java - 如何让内部类继承封闭类的泛型类型？

Question

我正在使用 Java 6。

我无法让我的内部类使用与其封闭类相同的泛型类。目前我有

public class TernarySearchTree < T > {
    ...
    protected class TSTNode < T > {
        // index values for accessing relatives array
        protected static final int PARENT = 0, LOKID = 1, EQKID = 2, HIKID = 3; 
        protected char splitchar;
        protected TSTNode < T > [] relatives;
        private T data;

        protected TSTNode(char splitchar, TSTNode < T > parent) {
            this.splitchar = splitchar;
            relatives = new TSTNode[4];
            relatives[PARENT] = parent;
        }
    }
}

现在我收到警告

The type parameter T is hiding the type T

如果我从内部类中删除类型参数(即从 <T> 行中删除 protected class TSTNode<T>)，那么我会在 relatives = new TSTNode[4] 行上遇到编译错误.

我怎样才能把一切都做好？

Answer 1

您可以:

• 删除 <T>来自 TSTNode 的类型参数(即，使其成为非通用的)——它仍然可以访问外部 <T> .

• 重命名 <T>类中的类型参数 TSTNode到(说)U .

[更新]

以下是重写代码的四种不同方法。他们都编译。我认为你应该考虑使用 EnumMap (见下文第 4 版)。

版本 1:在内部类中使用不同名称的类型参数。您需要使用列表而不是数组。

  public class TernarySearchTree<T> {

    protected class TSTNode<U> {
      // index values for accessing relatives array:
      protected static final int PARENT = 0, LOKID = 1, EQKID = 2, HIKID = 3;

      protected char splitchar;
      protected List<TSTNode<U>> relatives;
      private U data;

      protected TSTNode(char splitchar, TSTNode<U> parent) {
        this.splitchar = splitchar;
        relatives = new ArrayList<TSTNode<U>>();
        for (int i = 0; i < HIKID; ++i) {  // Allocate 4 slots in relatives
          relatives.add(null);
        }
        relatives.set(PARENT, parent);
      }          
    }

    private TSTNode<T> node; // When you use it, pass T as U

    public TernarySearchTree() {
      node = new TSTNode<T>(',', null);  // When you use it, pass T as U 
    }
  }

版本 2:从封闭类继承 T

  public class TernarySearchTree<T> {

    protected class TSTNode {
      // index values for accessing relatives array:
      protected static final int PARENT = 0, LOKID = 1, EQKID = 2, HIKID = 3;

      protected char splitchar;
      protected List<TSTNode> relatives;
      private T data;

      protected TSTNode(char splitchar, TSTNode parent) {
        this.splitchar = splitchar;
        relatives = new ArrayList<TSTNode>();
        for (int i = 0; i < HIKID; ++i) {  // Allocate 4 slots in relatives
          relatives.add(null);
        }
        relatives.set(PARENT, parent);
      }
    }

    private TSTNode node; 

    public TernarySearchTree() {
      node = new TSTNode(',', null);  
    }
  }

版本 3:使用 map (而不是列表)

  public class TernarySearchTree<T> {

    protected class TSTNode {
      // index values for accessing relatives array:
      protected static final int PARENT = 0, LOKID = 1, EQKID = 2, HIKID = 3;

      protected char splitchar;
      protected Map<Integer, TSTNode> relatives;
      private T data;

      protected TSTNode(char splitchar, TSTNode parent) {
        this.splitchar = splitchar;
        // Create a hash map. No need to pre-allocate!
        relatives = new HashMap<Integer, TSTNode>(); 
        relatives.put(PARENT, parent); // set -> put
      }
    }

    private TSTNode node; 

    public TernarySearchTree() {
      node = new TSTNode(',', null);  
    }
  }
}

版本 4:将索引定义为枚举 + 使用 EnunMap(而不是 HashMap )

  public class TernarySearchTree<T> {

    protected static enum Index {
      PARENT, LOKID, EQKID, HIKID;
    }

    protected class TSTNode {    
      protected char splitchar;
      protected EnumMap<Index, TSTNode> relatives;
      private T data;

      protected TSTNode(char splitchar, TSTNode parent) {
        this.splitchar = splitchar;
        // Create an EnumMap. 
        relatives = new EnumMap<Index, TSTNode>(Index.class);
        relatives.put(Index.PARENT, parent); 
      }
    }

    private TSTNode node; 

    public TernarySearchTree() {
      node = new TSTNode(',', null);  
    }
  }

[更新 2]要记住一件事:Use EnumMap instead of ordinal indexing