使用通用类比< T>在Java中实现二叉树数据？ [英] Implementing Binary Tree in Java with Generic Comparable<T> data?

问题描述

在下面的二叉树实现中，为什么编译器在

if（ data.compareTo（this.data）< = 0），

产生

错误：不兼容的类型：java.lang.Comparable< T>无法转换为T ？

data 和 this.data 的类型是 Comparable< T> ，并且应该能够使用或是compareTo（）方法的参数... right ？好吧，显然不是。但我真的不明白为什么。泛型仍然让我困惑。

  public class MyBinaryTreeNodeG< T> {
 Comparable< T>数据; 
 MyBinaryTreeNodeG< T>父母; 
 MyBinaryTreeNodeG< T>剩下; 
 MyBinaryTreeNodeG< T>对; 
 
 public MyBinaryTreeNodeG（比较< T>数据）{
 this.data = data; 
} 
 
 public MyBinaryTreeNodeG< T> addChild（Comparable< T> data）{
 if（data.compareTo（this.data）< = 0）{//这是编译器插入的行
 //检查是否存在左树节点为空。如果是这样，请添加。否则，递归。 
} else {
 //相同的树节点
返回null; 
 
 $ / code> 

下面是一个更加标准的二叉树实现的剪辑。这编译好。但我仍然不明白为什么这是一个更好（根据编译器）实施比我上面。

  public class MyBinaryTreeNodeG< ; T扩展了Comparable< T>> {
 T data; 
 MyBinaryTreeNodeG< T>父母; 
 MyBinaryTreeNodeG< T>剩下; 
 MyBinaryTreeNodeG< T>对; 
 
 public MyBinaryTreeNodeG（T data）{
 this.data = data; 
} 
 
 public MyBinaryTreeNodeG< T> addChild（T data）{
 if（data.compareTo（this.data）< = 0）{
 // left node stuff 
} else {
 // right node东西
返回null; 
 
  




解决方案

=https://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html#compareTo(T） =nofollow>可比较的的javadoc： compareTo（T）由 Comparable 接口提供。该方法允许将实现此接口的 T 类的对象实例与另一个 T 实例进行比较。在第一个示例中，您将比较 Comparable< T> 与 Comparable< T> T ）

这应该起作用：

  public class MyBinaryTreeNodeG< T> {
 
比较< T>数据; 
 
 // ... 
 
 public MyBinaryTreeNodeG< T> （this.data.compareTo（data）<= 0）{
} else {
} 
 // ... 
 
 
  

Q: In my implementation of a binary tree below, why does the compiler choke at

if (data.compareTo(this.data) <= 0),

producing

Error: incompatible types: java.lang.Comparable<T> cannot be converted to T?

Both data and this.data are of type Comparable<T> and should be able to use or be an argument to the compareTo() method...right? Well, clearly not. But I really don't understand why. Generics are still baffling me.

public class MyBinaryTreeNodeG<T>{
  Comparable<T> data;
  MyBinaryTreeNodeG<T> parent;
  MyBinaryTreeNodeG<T> left;
  MyBinaryTreeNodeG<T> right;

  public MyBinaryTreeNodeG(Comparable<T> data){
   this.data = data; 
  }

  public MyBinaryTreeNodeG<T> addChild(Comparable<T> data){
    if (data.compareTo(this.data) <= 0) { //this is the line on which the compiler chockes
    //check if left tree node is null. If so, add. Otherwise, recurse.
    } else {
    //same for the right tree node
    return null;
  }  

The following is a clip from a more standard implementation of a binary tree. This compiles fine. But I still fail to see why this is a "better" (according to compiler) implementation than mine above.

public class MyBinaryTreeNodeG<T extends Comparable<T>>{
  T data;
  MyBinaryTreeNodeG<T> parent;
  MyBinaryTreeNodeG<T> left;
  MyBinaryTreeNodeG<T> right;

  public MyBinaryTreeNodeG(T data){
   this.data = data; 
  }

  public MyBinaryTreeNodeG<T> addChild(T data){
    if (data.compareTo(this.data) <= 0) {
      //left node stuff
    } else { 
     //right node stuff
    return null;
  }

解决方案

Look at the javadoc of Comparable : compareTo(T) is provided by the Comparable interface. This method allows to compare a object instance of T class implementing this interface with another T instance. In your first example, you're comparing a Comparable<T> with a Comparable<T> (and not with T)

This should work :

public class MyBinaryTreeNodeG<T> {

    Comparable<T> data;

    // ...

    public MyBinaryTreeNodeG<T> addChild(final T data) {
        if (this.data.compareTo(data) <= 0) {
        } else {
        }
        // ...
    }
}

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