通过实例学习Java集合框架HashSet

package collection;
 
import java.util.HashSet;
 
public class TestCollection {
  public static void main(String[] args) {
     
    HashSet<String> names = new HashSet<String>();
     
    names.add("gareen");
     
    System.out.println(names);
     
    //第二次插入同样的数据，是插不进去的，容器中只会保留一个
    names.add("gareen");
    System.out.println(names);
  }
}

/**
 * It makes no guarantees as to the iteration order of the set;
 * in particular, it does not guarantee that the order will remain constant over time.
*/

package collection;
 import java.util.HashSet;
 public class TestCollection {
  public static void main(String[] args) {
    HashSet<Integer> numbers = new HashSet<Integer>();
 
    numbers.add(9);
    numbers.add(5);
    numbers.add(1);
 
    // Set中的元素排列，不是按照插入顺序
    System.out.println(numbers);
 
  }
}

package collection;
 
import java.util.HashSet;
import java.util.Iterator;
 
public class TestCollection {
  public static void main(String[] args) {
    HashSet<Integer> numbers = new HashSet<Integer>();
     
    for (int i = 0; i < 20; i++) {
      numbers.add(i);
    }
     
    //Set不提供get方法来获取指定位置的元素
    //numbers.get(0)
     
    //遍历Set可以采用迭代器iterator
    for (Iterator<Integer> iterator = numbers.iterator(); iterator.hasNext();) {
      Integer i = (Integer) iterator.next();
      System.out.println(i);
    }
     
    //或者采用增强型for循环
    for (Integer i : numbers) {
      System.out.println(i);
    }
     
  }
}

private static final Object PRESENT = new Object();
package collection;
 
import java.util.AbstractSet;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Set;
 
public class HashSet<E>
  extends AbstractSet<E>
  implements Set<E>, Cloneable, java.io.Serializable
{
  //HashSet里封装了一个HashMap
  private HashMap<E,Object> map;
 
  private static final Object PRESENT = new Object();
 
  //HashSet的构造方法初始化这个HashMap
  public HashSet() {
    map = new HashMap<E,Object>();
  }
 
  //向HashSet中增加元素，其实就是把该元素作为key，增加到Map中
  //value是PRESENT，静态，final的对象，所有的HashSet都使用这么同一个对象
  public boolean add(E e) {
    return map.put(e, PRESENT)==null;
  }
 
  //HashSet的size就是map的size
  public int size() {
    return map.size();
  }
 
  //清空Set就是清空Map
  public void clear() {
    map.clear();
  }
   
  //迭代Set,就是把Map的键拿出来迭代
  public Iterator<E> iterator() {
    return map.keySet().iterator();
  }
 
}

package collection;
 
import java.util.HashSet;
 
public class TestCollection {
  public static void main(String[] args) {
 
    String[] ss = new String[100];
    // 初始化
    for (int i = 0; i < ss.length; i++) {
      ss[i] = randomString(2);
    }
    // 打印
    for (int i = 0; i < ss.length; i++) {
      System.out.print(ss[i] + " ");
      if (19 == i % 20)
        System.out.println();
    }
 
    HashSet<String> result = new HashSet<>();
 
    for (String s1 : ss) {
      int repeat = 0;
      for (String s2 : ss) {
        if (s1.equalsIgnoreCase(s2)) {
          repeat++;
          if (2 == repeat) {
            // 当repeat==2的时候，就找到了一个非己的重复字符串
            result.add(s2);
            break;
          }
        }
      }
    }
 
    System.out.printf("总共有 %d种重复的字符串%n", result.size());
    if (result.size() != 0) {
      System.out.println("分别是：");
      for (String s : result) {
        System.out.print(s + " ");
      }
    }
  }
 
  private static String randomString(int length) {
    String pool = "";
    for (short i = '0'; i <= '9'; i++) {
      pool += (char) i;
    }
    for (short i = 'a'; i <= 'z'; i++) {
      pool += (char) i;
    }
    for (short i = 'A'; i <= 'Z'; i++) {
      pool += (char) i;
    }
    char cs[] = new char[length];
    for (int i = 0; i < cs.length; i++) {
      int index = (int) (Math.random() * pool.length());
      cs[i] = pool.charAt(index);
    }
    String result = new String(cs);
    return result;
  }
}