Java开发网 - 请问 Hashtable.containsKey(Object) 是怎样实现的？

Topic: 请问 Hashtable.containsKey(Object) 是怎样实现的？

1.请问 Hashtable.containsKey(Object) 是怎样实现的？

Posted by: dm356
Posted on: 2005-09-27 20:16

下面这段代码：很是奇怪
import java.util.*;

public class Hashtabletest {

  public static void main(String args[])
  {
    Hashtable hs = new Hashtable();
    for(int i = 0; i < 10; i++)
      hs.put(new Integer(i), new String(i + "aa"));

    if(hs.containsKey(new Integer(0)))
      System.out.println("OK");
    else
      System.out.println("SORRY");

    System.out.println((new Integer(1)) == (new Integer(1)));

    Hashtable hs1 = new Hashtable();
    for(int i = 0; i < 10; i++)
      hs1.put(new hashelement(i), new String(i + "aa"));

    if(hs1.containsKey(new hashelement(0)))
      System.out.println("OK");
    else
      System.out.println("SORRY");
  }
}

class hashelement
{
  int n = 0;
  hashelement(int i)
  {
    n = i;
  }
}

结果是：
OK
false
SORRY
为什么Integer与自定义的类不同产生的结果不同，如果想使自定义的类也作出Integer的结果，怎样修改自定义的类？

请各位赐教！

2.Re:请问 Hashtable.containsKey(Object) 是怎样实现的？ [Re: dm356]

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Posted by: bluecrystal
Posted on: 2005-09-28 00:10

1 Integer对象的比较用equals或者compareTo;
2 To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method 这是jdk文档的原话，通常要实现hashCode和equals方法
下面简单给出修改后的代码


import java.util.*;

public class Hashtabletest {

  public static void main(String args[])
  {
    Hashtable hs = new Hashtable();
    for(int i = 0; i < 10; i++)
      hs.put(new Integer(i), new String(i + "aa"));
    
    if(hs.containsKey(new Integer(0)))
      System.out.println("OK");
    else
      System.out.println("SORRY");
    
  System.out.println((new Integer(1)).equals(new Integer(1)));
    
    Hashtable hs1 = new Hashtable();
    for(int i = 0; i < 10; i++)
      hs1.put(new hashelement(i), new String(i + "aa"));
    
    if(hs1.containsKey(new hashelement(0)))
      System.out.println("OK");
    else
      System.out.println("SORRY");
  }
}

class hashelement
{
  int n = 0;
  hashelement(int i)
  {
    n = i;
  }
  public int getN() {
    return n;
  }
  public boolean equals(Object hs) {
    return (this.n == ((hashelement)hs).getN());
  } 
  public int hashCode() {
    return n;
  }
}

运行结果为：


OK
true
OK