildren
}
(注意这个方法cannot由于参数类型不同,通过继承重写上述方法。)
有一个解决方法。我们可以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
}
但使其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。这次,我们也尝试将其设为泛型类型参数。
首先定义一下一般情况Tree
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TreeManager
它管理着一个Tree
object.
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());
}
}
The TreeManager
初始化没有问题;不过队伍有点长。它也符合描述中的规则。
但是调用时出现编译错误Tree.addChild()
inside TreeManager
,如上图所示。
第三次审判
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
}
}
我已经使用与第二次试验非常相似的代码对其进行了测试。它创建时没有任何问题,就像第二次试验一样。 (甚至更长。)
(您可以跳过下面的代码块,因为它只是逻辑上的重复。)
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,当我尝试致电时出现问题TreeManager3.managerAddChild()
.
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)
means TreeManager3.managerAddChild(Tree<T>)
并且没有通配符Tree<? extends T>
在参数类型中,例如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。我解决这个问题的方向是错误的吗?我花了很多时间来输入这个问题,并尽最大努力使其更具可读性、更容易理解和遵循。我不得不说抱歉,它仍然很长而且冗长。但是如果您知道路的话可以帮忙吗,或者请给我您的任何想法吗?我们非常感谢您的每一次投入。多谢!
编辑#1(对于下面评论 https://stackoverflow.com/questions/18399922/build-a-generic-tree-with-inheritance#comment27433242_18399922)
总部设在一审, 只允许mChildren
修改为addChild()
(以及其他方法isAssignableFrom()
检查),所以即使允许用户继承Tree
并压倒一切addChild()
不会破坏树的完整性。
/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);
}
}
/用户/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?