HashSet和HashMap的区别比较

HashSet 实现的Set接口，集合中不允许出现重复的值（如果重复会覆盖）:

package com.wlf.base;public class Person{      public Person(String name, int age)        {            this.name = name;            this.age = age;        }        private String name;        private int age;        public String getName()        {            return name;        }        public void setName(String name)        {            this.name = name;        }        public int getAge()        {            return age;        }        public void setAge(int age)        {            this.age = age;        }        public String toString()        {            return "{" + name + ", " + age + "}";        }}

    public static void main(String[] args)     {        Collection<Person> set = new HashSet<Person>();        Person object = new Person("张三",22);        set.add(new Person("李四", 19));        set.add(new Person("王五", 22));         set.add(object);        object.setAge(34);        set.add(object);        print(set);    }    private static void print(Collection<Person> set)    {        Iterator<Person> it = set.iterator();        while (it.hasNext())        {            Person p = it.next();            System.out.println(p.toString());        }    }

输出结果是：
{张三, 34}
{李四, 19}
{王五, 22}

HashSet内部使用HashMap实现， 通过

  if (e.hash == hash && ((k = e.key) == key || key.equals(k)))来判断加入的元素是否相同。这里的hash是对象的hashCode，默认是通过地址计算的hashCode, 这里加入过的object相同，第二次加入的会覆盖前一次加入的

为了避免出现下面这种输出：

    public static void main(String[] args)     {        Collection<Person> set = new HashSet<Person>();        Person object = new Person("张三",22);        set.add(new Person("李四", 19));        set.add(new Person("王五", 22));         set.add(new Person("张三",22));        set.add(object);        print(set);    }    private static void print(Collection<Person> set)    {        Iterator<Person> it = set.iterator();        while (it.hasNext())        {            Person p = it.next();            System.out.println(p.toString());        }    }

输出
{王五, 22}
{张三, 22}
{张三, 22}
{李四, 19}

输出了两个张三，这两个加入的对象不一样，本意是不想出现这种情况，所以在加入HashSet前需要重写HashCode和equals方法：
Person类增加重写方法：

    public int hashCode()    {        return name.hashCode() + age;    }    public boolean equals(Object obj)    {        if (!(obj instanceof Person))            throw new ClassCastException("类型不匹配");        Person p = (Person)obj;        return this.name.equals(p.getName()) && this.age == p.getAge();    }

HashMap 实现了Map接口,Map中不允许重复的 键值（如果重复会覆盖），HashMap中允许key和value为null

        //测试HashMap        HashMap<Integer,Person> tmpMap = new HashMap<Integer,Person>();        Integer intObj = new Integer(3);        tmpMap.put(intObj, new Person("李四",14));        tmpMap.put(intObj, new Person("李四",16));        tmpMap.put(null, null);        printMap(tmpMap);

hashMap的三种输出方法:

    private static void printMap(HashMap<Integer,Person> tmpHash)    {        //Iterator<Entry<Integer,Person>> result = tmpHash.entrySet().iterator();        //1.通过Entry来遍历HashMap        for(Entry<Integer,Person> result:tmpHash.entrySet())        {            System.out.println("key="+result.getKey()+"value="+result.getValue());        }        //2.通过KeySet来获取        for(Integer key:tmpHash.keySet())        {            System.out.println("key="+key+"value="+tmpHash.get(key));        }        //3.通过entrySet的iterator来获取        Iterator<Entry<Integer, Person>> it = tmpHash.entrySet().iterator();            while (it.hasNext()) {             Entry<Integer, Person> entry = it.next();             System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());            }     }

输出结果：key=nullvalue=null
key=3value={李四, 16}

HashMap是非线程安全的，但是Collection提供了方法转成线程安全：

 Map<Integer, Person> tmpMap = Collections.synchronizedMap(new HashMap<Integer,Person>());