Set

Set 是一种用于保存不重复元素的数据结构。常被用作测试归属性，故其查找的性能十分重要。 三种set比较

HashSet

• 无重复数据
• 可以有空数据
• 数据无序

  Set<String> set = new HashSet<>();
  for (int i = 1; i < 6; i ++) {
    set.add(i + "");
  }
  set.add("1"); //不会重复写入数据
  set.add(null);//可以写入空数据
  Iterator<String> iter = set.iterator();
  while (iter.hasNext()) {
    system.out.print(iter.next() + " ");//数据无序 
  }// 输出(无序)为 3 4 1 5 null 2
  

  TreeSet

• 无重复数据
• 不能有空数据
• 数据有序

  Set<String> set = new TreeSet<>();
  for (int i = 1; i < 6; i ++) {
    set.add(i + "");
  }
  set.add("1"); //不会重复写入数据
  //set.add(null);//不可以写入空数据
  Iterator<String> iter = set.iterator();
  while (iter.hasNext()) {
    system.out.print(iter.next() + " ");//数据有序
  }// 输出(有序)为 1 2 3 4 5
  

  编程实现

Java

Set 与 Collection 具有安全一样的接口，通常有HashSet, TreeSet 或 LinkedHashSet三种实现。HashSet基于散列函数实现，无序，查询速度最快；TreeSet基于红-黑树实现，有序。

Set<String> hash = new HashSet<String>();
hash.add("billryan");
hash.contains("billryan");

在不允许重复元素时可当做哈希表来用。

HashSet

HashSet HashSet extends AbstractSet and implements the Set interface. It creates a collection that uses a hash table for storage.

A hash table stores information by using a mechanism called hashing. In hashing, the informational content of a key is used to determine a unique value, called its hash code.

The hash code is then used as the index at which the data associated with the key is stored. The transformation of the key into its hash code is performed automatically.

import java.util.*;

public class HashSetDemo {

   public static void main(String args[]) {
      // create a hash set
      HashSet hs = new HashSet();
      // add elements to the hash set
      hs.add("B");
      hs.add("A");
      hs.add("D");
      hs.add("E");
      hs.add("C");
      hs.add("F");
      System.out.println(hs);
   }
}

This would produce the following result: [D, E, F, A, B, C]

Iterate through hashset

How to Iterate over a Set/HashSet There are following two ways to iterate through HashSet: 1) Using Iterator 2) Without using Iterator

Example 1: Using Iterator

import java.util.HashSet;
import java.util.Iterator;

class IterateHashSet{
  public static void main(String[] args) {
     // Create a HashSet
     HashSet<String> hset = new HashSet<String>();

     //add elements to HashSet
     hset.add("Chaitanya");
     hset.add("Rahul");
     hset.add("Tim");
     hset.add("Rick");
     hset.add("Harry");

     Iterator<String> it = hset.iterator();
     while(it.hasNext()){
        System.out.println(it.next());
     }
  }
}

Output:

Chaitanya
Rick
Harry
Rahul
Tim

Example 2: Iterate without using Iterator

import java.util.HashSet;
import java.util.Set;

class IterateHashSet{
  public static void main(String[] args) {
     // Create a HashSet
     Set<String> hset = new HashSet<String>();

     //add elements to HashSet
     hset.add("Chaitanya");
     hset.add("Rahul");
     hset.add("Tim");
     hset.add("Rick");
     hset.add("Harry");

     for (String temp : hset) {
        System.out.println(temp);
     }
  }
}

Output:

Chaitanya
Rick
Harry
Rahul
Tim