HashSet & TreeSet 代码示例

HashSet

这个程序将从System.in读取单词，将他们逐个添加到HashSet对象中，最后打印20个单词。

package set;
import java.util.*;
/**
 *  这个程序打印System.in中的不同单词，用来测试HashSet类
 */
public class SetTest
{
   public static void main(String[] args)
   {
      var words = new HashSet<String>(); 
      long totalTime = 0;
      try (var in = new Scanner(System.in))
      {
         while (in.hasNext())
         {
            String word = in.next();
            long callTime = System.currentTimeMillis();
            words.add(word);
            callTime = System.currentTimeMillis() - callTime;
            totalTime += callTime;
         }
      }
      Iterator<String> iter = words.iterator();
      for (int i = 1; i <= 20 && iter.hasNext(); i++)
         System.out.println(iter.next());
      System.out.println(". . .");
      System.out.println(words.size() + " 个不同的单词. " + totalTime + " 毫秒.");
   }
}

可以在命令行中重定向系统输入：java set.SetTest < alice30.txt 。
下面这个名为 alice30.txt 的文本文件有5909个不同的单词
alice30.txt

TreeSet

下面的程序用两个TreeSet对象来组织Item对象，第一个按照编号排序，这是Item对象的默认排序顺序。第二个按照自定义的比较器来排序。

package set;
import java.util.*;
/**
 * 用两种方法来对Item对象进行排序
 */
public class TreeSetTest
{
    public static void main(String[] args) {
        var tree1 = new TreeSet<Item>();
        tree1.add(new Item("Toaster", 1234));
        tree1.add(new Item("Widget", 4562));
        tree1.add(new Item("Modem", 9912));
        System.out.println(tree1);
        var tree2 = new TreeSet<Item>(new AComparator());
        tree2.addAll(tree1);
        System.out.println(tree2);
    }
}
class AComparator implements Comparator<Item> {
   @Override
   public int compare(Item o1, Item o2) {
      return o1.getDescription().compareTo(o2.getDescription());
   }
}

package set;
import java.util.*;
/**
 * 定义一个物品类，包含两个实例字段description和partNumber
 */
class Item implements Comparable<Item>
{
   private String description;
   private int partNumber;
   /**
    * 构造一个物品类
    * @param aDescription 物品的描述
    * @param aPartNumber 物品的编号
    */
   public Item(String aDescription, int aPartNumber)
   {
      description = aDescription;
      partNumber = aPartNumber;
   }
   public String toString()
   {
      return "[description=" + description + ", partNumber=" + partNumber + "]";
   }
   public boolean equals(Object otherObject)
   {
      if (this == otherObject) return true;
      if (otherObject == null) return false;
      if (getClass() != otherObject.getClass()) return false;
      var other = (Item) otherObject;
      return Objects.equals(description, other.description) 
              && partNumber == other.partNumber;
   }
   public int hashCode()
   {
      return Objects.hash(description, partNumber);
   }
   public int compareTo(Item other)
   {
      int diff = Integer.compare(partNumber, other.partNumber);
      // 先按编号排序，编号相同按描述排序
      return diff != 0 ? diff : description.compareTo(other.description);
   }
   public String getDescription()
   {
      return description;
   }
}